Developments in Aquaculture and Fisheries Science - 33
DESIGN AND OPERATING GUIDE FOR AQUACULTURE SEAWATER SYSTEMSSECOND EDITION
DEVELOPMENTS IN AQUACULTURE AND FISHERIES SCIENCE The following volumes are still available:
9.
WATER QUALITY MANAGEMENT FOR POND FISH CULTURE By C.E. Boyd 1982 xii + 318 pages
17.
DISEASE DIAGNOSIS AND CONTROL IN NORTH AMERICAN MARINE AQUACULTURE Edited by C.J. Sindermann and D. V. Lightner 1988 xv + 412 pages
19.
CLAM MARICULTURE IN NORTH AMERICA Edited by J.J. Manzi and M. Castagna 1989 x + 462 pages
22.
FRONTIERS OF SHRIMP RESEARCH Edited by RE DeLoach, W.J. Dougherty and M.A. Davidson 1991 viii + 294 pages
23.
MARINE SHRIMP CULTURE: PRINCIPLES AND PRACTICES By A. W. Fast and L.J. Lester 1992 xvi + 862 pages
24.
THE MUSSEL MYTILUS: ECOLOGY, PHYSIOLOGY, GENETICS AND CULTURE By E. Cosling 1992 xiv + 589 pages
25.
MODERN METHODS OF AQUACULTURE IN JAPAN (2ND REVISED EDITION) Edited by H. Ikenoue and T. Kafuku 1992 xiv + 274 pages
26.
PROTOZOAN PARASITES OF FISHES By J. Lom and I. Dykova 1992 xii + 316 pages
27.
AQUACULTURE WATER REUSE SYSTEMS: ENGINEERING DESIGN AND MANAGEMENT Edited by M. B. Timmons and T. Losordo
28.
FRESHWATER FISH CULTURE IN CHINA: PRINCIPLES AND PRACTICE Edited by J. Mathias and S. Li 1994 xvi + 446 pages
29.
PRINCIPLES OF SALMONID CULTURE Edited by W. Pennell and B.A. Barton 1996 xxx + 1040 pages
30.
STRIPED BASS AND OTHER MORONE CULTURE Edited by R. M. Harrell 1997 xx + 366 pages
31.
BIOLOGY OF THE HARD CLAM Edited by J.N. Kraeuter and M. Castagna 2001 xix + 751 pages
32.
EDIBLE SEA URCHINS: BIOLOGY AND ECOLOGY Edited by J. M. Lawrence 2001 xv + 419 pages
33.
DESIGN AND OPERATING GUIDE FOR AQUACULTURE SEAWATER SYSTEMS (2nd EDITION) by J.E. Huguenin and J. Colt 2002 viii + 328 pages
D e v e l o p m e n t s in A q u a c u l t u r e and F i s h e r i e s S c i e n c e - 33
DESIGN AND OPERATING GUIDE FOR AQUACULTURE SEAWATER S Y S T E M S - S E C O N D EDITION
JOHN E. HUGUENIN Falmouth, Massachusetts U.S.A.
JOHN COLT
Northwest Fisheries Science Center National Marine Fisheries Service Seattle, Washington U.S.A.
2002
ELSEVIER
A m s t e r d a m - L o n d o n - New York - O x f o r d - Paris - S h a n n o n - T o k y o
ELSEVIER SCIENCE B.V. Sara Burgerhartstraat 25 RO. Box 211, 1000 AE Amsterdam, The Netherlands
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Contents
Chapter 1.
Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Chapter 2.
Problem definition and establishing requirements . . . . . . . . . . . . . . . . . . . . . . . . . .
5
2.1
Design process ........................................................................
5
2.2
Defining objectives . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
6
2.3
Quantifying requirements ..............................................................
7
2.4
Production cycle ......................................................................
2.5
Production modeling ..................................................................
12
2.5.1
Growth models ..............................................................
12
2.5.2
Mortality models ............................................................
13
2.5.3
Length-weight relationships
13
.................................................
9
2.6
Types of systems . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
16
2.7
System carrying capacity ..............................................................
20
...................................................................
22
2.7.1
Ammonia
2.7.2
Nitrite . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
24
2.7.3
Nitr ate . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
24
2.7.4
Dissolved oxygen ............................................................
24
2.7.5
Carbon dioxide ..............................................................
25
2.7.6
H y d r o g e n sulfide . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
25
2.7.7
Total gas p r e s s u r e . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
27
2.7.8
pH ..........................................................................
27
2.7.9
Residual chlorine . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
27
2.7.10
Temperature .................................................................
28
2.7.11
Salinity . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
28
2.7.12
Heavy metals ................................................................
28
2.7.13
Biocides .....................................................................
28
2.8
Carrying capacity guidelines ...........................................................
2.9
Design requirements ...................................................................
39
2.9.1
W a t e r flow . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
41
2.9.2
Rearing volume/area
42
2.9.3
R e a r i n g c o n t a i n e r size a n d n u m b e r . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
2.10
........................................................
Constraints ............................................................................
Chapter 3.
Site considerations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
29
43 43 47
3.1
Marine conditions .....................................................................
47
3.2
Terrestrial c o n d i t i o n s . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
54
3.3
Permitting ............................................................................
54
3.4
Site s e l e c t i o n . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
56
Chapter 4. 4.1
Seawater sources . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
59
Options and considerations ............................................................
59
VI 4.2
Artificial seawater . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
60
4.3
S e a w a t e r wells . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
62
4.4
M a r i n e intakes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
66
Chapter 5.
System planviews and elevations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
73
5.1
Generic s y s t e m . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
73
5.2
Elevations and head tanks . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
75
5.3
Intake and p u m p house considerations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
76
5.4
Discharge considerations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
79
Chapter 6. 6.1
Piping design and calculations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
81
M a j o r tradeoffs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
81 82
6.2
Biofouling control . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
6.3
Water h a m m e r . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
86
6.4
Frictional losses in pipes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
87 93
6.5
Frictional losses in fittings . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
6.6
O p e n channel flow . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
93
6.7
M o m e n t u m in pipes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
97
Chapter 7.
Pump Selection . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
101
P u m p options . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
101
7.2
Generic centrifugal p u m p . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
104
7.3
N P S H and d y n a m i c head . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
108
7.4
M a t c h i n g s y s t e m and p u m p . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
110
7.1
Chapter 8. Materials selection . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8.1 Biological constraints . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
117
8.2
Seawater constraints . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
120
8.3
Piping materials . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
122
8.4
P u m p materials . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
124
8.5
M a r i n e concrete . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
126
8.6
P r o b l e m areas . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
126
Chapter 9.
Seawater flow control . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
117
129
9.1
Gravity flow . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
129
9.2
Water level control . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
131
9.3
Control of flow rate . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
133
9.4
Flow measurement . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
134
Chapter 10.
Suspended solids removal . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
137
10.1
Considerations, tradeoffs and options . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
137
10.2
Cartridge filters . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
139
10.3
D i a t o m a c e o u s earth filters . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
140
10.4
Filter bags . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
10.5
Centrifuges and cyclones . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
141 141 141
10.6
Sand filters . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
10.7
Microscreens . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
144
10.8
Sedimentation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
145
VII
Chapter 11.
Heating and cooling . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
151 151
11.1
Setting requirements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
11.2
Heat exchangers .......................................................................
154
11.3
P r o b l e m areas . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
157
Chapter 12.
A e r a t i o n and degassing . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
12.3.1
A b s o r p t i o n efficiency . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
163 163 163 165 168 168 168 171
12.3.2
Blower selection . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
173
12.3.3
Pumping and mixing .........................................................
175
12.1
Aeration system requirements ..........................................................
12.2
G r a v i t y aer ator s . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12.2.1
12.3
Packed columns .............................................................
12.2.2
P e r f o r a t e d tray a e r a t o r . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
12.2.3
L a t t i c e aerator . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
S u b m e r g e d aer ators . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
12.4
Gas supersaturation and degassing .....................................................
178
12.5
Removal of other gases . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
180
Chapter 13.
Disinfection . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
13.1
Considerations and options ............................................................
13.2
Chemical compounds ..................................................................
13.3
Ozone ................................................................................
13.4
Ultraviolet (UV) radiation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Chapter 14.
Alarms, monitoring and automatic control systems . . . . . . . . . . . . . . . . . . . . . . . . .
14.1
Characteristics and options ............................................................
14.2
A l a r m points . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
14.3
Automatic control . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Chapter 15.
183 183 184 187 190 193
193 195 197
15.1
Water recycling . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Setting r e q u i r e m e n t s a n d o p t i o n s . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
15.2
Nitrification a n d biofilters . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
202
15.3
Foam fractionation ....................................................................
206
15.4
A c t i v a t e d c a r b o n a n d ion e x c h a n g e . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
207
15.5
Algae .................................................................................
210
Chapter 16.
201 201
16.1
Wet laboratory areas . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . G e n e r a l c o n s i d e r a t i o n s a n d trade-offs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
16.2
I n d o o r areas . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
212
16.3
O u t d o o r areas . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
215
Chapter 17. 17.1
Construction considerations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
211 211
217
Construction arrangements .............................................................
217
17.2
Construction cost estimating . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
217
17.3
Design changes .......................................................................
219
17.4
I n s t a l l a t i o n o f s e a w a t e r lines . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
219
17.5
Start-up . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
220
VIII
Chapter 18. 18.1 18.2 18.3 18.4 18.5 18.6
Operational considerations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Operating procedures . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . A s s i g n m e n t o f responsibilities . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Spares and redundancy . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Preventive maintenance . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Monitoring and control . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Operational p r o b l e m areas . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
223 223 224 224 226 226 228
Chapter 19.
Putting it all together . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
233
Chapter 20.
Summary commandments . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
249
References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
253
Appendices . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
263 264 264 265 269 272 276 281 284 287 291 295 298 306 309 314 320 322 322 323 323 324 324 324 325 325 325
A
B C D E F G H I J K L M N
Conversions, definitions and seawater properties . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . A- 1 Conversion factors . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . A-2 Definitions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . A-3 Seawater properties as a function o f temperature and salinity . . . . . . . . . . . . . . . . . . F l o w - t h r o u g h seawater system bibliography . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Reuse seawater system bibliography . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Water quality bibliography . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Biofouling bibliography . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Materials bibliography . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Suspended solids removal bibliography . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Temperature control bibliography . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Aeration and degassing bibliography . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Disinfection bibliography . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Culture unit shape, size and flow pattern/hydraulics bibliography . . . . . . . . . . . . . . . . . . . . . . . Feeder bibliography . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Indexes for equipment and supplies . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . C o m p u t e r data search . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . N-1 Traditional fee-based searches . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . N-2 Internet sources . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . N-2.1 Governmental sites (and sites with a lot o f free information) . . . . . . . . . . N-2.2 N o n - g o v e r n m e n t a l sites . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . N-2.3 D o c u m e n t delivery services . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . N-2.4 E-mail alerting service . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . N-2.5 Society web sites . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . N-2.6 On-line journals . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . N-3 Aquaculture information lists . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Subject Index . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
327
Chapter 1
Introduction
There is a strong and broad-based need from many educational, research and commercial organizations and individuals for information on the planning, design, construction and operations of seawater systems (salinities from freshwater to brines) with flow rates in the range of 10-1000 gallons per minute (40-4000 liters per minute). While the objectives of these systems vary widely, they all depend on a common technological and biological database. Since the seawater system is usually only a means to an end, most potential users have little prior practical experience or technical knowledge in this area. Practical information on seawater systems for culturing purposes tends to be fragmented and not readily available in usable form. Most conventional engineering experience is with marine systems which are orders of magnitude larger. This technology is often not readily scaled down, or directly useable and, more than likely, prohibitively expensive. Unfortunately, neither a good understanding of the biology nor the engineering alone is likely to result in a practical system. Biologists generally do not understand the mechanical and hydraulic aspects, engineers usually do not sufficiently appreciate the biological requirements, and usually neither appreciate the economic and regulatory constraints that can heavily influence decisions. In short, because success depends on a blend of expertise, avoidable mistakes with seawater systems are common. While many may claim expertise, there are, in fact, few experienced bioengineers available and they tend to be associated with small consulting companies specializing in these matters. Bioengineers generally spend much of their time 'educating' the user or client to the tradeoffs and consequences of major-system decisions. Not uncommonly, major-system decisions have already been made and fixed prior to seeking bioengineering assistance. This can be a serious problem. In addition, this bioengineering expertise may be either unavailable or unaffordable to many potential users. While considerable knowledge and experience has been gained with the design, construction, and operations of seawater culture systems, this accumulated experience has been very poorly documented and is therefore not readily available. In addition, much of this knowledge has been learned the hard way by trial and error. Since communications are poor, the same mistakes tend to be made over and over again. This book is intended to fill these gaps and hopefully reduce seawater systems problems. For simple systems and conditions, this book may be completely adequate. For more complex systems or conditions, this book may avoid the expensive mistakes often made by users early in a project and greatly reduce the 'user education' phase of system design. Even with this book as a guide, many projects will be sufficiently complex to require the user to seek outside technical expertise. There are usually a number of sources of 'free' technical help, such as universities, equipment manufacturers, or various governmental
extension services. This is all well and good if the needs are limited to advisory services. These sources will generally not provide any design services, such as systematic evaluations of alternatives, numerical sizing of systems or components, or providing drawings, construction specifications or cost estimations. At some project scope or stage of development, the user may have to seek professional aquacultural consulting services. There are a number of considerations, alternative approaches, tradeoffs, and potential problems in this complex matter (see Mayo, 1998). The seawater flow rate range of 10-1000 gallons per minute (gpm) or approximately 40-4000 liters per minute (lpm) has been chosen because it includes the vast majority of such systems currently in use. Many educational systems tend to the low side of the range while large research-oriented systems are towards the upper end. This also includes marine aquariums and most commercial and governmental molluscan, crustacean and marine fish hatcheries. The only major omission is large-scale commercial grow-out operations (as distinct from the hatchery phase). For economic reasons these systems tend to be much larger and often tidally pumped, either with ponds, pens or cages, and are outside the scope of this book. The stated flow rates of interest are very small by industrial water use standards. As an example, a single-unit 1000-megawatt nuclear power plant may pump 500,000 gpm (1,900,000 lpm) of water through its condensers. While the necessary equipment and techniques to handle the flows of interest to us are well within the mechanical and hydraulic state of the art, aquaculture imposes additional constraints on the design and operations which are not normally encountered in industrial applications. The primary additional constraints are due to the life support requirements of the culture organisms. This may include both maximum and minimum concentrations of dissolved gases, major ions, heavy metals, trace organic contaminates or particulate matter, to name just a few. Therefore, the available industrial water handling equipment and techniques must be used with caution when applied to seawater culturing systems. The primary objective of this book is to provide, in one place, basic information and considerations necessary to plan, build and operate seawater systems for culturing purposes. Due to the complexities of the subject and the fact that requirements of specific situations are highly variable, all the potentially important information cannot be fully developed or presented in a single book. Since a unit of aquatic animal or plant biomass, to a good first-order approximation, will react similarly under identical conditions, species differences have purposely been de-emphasized. This simplifies the presentation, broadens the usefulness of the text, and allows generalized conservative guidelines to be developed. Considerable thought and effort has been applied towards providing directions for more detailed efforts in the form of references and appendices in various technical areas. The book is not primarily intended for the experts or those already very experienced with seawater systems, although conditions vary widely and 'experts' may also find it to be of value. The primary value to 'experts' may be in the substantial accumulation of readily accessible data. The authors have found the first edition of their own book to be a valuable and frequently consulted reference. Of particular importance is Appendix A, which covers conversions of units, definitions, and seawater properties. The subject of this book crosses many different disciplines, interests, and backgrounds. This can cause considerable confusion with definitions and units of measure. Mixed systems of units are common and often unavoidable. Even within a system of measure,
there may be several different parameters and units commonly used for a particular purpose. Appendix A provides definitions, conversion factors between units and important constants to reduce this confusion. However, the problem is often more complicated than straightforward numerical conversion. As an example, a 1 in. pipe is not the same as a 2.54 cm pipe. A '1 in.' pipe is not a dimension, since neither the inside nor outside diameters are likely to be exactly this dimension, but rather a designation, which does not readily convert without knowing the type of pipe and the industrial standard involved. At least in the Western Hemisphere, industrial equipment and engineering practices are still predominantly in 'English' units while scientific matters are largely metric. The International System of Units (abbreviated SI in all languages) is intended as a basis for worldwide standardization of measurement units. Even in those countries using SI units, it is unlikely that all units will be SI (though they may be related to SI units). For example, the units of volumetric flow are cubic meters/second. A flow of 50 gpm is equal to 0.0032 m3/s, an inconveniently small number for practical use. The units of hour, day, month, liter, and hectare are not SI units. For practical applications, one must often cope with mixed units. For these reasons, this book has purposefully not standardized on either system. Numerical examples, provided to demonstrate methods, may be in either system or even have mixed units. In the United States, the conventional decimal marker is a dot ('.'). Outside of the United States, the comma (',') is sometimes used as a decimal marker. In the United States, the comma is also used to separate digits into groups of three (for example, 24,567), a practice that can cause confusion. The SI convention recommends separating digits into groups of three using a small space instead of the comma (for example, 24 567). This book will use the dot as a decimal marker and the comma as a separator. Another area of confusion arises in the use of parts per hundred (%) (or parts per thousand, %0). While this convention is convenient in many situations, it is important to understand that because the parameter is expressed as a percent, the actual number has not changed. A 2% growth rate is 2 parts per hundred or actually 0.02. A 1000 lb of fish growing at 2%/day, will grow 1000 x 0.02 = 20 lb, not 1000 x 2 -- 2000 lb. Computer spreadsheets can reduce the chance of errors as they allow numbers to be displayed as percentages, but retain the actual values for use in computations. The First Edition of this book was published in 1989. There have been a number of developments in this field since that time. Probably the most significant is the rapid growth in the industrial infrastructure specifically designed for aquatic life support activities. In the past, most of the equipment has had to be taken and adapted on an individual basis from other fields such as wastewater treatment, chemical process industries and various industrial and manufacturing applications. This change, while dramatic, has been more of an adaptation and modification process rather than the development of any radically new processes. In short, while there is now a considerably wider choice of parts and equipment and they are also easier to identify, the basic principles are unchanged. The changes that have occurred are reflected in the unit process chapters of this Second Edition. Computer use has accelerated rapidly in our society. This has had some impact in the form of greater computer usage in the monitoring and control of life support system and for data acquisition and dissemination. Chapter 14 has been renamed (Alarms, Monitoring and Control Systems) and expanded and Appendix N (Computer Data Search) added to include these changes. Since the first edition of this book was published, three important
changes have impacted the organization of this revised edition: the widespread availability of computer spreadsheets (EXCEL and 123), the consolidation of the software industry, and the ability to communicate rapidly over the Internet. For routine computations, the spreadsheet has largely replaced both the remote mainframe computer and to a significant degree, the use of hand-held calculators. (Newer hand-held calculators are much more user-friendly and many have the ability to plot results.) The combination of these two technologies allow the posting of useful spreadsheets on a website, the downloading (or e-mailing) of these programs to personal computers, and use of these programs. The common spreadsheet programs are available on a wide number of platforms (IBM, Macintosh, Sun, etc.) and commonly can read other spreadsheets (for example, EXCEL can read Lotus 123 spreadsheets). The regulatory and permitting environment has, in general, gotten a lot more severe since the First Edition. This is due to a lot of factors, such as reduced site availability, expanding populations, competition and conflicts in the near-shore environment, and increased legislation. The effects can differ considerably between regions and can be quite local in their specific characteristics. Increased constraints exist on both the supply side, for access to good water, and on the discharge side, relative to discharge permitting. Treatment prior to discharge is becoming much more common and more extensive. Water problems have increased interest and development in greater water reuse. While considerable progress has been made in high reuse system technology, these systems are not suitable for some applications and are still economically constrained for other applications. Biosecurity, in terms of reducing the transmission of pathogens between internal units as well as with the outside, has gained in importance in some applications. These include marine mammal rehabilitation facilities and for hatcheries of various kinds. This can have major design impacts. Physical security has also increased in importance due to possible treats of vandalism or terrorism as well as due to liability concerns in our increasingly more litigious society. The numerical problems contained in the First Edition have proven useful. The Second Edition includes even more examples in the various subject areas. However, practical problems rarely include only one subject area but more often require application and synthesis of a number of different technical and non-technical aspects. For this reason, Chapter 19 (Putting It All Together), dealing with integrating problems and examples, has been added in the Second Edition.
Chapter 2
Problem Definition and Establishing Requirements
2.1 Design process The design process is a complex and iterative operation. It has many characteristics that are common to all hardware systems, whether they are seawater systems or space shuttles. There exists many simplified schematics of the process. Fig. 2.1 has many of the common components of such models and has been chosen to be compatible with the format of this book. All human activities are motivated by subjective human needs. These are basic needs such as for food, shelter, clothing, security/defense, educating and rearing of the young and, by extension, making money. These needs have to be translated into specific objectives (see Section 2.2) which might be satisfied by hardware systems, in our case seawater systems. These statements of objectives generally need considerable greater refinement and quantification before any design calculations or dimensioning can be done (see Sections 2.3
Subjective Human Purpose
Define Objectives
C--'--
Quantify Requirements
Environment -Physical -Economic - Legal / Political/Social
_.
,~
System Synthesis and Evaluation
Implementation I and Construction J
...
~
Operations
Benefits
Fig. 2.1. Simplified design process.
and 2.4). Systems synthesis or, if preferred, systems design, is done in the next phase. However, it cannot be done in isolation. The total environment (physical, economic and legal/political/social) within which the system is expected to operate must be considered. The environmental factors impose limitations or constraints on system design. These constraints may be physical, economical, legal, time or other factors. It is not at all uncommon that the constraints are sufficiently severe to make the design of a workable system impossible or impractical. In this case, the only option is to reexamine the statements of objectives or requirements. This evaluation and feedback will often occur, even when it is not a matter of infeasibility. It is often done to improve performance or lower initial or operating costs. Many cycles of design, evaluation and redesign may occur before the decision to build is made. Feedback often occurs into the construction and operating phases, but is hopefully limited to matters of 'fine tuning'. Major reevaluations in the later phases can be serious matters that can be fatal to projects. The benefits that result from the operations of the system hopefully meet the needs that initiated the project in the beginning, assuming that these have not appreciably changed with time.
2.2 Defining objectives A critical first step in the design process is the clear definition of the objectives of the system. While this book is directed towards applications with educational, research or commercial objectives, what does this actually mean? Is the education to be of the 'show and tell' variety common to public aquariums, or of the participatory type more characteristic of field stations or in a more formal academic context and for what age group or audience. Is it for tropical or temperate species, how many species, both animals and plants, which species, how many, etc.? What specifically are the educational goals? How many people at a time and for how long? Similar questions can be developed for research and commercial systems. While the answers may vary widely, they will strongly influence the specification and design of the system. Wrong directions or assumptions at the beginning of a project can seldom be reversed later without high cost, extensive delays, or poor system performance. Such mistakes often prove fatal to the project. System objectives may be based on (1) a clear present need, (2) a potential need that must be addressed, (3) an anticipated future need, or (4) a combination of the first three. While it is very difficult to clearly define in quantitative terms, present and future objectives, it has to be done. Considerable time and effort should be devoted to checking all assumptions, identifying implicit assumptions and resolving differences in objectives between the participants. Many doomed projects have been launched based on a false premise, which no amount of engineering ingenuity or operator expertise can overcome. It is also not uncommon to have different participants in a project working towards different and sometimes conflicting objectives. In addition, there is often the problem of unstated or 'hidden' objectives. These might involve aspects of 'prestige', bureaucratic needs, or even individual egos. What can considerably complicate the matter is that it is very rare to have just a single well defined objective. The more common pattern is a series or a hierarchy of objectives in decreasing priority. Educational systems are often also used for research purposes, commercial systems usually have active research aimed at improving production, and research systems may have limited holding or production requirements. The problems are generated by the fact
that the requirements to support the different objectives may be in direct conflict or indirectly competing for scarce resources (time, money, water flow, etc.). The ordering and relative priorities of the various objectives can significantly influence design decisions. Since relative priorities of objectives is by necessity subjective, personnel changes can dramatically alter project objectives. Not uncommonly an important requirement or objective is not considered during design. The discovery of new requirements can prove to be seriously disrupting and expensive. An example is provisions for visitors, both formal and informal. While considerably removed from their basic objectives, research and commercial systems often entertain visitors for educational, promotional or 'public relations' reasons. These visitors can easily get in the way, pose a biosecurity hazard, threaten the work in progress or present a liability risk, if these potential problems have not been foreseen. Making a good impression and 'looking good' does have some value and might be considered during design, even for these types of systems. During the early stages of a project, the objectives are not fixed but dynamic. They change as new evaluations are made and design feedback is provided. They characteristically proceed from an initial general statement to greater and greater refinement and detail, which consider all of the objectives, priorities, and given conditions. A simple example for a commercial shellfish hatchery is given below:
Initial statement 9 Raise oysters for sale. Rising enthusiasm for project 9 Raise 25 million oyster larvae, 20 million clam and 5 million bay scallop larvae a month, and produce 1 million half-shell oysters per year. Consideration of seasonal demands and costs 9 Raise 100 million oyster larvae a month during June to September, 25 million oyster larvae during the rest of the year, and produce 1 million half-shell oysters per year. After first detailed cost estimate and discussion with banker 9 Raise 40 million oyster larvae a month during June to September, 10 million during the rest of the year and produce 1 million half-shell oysters a year. Mortgage house, postpone buying new car, and get spouse a secure job. After discussion with lawyers on permitting problems and additional inputs from engineers and banks 9 Restructure program to small-scale part-time 'Pilot Project' pending finding high rolling venture capitalist.
2.3 Quantifying requirements The problems of establishing requirements for a new culture system and defining them quantitatively is a very difficult task. It is the source of the biggest and costliest mistakes. The basic statements of needs, which launch the project, are often quite subjective and not quantified. These statements of need must go through many iterations of progressively greater detail, refinement, and evaluation before anything can be built and operated. The problems of translating basic objectives into quantitative terms necessary for construction are usually not given the attention and scrutiny they deserve.
TABLE 2.1 Major systems decisions 9 Species 9 Site 9 Capacity or scale (initial and future) 9 Seawater source 9 Freshwater source 9 Flow-throughor reuse options 9 Biomass loading (normal and maximum) 9 Operating schedule (seasonal or year-round) 9 Power sources 9 Monoculture or Polyculture 9 System Lifetime 9 Pumping schedule 9 Redundancy and reliability 9 Operator skill levels 9 Future options
One common and frequently encountered basic mistake is that the user at the very beginning of the project makes a number of major system's decisions without knowing the system consequences, without any sort of systematic evaluation and based solely on a priori judgment. These types of decisions are listed in Table 2.1. Such decisions may be irreversible, such as the site having already been purchased or otherwise secured. These decisions and the specifics of any given situation are so interactive that, if these decisions have to be accepted as 'givens', the project may be doomed before it starts. The tradeoffs involved with these decisions can be long and complex. Species selection may be fixed by research or educational needs. Criteria for selection of species for commercial culturing can be more involved (Webber and Riordan, 1976a) but the decision is often erroneously made on marketing considerations only. Site selection is equally complex with many important considerations (see Chapter 3). While there is no such thing as a perfect site, a bad site can easily doom a project. Bad choices with respect to the other decisions in Table 2.1 can, depending on circumstances, be equally serious. The probability of irreconcilable conflicts between the 'fixed' system's decisions and the project objectives and criteria is greatly increased if there exists strong economic constraints. It is common to have 'no acceptable solution' without fundamental changes in the project. If not recognized early, this can prove fatal to the project. The considerations and tradeoffs involving many of these decisions will be considered further. Another basic mistake is for the user to provide the general objective statements and then bow out of the process expecting someone else to 'engineer' the system. This can result from the users inability to answer detailed quantitative questions about both present and future needs. The user then assumes that the questioning engineer is better qualified to quantitatively define the requirements. The end result can be equipment and facilities which, in their approach, dimensions and capabilities, have implicit statements about not only present and future objectives and requirements but also on priorities, future growth, and management and operating philosophies. In addition, if the engineers do not have considerable prior experience with biological systems, 'sound engineering judgment' can produce systems that cannot adequately support the culture organisms and may hurt them directly. Sometimes such
mistakes can be remedied, but often can only be reversed at unacceptable cost. Working around such built-in problems often results in accepting reduced capacity, reduced efficiency, operating constraints, increased cost and increased risks. It often happens that even after considerable iteration between users and engineers, future requirements cannot be precisely defined. This is not surprising, since a fixed major seawater system and associated facilities may have a useful lifetime of 50 years or even more, and external conditions can dramatically change several times in this period of time. The problem is likely to be most acute for research oriented systems, due to the rapidity with which their requirements can change. Philosophically there are two extreme approaches to this problem. One is to purposely build the system for only the limited period of time for which the requirements can be clearly defined. This can be the best approach, especially for some seasonal or temporary systems, which may be entirely reconfigured and rebuilt from year to year, or if intended for some highly specialized purpose. This approach generally has the lowest initial cost. The dangers are that the requirements may change faster than anticipated or the system may not be abandoned or replaced when originally planned. The result may leave the user with an expensive to maintain system with low reliability and marginal usefulness. Under these conditions the overall cost could be very high. The alternate extreme approach is to hedge against all uncertainty by providing extra capabilities to match the range of uncertainties and by providing design features which will simplify future additions and modifications. Experience has shown that system requirements can change very rapidly with time and are not entirely predictable. However, the vast majority of changes in system requirement's are usually in directions that were at least considered during design, and often even anticipated. Changes in requirements are likely to occur late in the design phase or early in the operational phase and they can be very disruptive if they were not foreseen. The problems with this approach are that future requirements may develop in completely unanticipated directions and that, in the extreme, this approach inevitably becomes money limited. A flexible or adaptive system can cost much more than one whose requirements can be precisely stated with confidence.
2.4 Production cycle Based on the system objectives, the production cycle must be defined and may include the following items. (1) The total number of steps in the production cycle. For many species, a separate rearing or holding unit may be needed for each step. For short-term holding of a single adult species, there may be only one step. In contrast, a hatchery for even a single species might include broodstock holding, adult maturation, spawning, larval rearing and rearing of juveniles. Each of these steps or phases might have distinctly different requirements and facilities. (2) The total amount of time required for each step. This may be highly variable, depending on assumed environmental conditions, such as temperature, food sources and other factors. (3) The reproductive potential of the adults and the survival during each step. The reproductive potential in many species can be strongly influenced by conditions during the maturation process. Survival can also be highly variable depending on the system used, culturing conditions, and the skill and knowledge of the operators.
10
Example 2.1. Computation of required capacity per stage It is desired to produce 100,000 of the rare alpha fish, with an average weight of 1.5 pound each. The following data on alpha fish have been derived from previous operations. The tendency is to use for design the best of previous results, with the consequences that expectations are extremely over-optimistic relative to initial operations of a new system. Compute the number of individuals needed at the start and end of each rearing stage. Eggs/female Females/male Broodstock margin Survival data: juvenile fry eggs
10,000 4/1 300%
Number at end Number at start, end/survival, 100,000/0.80
100,000 harvested fish 125,000 juveniles
Number at end Number at start, end/survival, 125,000/0.70
125,000 juveniles 178,572 fry
Number at end Number at start, end/survival, 178,572/0.20
178,572 fry 892,858 eggs
Number females, 892,858/10,000 = 90 x 3 (margin) Number males, 270/4
270 females 68 males
80% 70% 20%
Starting with the production objective and survival data for the last step, the number of animals needed at the start of the last step can be computed. This process is repeated for each production step in reverse order and will result in the total number of animals or adult spawners needed at the front end (see Example 2.1). This information for each stage will also be used to compute the required water flows and tank volumes needed, as shown in a more detailed example for a shrimp hatchery (Example 2.2). However difficult, these parameters must be quantified. Since the inputs for these calculations are functions of assumed environmental conditions, operating procedures, and expertise of personnel, the numerical results usually have high uncertainty. The planning numbers often prove to be very optimistic relative to actual subsequent performance. If multiple species are being considered, the problems and uncertainties are increased. If the species are closely related, the differences may be important to the project but fortunately are often of minor importance in terms of impacts on equipment requirements and procedures. Since the parameters are interdependent, there are also a number of possible tradeoffs between production cycle values and water quality, environmental conditions, food sources, procedures, loading densities, economics and acceptable risks. One of the most common tradeoffs is the faster growth versus increased costs of more optimum temperature control. Other common tradeoffs are the increased cost of higher water quality or greater feed quantity versus the faster growth they may produce. In the absence of hard data directly relevant to the conditions of a given project, these tradeoffs can become highly subjective with a high premium on prior direct personal experience.
11
Example 2.2. Shrimp hatchery capacity and sizing A marine shrimp hatchery is designed to produce 1,000,000 post-larval (about P-20) shrimp per month. The total process takes about a month. The following data for a 'large tank' hatchery have been compiled from similar operations. Eggs/female Females/male Brood stock margin Stocking density: brood stock Naupli P-1 P-20 Survival data: Eggs-Naupli Naupli-P- 1 P-1 to P-20
= 400,000 = 1/1, average weight 90 g -- 700% (factor of 7) = = = =
2/m 2 60/1 = 60,000/m 3 20/1 = 20,000/m 3 15/1 = 15,000/m 3
= 50% -- 60% = 80%
(A) Compute the number of animals at the beginning and end of each stage. P-1 to P-20, at end at beginning Naupli to P-I, at end at beginning Eggs to Naupli, at end at beginning Brood stock: females 4,166,666/400,000 x 7 males
1,000,000 1,250,000 1,250,000 2,083,333 2,083,333 4,166,666 73 73
(B) The spawning females are temporarily moved to the spawning tanks and then quickly returned to the maturation facility. If the brood stock animals average 90 g each and the maturation tanks are shallow (typical water depth 12-18 in., use 12 in.), estimate the total maturation tank bottom area, water volume, and required seawater flow rate using research criteria. Tank bottom area (based on 2 animals/m 2) = 146/2 = 73 m 2 (2580 ft 2) For 1 ft water depth, volume = 22.25 m 3 (786 ft 3) Interpolating Fig. 2.7C for 100 g animals, 20~ = 0.1 kg/lpm Total biomass = 146 x 90 g = 13.1 kg of animals Required flow -- 13.1 kg/0.1 kg/lpm = 131 lpm (C) If the culturing from eggs through P-20 takes place in the same tank, estimate the total required tank volume at each stage and identify the limiting condition. Stage
Max. No.
Density
Volume
Naupli P- 1 P-20
2,083,333 1,250,000 1,000,000
60,000/m 3 20,000/m 3 15,000/m 3
34.7 m 3 62.7 m 3 66.7 m 3
The most limiting time from a loading standpoint is, not surprisingly, at the end of the culturing cycle. Note: While the numbers in this problem have been taken from several sources on 'large tank' hatcheries, the example presented in Fig. 5.2 is for a similar 1,000,000 PL/month penaeid shrimp hatchery.
12 2.5
Production
modeling
The space and water requirements of aquatic species depend strongly on size of the animals. Therefore, in cases where significant growth occurs, it is necessary to estimate total biomass, space, and water requirements on a weekly or monthly basis. Production modeling typically requires the following input components: - Time-step for the model; Estimated starting and ending dates; Initial and final weights (or lengths); - Growth model; - Mortality model; Water flow and animal density requirements; Metabolic production functions (if needed). The output of the production model is an estimate of the total biomass in the group as well as the water and space requirements. This information can be used to estimate the ending date of the production cycle or estimate what temperature changes would be necessary to achieve a needed ending date. For commercial operations, this type of analysis is used to adjust the starting dates of multi-batch production facilities to achieve maximum output or relative constant harvest rates (Watten, 1992). There are a vast number of different types of growth models for different life stages, or for different species. Three simple models will be discussed: A L method, degree day method, and the exponential method. Additional information on length-weight relationships, presented as conversions between lengths and weights, is commonly needed by species. 2.5.1 Growth Models The AL method is based on Haskell's work on trout (Haskell, 1959). He observed that at a constant temperature and adequate food supply, the increase in the length of a fish is a constant. This same relationship has been commonly observed for mollusks and crustaceans: Lt+l
--
Lt + AL
(2.1)
where Lt+l is equal to the length at time t 4- 1, L is equal to length at time t, and AL is the change in length/unit time. The AL is typically expressed in inches per day or inches per week. It is also possible to derive the following relationships for specific growth rate and specific feeding rate: Specific growth rate (%) -
[AW] 3xALxl00 -100 L
(2.2)
Specific feeding rate (%) -
IF ] 3 x AL x 100 x FCR ~ 100 L
(2.3)
where A W is change in weight, W is weight, F is wet weight of feed/time period, and FCR is feed conversion ratio (feed input/change in weight).
13 If the AL is expressed as inches per day, the specific growth and feeding rates will be in %/day. Once the A L is known, growth and feeding rates can be computed if the feed conversion ratio and length are know. The AL method can be applied to variable temperatures by the use of what is commonly called the temperature unit theory. This 'theory' is based on the assumption that over some temperature range, growth is directly proportional to temperature. The dependence of AL on temperature has the general form of AL = ( T U G R ) ( T - To)
(2.4)
where TUGR is equal to temperature unit growth rate (inches/day per ~ T is the water temperature (~ and To is the temperature (~ that results in no growth (AL = 0). For many cold-water species, To is generally assumed to be equal to 0~ (Westers, 1981). Eq. 2.4 can be used to modify Eqs. 2.2 and 2.3 for changes in temperatures. This discussion has been based on daily ~ temperature units; other forms of the temperature unit have been used (monthly temperature units and temperature units based on ~ An example of production planning with the AL method is presented in Example 2.3. When weight is plotted against time, the growth of many species can be described by an exponential curve of the form:
Wt = Woe Gt
(2.5)
where Wt is weight at time t (g), e is base e (2.71828183 .... ), W0 is weight at t -- 0 (g), G is specific growth rate (%/day), and t is time (days). In general, a single value of G may be valid for only part of the growth curve, although the growth over the whole period may be modeled by dividing the growth curve into successive exponential segments. Many aquatic species are only reared over short periods of their total life, and a single value of G may be entirely adequate. The value of G will depend on species, temperature, and the specific rearing conditions (see Example 2.4). If the weight is known at two times, Eq. 2.5 can be solved for the growth rate (G):
G -- [ln(W2) - ln(W1) tl I
(2.6)
where In is the natural logarithm, W1 = weight at tl, and W2 is the weight at t2. 2.5.2 Mortality Models Mortality data are typically quite site- and system-dependent. For modeling purposes, it is common to assume a fixed mortality in terms of %/day or %/week. 2.5.3 Length-Weight Relationships The relationship between length and weight for aquatic animals has the form of:
W = cL"
(2.7)
where W is wetweight in grams, L is length in mm, and c and n are species-specific constants. Typical values for c and n are presented in Table 2.2 for important culture species.
Example 2.3. Growth model using the A L method Compute the total biomass, specific feeding rate, and total feed requirement for 100,000 chinook salmon using the AL method (Eqs. 2.1-2.4). The temperature (~ is equal to 11.5 § (7.5) sin (0.985d + 238.4), where d is the Julian day (1-366). The initial length = 140 mm, c = 8.1910 -6 mm/g, n = 3, FCR = 1.3, mortality -- 0.25%/week, TUGR = 0.08 mm/day per ~ and To = 0~ The fish are ponded on 4/19/95 and the final target weight is 300 g. With the given conditions and equations, the following table of predictions can be developed. The final weight of 300 g is estimated to be reach on 9/13 after 21 weeks of rearing. Date
Temperature (~
Number #
Length (mm)
AL (mm/week)
4/19/95 4/26/95 5/ 3/95 5/10/95
9.66 10.54 11.44 12.34
100,000 99,750 99,501 99,252
140 146 152 159
n/a 5.90 6.41 6.91
Weight (g) 22 25 29 33
Continue computations on w e e n y time step until massis equalto 300 g 9/13/95 17.19 94,879 334 9.63 305
#/kg
Total biomass (kg)
Specific feeding rate (%/day)
Total feeding rate (kg/day)
44 39 35 30
2,247 2,537 2,879 3,281
n/a 2.25 2.34 2.42
n/a 57 67 79
3
28,904
0.36
210
Initial conditions (4/19/95):
Julian Day Number of fish Length Mass, 8.19 x 10 -6 (140) 3
109 100,000 fish 140 mm 22.47 g
End of week 1 (4/26/95):
Julian Day Temperature, 11.5 + (7.5) sin (0.985 • 116 + 238.4) Number of fish, 100,000 • (1.00 - 0.0025) A L, 0.08 x ( 1 0 . 5 4 - 0) x 7 Length, 140 mm + 5.9 mm Mass, 8.19 x 10-6 (145.90) 3 #/kg, 1000/25.44 Total biomass, 99,750 x (25.44/1000) Specific feeding rate, 3 x (5.90/7) x 100 • 1.3/145.90 Total feed, 2537.64 kg x 0.02253
116 10.54~ 99,750 fish 5.90 mm 145.90 mm 25.44 g 39.31 fish/kg 2537.64 kg 2.253%/day 57 kg/day
Repeat computations on weekly time step until mass is equal to 300 g (9/13/95)
15
The weight term in Eq. 2.7 is generally reported on a wet weight basis. For mollusks, wet body weight (excluding shell weight) may be reported. For crabs, the width of the carapace is used. For crayfish, shrimp, or lobster, length is measured in terms of carapace length or total length. Carapace length is typically measured from the back of the eye socket. The value of n for fish is near 3.0. The value of c for fish typically ranges from 5 x 10 -6 to 15 x 10 -6. Both n and c may vary with sex, especially for mature animals (see Example 2.5).
Example 2.4. Comparison of exponential growth curve with AL methods The following growth curve was observed over a 360-day trial (Actual weight column). Compute the exponential growth rate from Eq. 2.5 and compute the predicted weight for each time step. Compare results with AL method (c -- 11.22 x 10 -6 and n -- 3.00) using the mean AL for the whole production cycle. Days
Actual weight (g)
Actual lengths (mm)
Actual AL (mm/15 days)
0.0 15.0 30.0 45.0 60.0 75.0 90.0 105.0 120.0 135.0 150.0 165.0 180.0 195.0 210.0 225.0 240.0 255.0 270.0 285.0 300.0 315.0 330.0 345.0 360.0
5.0 8.4 13.0 19.3 27.4 37.7 50.4 65.8 84.2 105.9 131.4 160.8 194.5 232.9 276.4 325.2 379.7 440.3 507.5 581.4 662.6 751.4 848.3 953.5 1067.6
76.4 90.8 105.0 119.8 134.7 149.8 165.0 180.3 195.8 211.3 227.1 242.9 258.8 274.8 291.0 307.2 323.5 339.8 356.3 372.8 389.4 406.1 422.8 439.7 456.5
. 14.4 14.2 14.8 14.8 15.1 15.2 15.3 15.4 15.6 15.8 15.8 15.9 16.0 16.1 16.2 16.3 16.4 16.5 16.5 16.6 16.7 16.8 16.8 16.9
Mean AL =
.
Computed weight using G . 6.3 7.8 9.8 12.2 15.3 19.1 23.9 29.9 37.4 46.7 58.4 73.1 91.4 114.2 142.8 178.6 223.4 279.3 349.2 436.7 546.0 682.8 853.8 1067.6
Computed L using G
Computed L using mean AL
Computed weight using mean AL
82.3 88.7 98.5 102.9 110.9 119.4 128.7 138.6 149.3 160.9 173.3 186.7 201.2 216.7 233.5 251.6 271.0 292.0 314.6 338.9 365.1 393.3 423.8 456.5
92.2 108.1 123.9 139.7 155.6 171.4 187.3 203.1 218.9 234.8 250.6 266.5 282.3 298.1 314.0 329.8 345.7 361.5 377.3 393.2 409.0 424.9 440.7 456.5
8.8 14.2 21.3 30.6 42.3 56.5 73.7 94.0 117.7 145.2 176.6 212.3 252.4 297.3 347.3 402.5 463.4 530.0 602.8 681.9 767.7 860.4 960.3 1067.6
.
15.84
Note that it is always possible to fit an exponential growth curve between any initial and final weights of any growth curve. This does not mean that it is a good fit. For this example, the AL method (Eq. 2.1; column 7) gives a far better fit than the exponential method (Eq. 2.5; column 5). See the following plot of the actual weights versus the predicted weights from the exponential and AL methods.
16
Example 2.4. (continued) 1200
1000 -
[]
Actual Weight (g)
O
ComputedWeight (G)
4"
ComputedWeight (AL)
t3
~o
800 -
~ o ,~
600-
~
,~
o o
400o o o
200 -
04 0
o
~,.,no08~ooO~176176176 i
I
I
100
200
300
400
Days
Step 1.
Compute G using Eq. 2.6 G = 100[ln(1067.6) - ln(5)]/360 - 0
= 1.490%
Step 2.
Mean A L (from above spreadsheet) A L = 15.84 mm/15 days
Step 3.
Compute length at day 0 (solve Eq. 2.7 for L) Length = (5/11.22 • 10-6) 0.33333
= 76.38 mm
Step 4a.
Compute weight at day 15 (G method, use Eq. 2.5) Weight = 5e (0"0149)(15)
= 6.252 g
Compute length at day 15 (solve Eq. 2.7 for L) Length = (6.252/11.22 x 10-6) 0.33333
- 82.29 mm
Step 5a.
Compute length at day 15 (AL method, use Eq. 2.1) Length - 76.38 + 15.84
= 92.22 mm
Step 5b.
Compute weight at day 15 (use Eq. 2.7) Weight = 11.22 • 10 -6 • (92.22) 3
= 8.80 g
Step 4b.
Go back to Step 1" continue computations until day 360.
2.6 Types of systems
There are several possible types of seawater systems and these are classified at two extremes as open cycle and closed cycle systems. Open or flow-through systems depend on relatively large quantities of good quality incoming seawater to provide life support to
17 TABLE 2.2 Constants for length-weight relationships Common name
Species
c x 106
n
Length basis
Atlantic sturgeon Channel catfish Largemouth bass Chinook salmon Muskellunge Northern pike Rainbow trout Walleye European eel Milkfish Pacific bonita American lobster Blue crab
Acipenser oxyrhynchus Ictalurus punctatus Micropterus salmoides Oncorhynchus tshawytscha Esox masquinongy Esox lucius Oncorhynchus mykiss Stizostedion vitreum Anguilla anguilla Chanos chanos Sarda chiliensis Homarus americanus Callinectes sapidus, female Callinectes sapidus, male Procambarus acutus acutus Macrobrachium rosenbergii Procambarus clarkii Cancer irroratus Penaeus stylirostris Penaeus vannamei Artica islandia Loligo pealai Chelonia mydas
1.1402 5.160 12.748 8.190 4.429 5.012 11.224 8.303 0.04302 8.989 7.729 589 287.4 181.4 8.026 1.305 8.837 87.10 15.2 9.88 68.436 1809 1659.0
3.18 3.11 3.00 3.00 3.00 3.00 3.00 3.00 3.63 2.99 3.09 3.07 2.64 2.78 3.32 3.42 3.28 3.14 3.10 3.05 2.89 2.15 2.54
total length total length total length total length total length total length total length total length total length total length total length carapace length carapace width carapace width total length total length total length carapace width total length total length shell length dorsal mantle length carapace length
Crayfish Freshwater prawn Red swamp crayfish Rock crab Shrimp Shrimp Ocean quahog Squid Sea turtle
W - cL n, where length is in mm and weight in grams. Note that the actual value of c is the value of c in the table • 106. The value of n does not depend on the units of weight or length, but the value of c depends on the units of both. Much of the length-weight data reported in the literature are based on other units. The value of c in other units can be converted to c based on millimeters and grams by observing that if L = 1 mm then W = c. The value of c is equal to the weight in grams of a 1 mm animal. Therefore, in a system based on inches and pounds, convert 1 mm to inches and solve for weight. Convert weight in pounds to grams - - this is the value of c that should be used in Eq. 2.7.
the culture organisms. When the seawater's properties become unacceptable, the water is discharged. The advantages of open systems are proportional to the input water quality and quantity. If the input water parameters are consistently good and no discharge treatment is required, these systems are clearly the simplest, cheapest, most reliable and have the least risk (see Fig. 2.2A). Few, if any, completely open systems exist. The incoming seawater may require some treatment before use. This may include filtering to protect pumps or remove eggs, larvae or debris, heating/cooling, aeration/degassing or settling of solids. In addition, it is very rare that there would not be some internal water treatment in the system (see Fig. 2.2B). This might include adding oxygen, removing uneaten food and fecal matter or reducing the concentrations of pathogenic organisms. Flow-through systems can become quite complex. Depending on physical and regulatory conditions, it may be necessary to treat process water prior to discharge (see Fig. 2.2C) and such requirements are becoming much more common. Systems that are predominantly open or flow-through systems are by far the
18
Example 2.5. Computation of weight from length-weight relationship Compute the weight of a 250 mm largemouth bass, chinook salmon, and European eel in grams and #/kg. From Table 2.2 Species
c
n
Largemouth bass Chinook salmon European eel
12.748 x 10 -6 8.190 x 10 -6 0.04302 x 10 -6
3.00 3.00 3.63
From Eq. 2.7 W = 12.748 x 10 -6 (250) 3.00 W = 8.190 x 10 -6 (250) 3.oo W = 0.04302 x 10 -6 (250) 3.63
A
Seawater
B
Seawater~ Coarse I v[ Filtration I
= 199 g or 5.02/kg (largemouth bass) = 127 g or 7.87/kg (chinook salmon) = 21.8 g or 45.9/kg (European eel)
URearing Unit
Discharge ,~
U Rearing Unit
Discharge ,~
iv
iv
~-Aeration C
'
SeawaterJ Pre! "1 treatments I
J Rearing Unit 9
Posttreatment
Discharge ,~ v
....
~--Aeration Fig. 2.2. Open or flow-through systems. (A) Completely open system. (B) Limited pretreatment and internal aeration. (C) More extensive pretreatment, internal aeration and treatment of discharge water. Many other variations may also exist.
most common type, although this is not reflected in the available literature (see Appendix B). The available literature on flow-through systems is sparse, unbalanced in representation and biased towards research oriented systems. A completely closed cycle system, such as a well balanced aquarium, has no water inflow requirement, except to make up for evaporation, and no discharge (see Fig. 2.3A). The water is treated and reconditioned internal to the system by a series of complex biochemical and physical processes. If everything is working well, all the processes are in equilibrium and good water quality is maintained. Any changes, such as increased feeding, chemical additions, adding or removing animals, or even cleaning parts of the system, can destroy the delicate balances. For most practical uses, a completely closed system of any appreciable size is difficult to monitor and manage and is often uneconomic if any alternatives exist. There
19
Water Treatments A
Distilled Water
Rearing~Unit
"1 Water Treatments~, ~ B Seawater J Prey[ treatments
RearingUnit
I POst] Discharge treatment
Fig. 2.3. Closed or reuse systems. (A) Pure closed system, with water addition only to make up for evaporation. (B) More common reuse system with some flow-through.
can be depletion of necessary trace materials and build-ups of other persistent contaminants to troublesome levels. Periodic recharging with new seawater is often required. Thus, even closed systems need access to good quality seawater or use high cost artificial seawater. Therefore, some net flow through the system is usually present and a better label for most systems would be a water reuse system (see Fig. 2.3B). Generally a 90% water flow reuse (10% new water and 90% reconditioned water) can be achieved with reasonable processing. Technical and management difficulties progressively increase as one approaches 100% reuse (completely closed system). The difficulties and their severity are conditional on the specific circumstances. A reuse system being operated at a fraction of its carrying capacity is likely to be much less trouble than one being operated at its limits. In spite of the difficulties, complexities and costs of seawater reuse systems, there is considerable prior experience with such systems in the size range of interest (see Appendix C). Closed systems have primarily been used in marine aquariums, which are often far from the sea or in urban areas with highly polluted seawater. In marine aquariums small numbers of organisms are held in good water quality conditions. The operating costs of these systems may be in the range of US$10-1000 per year per pound of animals. Much of this expense is due to maintaining the appearance of the water (turbidity and color) required for public viewing, rather than for the life support requirements of the animals themselves. High reuse systems, under some conditions, may be economical for hatchery operations, short-term holding, or for very high value organisms, but have generally proven uneconomical for the growth of shrimp, lobsters and fish to marketable size for human food unless heavily subsidized. Thus, there is a spectrum of possibilities between completely open and completely closed systems. If sufficient quantities of good quality water are readily available, the decision will be towards the open systems due to their greater simplicity, reliability and lower costs. However, if sufficient water is not available or of poor quality, or if environmental considerations are paramount, there will be a tendency towards greater water reuse. While leaning towards one type or the other, most seawater systems have some net water flow through the system as well as some degree of water reconditioning internal to the system and increasingly some required post treatment of discharge water. It is, therefore, useful to consider the systems externally as
20 open or flow-through systems while considering various water reuse or reconditioning options internally. In short, internal reuse loops in a flow-through system (Fig. 2.3B) or in a more elaborate version as shown in Fig. 5.1.
2.7 System carrying capacity The carrying capacity of a flow-through system is dependent on many parameters and the specifics of any given application. Table 2.3 lists just some of the factors that might be critical. These factors cannot be considered independently, but must all be evaluated relative to the biology of the culture organisms. This complex evaluation of environmental parameters and organism biology is often termed 'water quality'. While much is known relative to economically important species, there are nevertheless a number of important data voids for culturing purposes (Huguenin and Colt, 1986). One of the most significant problems is that most of the data on water quality criteria are based on short-term experiments, and in many cases, lethal effects. Detailed criteria based on chronic effects and long-term experiments are unavailable for many species over their life spans. Low risk and fast growth require not stressing the organisms. The acceptable values for 'no stress' are often not precisely known. Determining the water quality requirements and inherent tradeoffs for specific culture organisms and life stages is critical and essential to the specification of the seawater system (see Appendix D). A very brief review of water quality parameters and water quality considerations is presented in this section. Water quality criteria for use with marine systems will be needed in the site selection process and loading computations. They may be quite different from published environmental protection criteria (for example, USEPA, 1976, 1986), as these criteria are formulated to protect a wide range of ages and species. In a specific marine system, (a) only a single species TABLE 2.3 Seawater properties affecting carrying capacity Physical parameters
Chemical parameters
Temperature range (daily and seasonal pH and alkalinity variability) Gases Salinity range (tidal and seasonal variability) total gas pressure oxygen Particulates (solids) composition (organic and inorganic) nitrogen size carbon dioxide hydrogen sulfide concentration Color Nutrients nitrogen compounds Light artificial or natural phosphorus compounds trace metals and speciation total annual incident energy Organic compounds intensity of radiant energy biodegradable quality of light photoperiod (daily cycles) non-biodegradable Toxic compounds heavy metals biocides
Biological parameters Bacteria (type and concentrations) Virus Fungi Others
21 TABLE 2.4 Preliminary water quality screening and production levels for marine applications Parameter
Screening level
Production level
Ammonia (except for plants)
< 1 Ixg/1 NH3-N
Nitrite Dissolved oxygen (except for plants) Total gas pressure Carbon dioxide (except for plants) Hydrogen sulfide Chlorine residual pH Temperature Salinity
0.05 mg/1 NO2-N 90% of saturation 76 mm Hg 5 mg/1 CO2 2 ~g/1 as H2S 10 ~g/1 7.9-8.2 Depends on life stage and species Depends on life stage and species
< 1 Ixg/1 NH3-N research <10 l~g/1 NH3-N production <40 Ixg/1 NH3-N holding, little or no feeding 0.10 mg/1 NO2-N 6 mg/1 20 mm Hg 10 mg/1 CO2 1 Ixg/1 as H2S 1 ~g/1 7.9-8.2 - 1 to 4 0 ~
1-40 g/kg
Metals (total)
Cadmium Chromium Copper Iron Mercury Manganese Nickel Lead Zinc
1 ~g/1 10 Ixg/1 1 Ixg/1 300 Ixg/1 0.05 ~g/1 50 ~g/1 2 Ixg/1 2 Ixg/1 10 Ixg/1
3 ~tg/1 25 ~tg/1 3 ~tg/1 100 Ixg/1 0.1 lxg/1 25 Ixg/1 5 Ixg/1 4 Ixg/1 25 Ixg/1
may be cultured and for a only a part of its life, (b) the animals may be held for only a short time (example, holding of lobsters or test animals), or (c) the condition of the animals may not be too important as long as they do not die too rapidly. For some key parameters such as ammonia, the formulation of a single fixed criterion based on a no-effects level may be inappropriate or at least costly for many applications. Water quality criteria for research applications may be more restrictive than for environmental protection. For the small volumes of seawater flow involved in most research projects, the cost of maintaining exceptionally high water quality is usually not significant compared to labor, equipment, and supplies. Two types of water quality criteria may be specified: screening criteria and production criteria. Preliminary screening and production criteria for fish and crustaceans are presented in Table 2.4. The screening criteria are used to screen potential sites or seawater sources. If measured water quality does not satisfy the screening criteria for all components, small-scale rearing experiments may be needed prior to construction or more extensive water treatment will be required prior to use. High concentrations of ammonia, nitrite, and metals may make a site unacceptable as they are expensive and difficult to remove. On the other hand, low dissolved oxygen, high concentrations of nitrogen gas or hydrogen sulfide, while not desirable, can sometimes be economically treated. Production criteria are the criteria used in the actual culture system. They may be significantly different for marine algae than for fish and crustaceans. In particular, carbon dioxide, ammonia, and phosphorus compounds may
22 have to be enriched m a n y times to support rapid growth of aquatic plants. The important water quality criteria p a r a m e t e r s are given in Table 2.3 and are discussed below. 2.7.1 A m m o n i a A m m o n i a is the m a j o r end-product of protein m e t a b o l i s m in most aquatic animals. The toxicity of a m m o n i a is due to the free or un-ionized form (NH3) while the ionized form (NH +) has little toxicity. U n - i o n i z e d a m m o n i a is typically reported as nitrogen and written as NH3-N. A m m o n i a concentration expressed as the NH3 c o m p o u n d can be converted to a nitrogen basis by m u l t i p l y i n g by 0.822. Wet chemical tests m e a s u r e the sum of the un-ionized and ionized a m m o n i a or the total a m m o n i a (called total a m m o n i a nitrogen or TAN when expressed on a nitrogen basis). The c o n c e n t r a t i o n of un-ionized a m m o n i a depends primarily on total a m m o n i a , pH, temperature, and salinity. The m o l e fraction of un-ionized a m m o n i a is equal to the percent un-ionized a m m o n i a / 1 0 0 . The concentration of un-ionized a m m o n i a is equal to: U n - i o n i z e d a m m o n i a (mg/1 as NH3-N) - otTAN
(2.8)
U n - i o n i z e d a m m o n i a (Ixg/1 as NH3-N) -
(2.9)
or 1000otTAN
TABLE 2.5 Mole fraction of un-ionized ammonia in flesh and brackish waters based on NBS pH scale Temperature
pH
(~
7.0
7.8
7.9
8.0
8.1
8.2
8.3
9.0
0.0012 0.0019 0.0027 0.0039 0.0056 0.0080 0.0111 0.0153
0.0078 0.0116 0.0169 0.0243 0.0346 0.0483 0.0663 0.0894
0.0098 0.0145 0.0212 0.0304 0.0431 0.0600 0.0820 0.1100
0.0123 0.0182 0.0266 0.0380 0.0537 0.0744 0.1011 0.1345
0.0154 0.0229 0.0332 0.0474 0.0667 0.0919 0.1240 0.1638
0.0193 0.0286 0.0415 0.0590 0.0825 0.1130 0.1513 0.1978
0.0242 0.0357 0.0516 0.0731 0.1017 0.1382 0.1833 0.2367
0.1107 0.1567 0.2144 0.2833 0.3621 0.4455 0.5293 0.6088
0.0009 0.0013 0.0019 0.0028 0.0040 0.0057
0.0055 0.0082 0.0121 0.0174 0.0247 0.0347
0.0070 0.0103 0.0151 0.0218 0.0310 0.0433
0.0087 0.0130 0.0190 0.0273 0.0387 0.0539
0.0110 0.0163 0.0238 0.0341 0.0482 0.0669
0.0138 0.0204 0.0297 0.0426 0.0599 0.0828
0.0173 0.0256 0.0371 0.0530 0.0743 0.1020
0.0810 0.1162 0.1620 0.2191 0.2868 0.3629
Salinity = 0 g / k g (freshwater)
5 10 15 20 25 30 35 40 Salinity = 5 g / k g
5 10 15 20 25 30
Notes: For normal applications, interpolation within and between Tables 2.5 and 2.6 may be adequate. Based on freshwater equilibrium constants developed by Emerson et al. (1975), seawater relationship found by Khoo et al. (1977), and the equation for the computation of ionic strength presented by Whitfield (1974). The NBS pH was converted to the pHT scale (Hansson, 1973) by subtracting 0.0007 x Salinity + 0.131 (Millero, 1986).
23 TABLE 2.6 Mole fraction of un-ionized ammonia in marine waters based on NBS pH scale Temperature
pH
(~
7.0
Salinity = 5 10 15 20 25 30
15
7.8
7.9
8.0
8.1
8.2
8.3
9.0
0.0051 0.0075 0.0111 0.0160 0.0228 0.0320
0.0064 0.0095 0.0139 0.0200 0.0285 0.0399
0.0080 0.0119 0.0174 0.0251 0.0356 0.0497
0.0101 0.0149 0.0218 0.0314 0.0444 0.0618
0.0126 0.0187 0.0273 0.0392 0.0553 0.0766
0.0159 0.0235 0.0342 0.0489 0.0686 0.0946
0.0747 0.1075 0.1506 0.2048 0.2697 0.3436
0.0042 0.0063 0.0093 0.0134 0.0192 0.0270
0.0053 0.0079 0.0117 0.0169 0.0240 0.0338
0.0067 0.0100 0.0146 0.0211 0.0301 0.0422
0.0084 0.0125 0.0183 0.0265 0.0376 0.0525
0.0106 0.0157 0.0230 0.0331 0.0469 0.0652
0.0133 0.0197 0.0288 0.0413 0.0583 0.0808
0.0631 0.0915 0.1293 0.1776 0.2367 0.3057
g/kg 0.0008 0.0012 0.0018 0.0026 0.0037 0.0052
Salinity = 35 g / k g
5 10 15 20 25 30
0.0007 0.0010 0.0015 0.0022 0.0031 0.0044
Sames notes as Table 2.5.
where ol is mole fraction of un-ionized ammonia (%/100), and TAN is total ammonia nitrogen (mg/1 as N). Values of the mole fraction of un-ammonia are presented in Table 2.5 for freshwater and brackish waters and in Table 2.6 for marine waters. At a given pH and temperature, the concentration of un-ionized ammonia is significantly less in seawater (see Example 2.6). For equal concentrations of total ammonia, the concentration of un-ionized ammonia is typically higher in seawater because of the generally higher pH compared to freshwater. Therefore, the carrying capacity in seawater is generally lower relative to freshwater (see Section 2.7). There are situations where one may wish to increase or adjust the un-ionized ammonia concentration (see Example 2.7). The concentration of un-ionized ammonia is strongly dependent on pH. In freshwater, pH meters are calibrated with low ionic strength buffers, commonly called 'NBS' or National Bureau of Standards buffers. Tables 2.5 and 2.6 are based on the use of these buffers. The National Bureau of Standards has been renamed as the National Institute of Standard and Technology (NIST), but commonly these buffers are still called 'NBS' buffers. In seawater, many chemists use a 'tris' seawater buffer (Hansson, 1973). While the use of this buffer has important thermodynamic advantages, it is not commercially available. Experimental work on the ammonia equilibrium in seawater (Khoo et al., 1977) is based on this pH scale. A number of researchers have published tabular listings or computer programs for the computation of percent un-ionized ammonia in seawater based on this work (Hampton, 1977; Bower and Bidwell, 1978; Spotte and Adams, 1983). The computation of un-ionized ammonia from these articles must be based on the use of the 'tris' seawater buffer. The error resulting from the use of 'NBS' type buffers with these data will be greater than the use of equilibrium constants for freshwater.
24
Example 2.6. Computation of un-ionized ammonia nitrogen Given a water temperature of 20~ pH of 7.9, salinity of 35 g/kg (ppt) and a measured total ammonia nitrogen (TAN) of 0.82 mg/1, what is the concentration of un-ionized ammonia nitrogen in Ixg/l? Is the concentration of un-ionized ammonia greater in freshwater or seawater at the same pH and temperature. (A) Using the pH and water temperature read the mole fraction (or) from Table 2.6 and Eq. 2.9. c~ = 0.0169 (dimensionless) NH3-N = 1000 (c~) (TAN) = 1000 (0.0169) (0.82) = 14 Ixg/1 (B) Comparison of un-ionized ammonia in seawater and freshwater Mole fraction (or) for freshwater - 0.0304 (Table 2.5) Mole fraction (or) for seawater = 0.0169 (above) From Eq. 2.8, the concentration of un-ionized ammonia is proportional to mole fraction and therefore the concentration in freshwater is greater by: { 0"0304 0.169- 0"0169 ] 100 = 79%
2.7.2 Nitrite Nitrite, an intermediate in the conversion of ammonia to nitrate, can oxidize the iron in hemoglobin such that it is unable to transport oxygen. The toxicity of nitrite, while still formidable, is significantly reduced in seawater due to the high concentration of chloride and calcium (Lewis and Morris, 1986). 2.7.3 Nitrate Nitrate is the final compound produced by the oxidation of ammonia. This compound is largely nontoxic, except at very high concentrations, which can occur in closed systems. 2.7.4 Dissolved Oxygen The equilibrium or saturation concentration of dissolved oxygen depends on temperature and salinity. Saturation concentrations of dissolved oxygen are presented in Table 2.7 for salinities in the range of 0 - 4 0 g / k g and water temperatures of 0-40~ Photosynthesis and solar heating can produce dissolved oxygen concentrations higher than saturation (supersaturation). Dissolved oxygen concentrations significantly less than saturation are commonly produced by pollution. Dissolved oxygen concentration must be measured immediately after sampling to avoid changes with time in the sampling container. For fish and crustaceans, growth will be reduced when the dissolved oxygen falls below 6 mg/1 and significant mortality will result below 2 - 3 mg/1. Bivalve shellfish may be able to tolerate lower dissolved oxygen concentrations for extended periods of time, especially if the temperature is low. Water
25
Example 2.7. Adjustment of un-ionized ammonia A bioassay tank contains 40 1 of water with a temperature of 5~ salinity of 35 g/kg and a pH = 8.0. How much NH4C1 must be added to produce an un-ionized ammonia concentration of 1000 Ixg/1 NH3-N? (A) Compute concentration of TAN From Table 2.6, From Eq. 2.9,
mole fraction (or) --- 0.0067 1000 Ixg/1 = (1000) (0.0067) TAN
TAN = 149.3 rag/1 (B) Compute concentration and weight of ammonia chloride. TAN or total nitrogen is based on nitrogen with a molecular weight of 14.0 g/mole, therefore, molecular concentration is equal to: 149.3 x 10-3/14.0 = 1.066 x 10 -2 moles/1 The molecular weight of NH4C1 is:
N H C1
(1 • 14.00) (4 • 1.01) (1 • 35.45)
--=
14.00 4.04 35.45 53.49 g/mole
NHaC1 = (1.066 x 10 -2) (53.49) = 0.570 g/1 Total amount of NHaC1 required = (0.570 g/l) (40 1) -- 22.8 g
temperature, even within seasonal variations, can significantly impact the oxygen saturation concentration (see Example 2.8). 2.7.5 Carbon Dioxide High carbon dioxide concentrations reduce the ability of animals to transfer oxygen. Carbon dioxide concentrations should be maintained below 10 mg/1 (Smart et al., 1979), although many animals can tolerate higher levels, especially if the dissolved oxygen is high. For marine plants, the more dissolved carbon dioxide the better, as it is often the growth limiting substance. In fact, in the mass culturing of phytoplankton, such as may be needed in bivalve or shrimp hatcheries, direct carbon dioxide injection into the water may be used to increase growth rate. Some care must be taken to control carbon dioxide concentrations even in aquatic plant culture as it is possible to reduce the pH to below 5 at extremely high concentrations. 2.7.6 Hydrogen Sulfide High concentrations of hydrogen sulfide may be present in well waters or the bottom water of some marine bays with restricted circulation. Samples used for total sulfides analysis must be specially treated to prevent loss of hydrogen sulfide gas during transport and analysis. A criterion of 1 t~g/1 of hydrogen sulfide gas is suggested as a maximum for fish and crustaceans.
TABLE 2.7 Air solubility of oxygen (Cx)in seawater (rng/l), 0 4 0 g/kg at 760 rnm Hg (1 atm pressurej Temperature
Salinity (g/kg)
("'4
0
5
10
15
20
25
30
35
40
0 I 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40
14.621 11.216 13.829 13.460 13.107 12.770 12.447 12.139 11.843 11.559 11.288 11.027 10.777 10.537 10.306 10.083 9.870 9.665 9.467 9.276 9.092 8.914 8.743 8.578 8.418 8.263 8.113 7.968 7.827 7.691 7.558 7.430 7.305 7.183 7.065 6.949 6.837 6.727 6.619 6.514 6.412
14.120 13.733 13.364 13.011 12.674 12.352 12.043 11.748 11.465 11.194 10.933 10.684 10.444 10.214 9.993 9.780 9.575 9.378 9.188 9.005 8.828 8.658 8.493 8.334 8.181 8.032 7.888 7.748 7.613 7.482 7.354 7.230 7.110 6.993 6.879 6.767 6.659 6.553 6.449 6.348 6.249
13.636 13.266 12.914 12.577 12.255 1 1.047 11.652 1 1.369 11.008 10.839 10.590 10.351 10.121 9.901 9.689 9.485 9.289 9.099 8.917 8.742 8.572 8.408 8.250 8.098 7.950 7.807 7.668 7.534 7.404 7.278 7.155 7.036 6.920 6.807 6.697 6.590 6.485 6.383 6.283 6.186 6.090
13.167 12.815 12.478 12.156 1 1.849 11.554 11.272 l 1.002 10.743 10.495 10.257 10.028 9.808 9.597 9.394 9.198 9.010 8.829 8.654 8.486 8.323 8.166 8.014 7.867 7.725 7.588 7.455 7.326 7.201 7.079 6.961 6.846 6.735 6.626 6.520 6.417 6.316 6.218 6.121 6.027 5.935
12.714 12.378 12.057 11.750 11.456 11.175 10.905 10.647 10.399 10.162 9.934 9.715 9.505 9.302 9.108 8.921 8.740 8.566 8.399 8.237 8.081 7.930 7.785 7.644 7.507 7.375 7.247 7.123 7.003 6.886 6.772 6.602 6.555 6.450 6.348 6.248 6.151 6.056 5.963 5.873 5.783
12.277 11.956 11.650 11.356 11.076 10.807 10.550 10.303 10.066 9.839 9.621 9.412 9.210 9.017 8.830 8.651 8.478 8.311 8.151 7.995 7.846 7.701 7.561 7.126 7.295 7.168 7.045 6.926 6.810 6.698 0.589 6.483 637rl 6.278 6.180 6.084 5.991 5.899 5.810 5.722 5.636
11.854 11.548 11.256 10.976 10.708 10.451 1O.ZOh 9.970 9.744 9.526 9.318 9.117 8.925 8.739 8.561 8.389 8.223 8.061 7.910 7.761 7.617 7.479 7.344 7.214 7.089 6.967 6.849 6.734 6.623 6.515 6.410 6.308 6.208 6.1 1 1 6.017 5.924 5.834 5.746 5.660 5.575 5.492
11.445 11.154 10.875 10.608 10.352 10.107 9.872 9.647 9.431 0 223 9.024 8.832 8.648 8.470 8.300 8.135 7.976 7.823 7.676 7.533 7.395 7.262 7.134 7.009 6.888 6.771 6.658 6.548 6.441 6.337 6.236 6.137 6.042 5.948 5.857 5.768 5.681 5.597 5.513 5.432 5.352
11.051 10.773 10.507 10.252 10.008 9.774 9.550 9.335 9.128 8.930 8.739 8.556 8.379 8.210 8.046 7.888 7.737 7.590 7.449 7.312 7.180 7.052 6.929 6.809 6.693 6.581 6.472 6.366 6.263 6.104 6.066 5.972 5.880 5.790 5.702 5.617 5.533 5.451 5.371 5.292 5.215
Based on Benson and Krause (1984).
27
Example 2.8. Air solubility of oxygen in seawater
It is important to understand the air solubility effects created by seawater temperature changes. Assume a salinity of 20 g/kg and atmospheric pressure. (A) Increasing the seawater temperature from 5~ to 20~ will increase or decreases the air solubility of oxygen and by what percentage? Table 2.7, for 5~ C* = 11.175 mg/1; for 30~ C* -- 6.772 mg/1 [6.772-11.17511.175] 100=-39% (decrease in solubility) (B) Assuming oxygen saturation at 5~ and none of the dissolved oxygen is allowed to escape during the heating to 30~ what is the percent saturation of oxygen at 30~ The final dissolved oxygen concentration is the same = 11.175 mg/1 11.1751 100 : 165% of saturation 6.772 J
2.7.7 Total Gas Pressure Gas supersaturation can be produced by a variety of natural and man-made conditions (see Section 12.4) and many surface waters and seawater wells are, at least at times, supersaturated with dissolved gases. Lethal gas pressure ranges from 100 to 200 m m Hg, but levels in the range of 4 0 - 5 0 m m Hg may increase the mortality of fish on a chronic basis (Colt, 1986). Gas supersaturation must be measured in situ. A maximum criterion of 20 m m Hg is suggested for fish and crustaceans. Dissolved oxygen is n o t a good measure of total gas supersaturation. 2.7.8 pH The normal pH of seawater ranges from 7.9 to 8.2. Values outside of this range may indicate pollution or excessive freshwater runoff. Animals that typically live in full-strength seawater can be very sensitive to changes in pH outside the normal range. Animals that live in tidal streams or estuaries may be much more tolerant to changes in pHs. A criterion of 7.9 to 8.2 is suggested. As discussed in the ammonia section, pH meters may be calibrated by use of two different types of buffers. The low ionic strength buffer ('NBS' type) is widely available for general use and most pH readings are based on this buffer. 2.7.9 Residual Chlorine Chlorine is widely used to disinfect drinking water, wastewater prior to discharge, and prevent fouling of heat-transfer systems in power plants. It is also used to control fecal coliform counts in marine m a m m a l pools and to disinfect water used in molluscan and shrimp hatcheries. A m a x i m u m criterion for chlorine residual of 1 Ixg/1 is suggested for fish and crustaceans. Marine mammals can tolerate considerably higher concentrations without ill
28 effects. Analysis of chlorine residual at this low level is difficult and expensive. Common wet chemical tests for chlorine residual are not reliable below 20-100 Ixg/1. 2.7. l0 Temperature The temperature of seawater may vary from - 1 to greater than 40~ No single criterion can be suggested, as some animals can survive over this full range, while most will be restricted to only a portion of this range. The water quality criteria for temperature will depend on the species and the life-stage. Rapid temperature changes are entirely another matter. Very few animals can tolerate even modest temperature changes if rapid and prolonged. For prolonged temperature changes, a change up or down of not more than I~ per day is recommended. 2.7.11 Salinity The salinity of seawater is expressed in terms of parts per thousand (g/kg). In older literature, the symbols %o or ppt (parts per thousand) are often used. The salinity of seawater may vary from 1 to greater than 40 g/kg. Salinity can be easily measured by use of conductivity or refractive methods. The water quality criteria for salinity will also depend on the species and life-stage. As with temperature, different organisms may have broad or narrow tolerances and rapid changes can be very stressful, even within the acceptable range. 2.7.12 Heavy Metals Heavy metals include cadmium, chromium, copper, iron, mercury, manganese, nickel, lead, and zinc. High levels of manganese and iron may occur in well water low in dissolved oxygen. Samples used for heavy metal analysis should be treated with acid to reduce loss of metals during travel and storage. The presented criteria (Table 2.4) are based on total metals (particulate § dissolved forms) as the toxic mode for iron and manganese may be physical blocking of the gills by particulate matter (Chen et al., 1985). If water is aerated and allowed to stand for days before analysis, the majority of the iron and manganese will precipitate out of solution. Therefore, high iron and manganese concentrations may not be reported if water samples are allowed to stand for sometime prior to analysis. The addition of 1-10 mg/1 of EDTA (ethylenediaminetetraacetate, a common chelating agent) can significantly reduce the toxicity of heavy metals and it is commonly used in some crustacean and molluscan hatchery facilities. Screening levels for metals may be several times higher than those listed in Table 2.4 if use of EDTA is anticipated. 2.7.13 Biocides A vast number of organic chemicals are introduced into the natural environment. These include herbicides, pesticides, and industrial chemicals. Rare storm events may introduce significant quantities of such chemicals from agricultural runoff or resuspend them from the bottom. Such storm drainage problems can be anticipated but are not obvious site selection considerations (see Chapter 3). Depending on the purpose of the seawater systems, it may be necessary to conduct detailed monitoring of water, sediment, animal tissue, or conduct a
29 pilot-scale operation to assess the potential problems from biocides and industrial chemicals. Acutely lethal concentrations of biocides can be introduced into streams and ponds from transport accidents (crop duster crashes, truck crashes, or railcar derailments). Extreme water levels may also result in destruction of agricultural and industrial distribution facilities and float containers of biocides and toxic chemicals for significant distances.
2.8 Carrying capacity guidelines While the carrying capacity of flow through systems are dependent on species biology and specific conditions, there are some generally valid 'rules of thumb' that can be used for planning purposes, assuming good quality input water. The intensity or carrying capacity of an aquaculture system can be described by a number of parameters. The most common parameters are: Volumetric density (lb/ft 3) -
Areal density (lb/ft 2) = Loading (lb/gpm) =
mass of animals (lb) volume of rearing unit (ft 3)
mass of animals (lb) area of rearing unit (ft 2) mass of animals (lb)
flow to rearing unit (gpm)
(2.10)
(2.11)
(2.12)
(60) (flow to unit, gpm) (2.13) volume of unit (gal) Loading, exchange rate and volumetric density are related by: 8.02 • density Loading (lb/gpm) = (2.14) exchange rate Another important measure of rearing intensity is cumulative oxygen consumption (COC). For a single rearing unit, COC is equal to D O i n - DOout. For n rearing units in series, the COC is equal to: i=1 COC (mg/1) - ~ (DOin DOout) (2.15) Exchange rate (exchanges/h) =
-
-
The COC depends strongly on animal size and temperature and therefore integrates both animal size and metabolic activity. If loading rates are maintained at low values (high exchange rate), densities in small-scale experimental systems have been as high as 34 lb/ft 3 (545 kg/m3). These numbers are well above any practical values. The maximum practical density will depend on water quality considerations, management skills and the ability of the particular species to tolerate crowding. Maximum loading rates in production systems typically range from 4 to 10 lb/gpm (0.5-1.2 kg/lpm). For research purposes maximum loading rates typically range around 1 lb/gpm (0.125 kg/lpm) or less. Carrying capacity may not be limited by water flow but by volumetric or area density. Surface area limitations often apply for plants due to their need for sunlight and to organisms that require a substrate, such as some shellfish. A substrate requirement may also be combined
30 with quantifiable territorial needs of the organisms. As an example, juvenile or adult lobsters and some crabs are cannibalistic bottom dwellers and generally must be individually isolated to prevent unacceptable mortality. In surface-limited systems, the capacity can sometimes be increased by stacking substrate layers or clever packaging. For organisms that tolerate communal crowding, volumetric density is an important economic parameter, having a direct impact on the required rearing volume and a major impact on capital costs. The maximum density depends on both the loading and behavior characteristics of the animals. Much of the available volumetric data are of questionable use because of the interactions between biomass, density and loading. As an example, if 1000 lb of fish is held in 1000 ft 3 of rearing volume with 1000 gpm of flow, this results in a loading of 1 lb/gpm and a density of 1 lb/ft 3. If the biomass of fish is doubled both the loading and the density are also doubled. If the flow is cut by half, the loading doubles but the density remains unchanged. If the loading is kept low (high exchange rate), the maximum density is only limited by the ability of the organisms to tolerate crowding. As a general rule, assuming good water quality and amenability to crowding, the following are suggested as maximum densities for organisms that are spread throughout the water column, such as most fish: Research: 0.01-0.1 lb/ft3; Production: 1-2 lb/ft3; Holding: 2-5 lb/ft 3. The consumption of oxygen by aquatic animals has been studied extensively by physiologists because it can be used to estimate energy expenditure. The respiration or oxygen consumption of aquatic animals is composed of three components T = Tstandard --[- Tactivity -3I- Tsd a
(2.16)
where T is total respiration (mg/h per individual), Tstandard is oxygen consumption of an unfed and resting animal (mg/h per individual), Tactivity is additional oxygen consumption due to swimming or movement (mg/h per individual), Tsda is additional oxygen consumption required for digestion, assimilation, and storage of material (mg/h per individual). The standard metabolic rate can be determined by extrapolation to zero activity from determination of oxygen consumption at various levels of forced activity. The sum of Tstandard + Tactivity is the routine metabolic rate and is a measure of the random activity of the animal. The routine metabolism of an individual animal may vary more widely than its standard metabolic rate. The active metabolic rate is the maximum sustained metabolic rate of an animal swimming or moving steadily. Most oxygen consumption studies are conducted on unfed animals and measure either the standard or routine metabolic rate. The effect of weight on oxygen consumption of aquatic animals has been studied extensively. At a given temperature and feeding level (typically a zero feeding rate), the oxygen consumption of a single animal is equal to: T ' = a' W b'
(2.17)
where T' is oxygen consumption in mg/h per individual, a' and b' are constants, W is weight of animal in grams.
31 The oxygen consumption may also be expressed in mg/h per kg biomass basis. Letting a = 1000a' and b = b' - 1, Eq. 2.17 can be rewritten as (2.18)
T = aW b
where T is the oxygen consumption in mg/h per kg. Typical values of a and b are presented in Table 2.8 for aquatic species. The value of b ranges typically from -0.100 to -0.300. For many species, the value of b is independent of temperature and feeding level. The value of a depends primarily on temperature, but feeding levels and activity can also have significant effects. The impact of temperature on the a value can be modeled as an exponential: a = ott ~ or T = o t t ~ W b
(2.19)
where ot and/3 are constants for a specific species and activity level, T is temperature (~ When available, values of ot and/3 are presented in Table 2.8. When oxygen consumption data were collected at a single temperature, only the a value has been listed. The impact of activity level and feeding is much more difficult to estimate. Commonly, the maximum oxygen consumption rate of fed fish is twice the standard rate. The average oxygen consumption rate of fed fish is about 1.4 the unfed routine rate. The impact of activity depends very strongly on species and culture system. For very active fish such as tuna or salmon, the active rate can be as high as 10 times the standard rate. The metabolic activity of animals depends strongly on size, temperature, and activity level. For example, the oxygen consumption of the American lobster (McLeese, 1964) is equal to: Oxygen consumption (mg/h per kg)
-- 5.52T~
-0"120
where T is temperature (~ and W is wet weight (g). At 12~ kg of 1, 50, and 1000 g lobsters is equal to: Size (g)
Oxygen consumption (mg/h per kg)
(2.20) the oxygen consumption of 10
Oxygen consumption (mg/h)
1
66
661
50
41
413
1000
19
192
Therefore, a given flow of water that would support the oxygen requirements of 10 kg of 50 g lobsters would also support 22 kg of 1000 g lobsters. An alternative approach to the computation of oxygen demand is based on the ingested ration. For trout, the average daily oxygen demand (Haskell, 1955; Willoughby, 1968) is proportional to the total ration: Average daily oxygen demand (lb/d) - OFR • R
(2.21)
where OFR is oxygen/feed ratio (lb/lb), R is total ration (lb/d). The oxygen requirement to process a given mass of feed depends on animal size, feeding rate, composition of the ration, digestibility of the feed components, and moisture content and can be described by the oxygen/feed ratio (OFR).
TABLE 2.8 Constants for the computation of oxygen consumption rates of aquatic animals Species
Form a
temp. (~ Freshwater fish Carp (Cyprinus carpio) Channel catfish (Ictalurus punctatus)
Mozambique tilapia (Sarotherodon
S R F R
Reference
Variable range
a
wt. (g)
2.157 2.685 1.540 2.078
-0.106 -0.200 -0.200 -0.348
10-35 24-30 24-30 16-37
30-400 2-1000 2-1000 10-150
Beamish, 1964 Andrews and Matsuda, 1975
0.866 0.903 0.944 1.514
-0.196 -0.142 -0.118 -0.250
4-10 12-22 5-20 8-24
12-900 12-900 2-2000 0.3-40
Muller-Feuga et al., 1978
[ 140 mg/h per kg] [57.6 mg/h per kg] 85.8 0.372 87.5 0.665 39.1 0.908 [288 mg/h per kg] [ 118 mg/h per kg] 1.14 1.759 1.87 2.15
-0.213 -0.180 -0.159 -0.207 -0.374 -0.376 -0.190 -0.145 -0.252
23 15-19 3-15 3-15 10-20 10 23-25 13-33 7-16
10-80 6000-13,000 90-3200 90-3200 4-50 1-320 400-6000 3-90 4-1000
Muir and Niimi, 1972 Graham and Laurs, 1982 Saunders, 1963
[114.2 mg/h per kg] [210.7 mg/h per kg] 5.52 0.999 10.9 0.785 17.4 1.11 [52.9 mg/h perkg] [275 mg/h per kg] 2.23 1.213
-0.390 -0.350 -0.120 -0.289 -0.194 -0.139 -0.293 -0.163
22 22 12-25 10-25 5-35 10 22-23 17-26
0.123 0.051 3.06 0.629
Caulton, 1978
mossambicus) Rainbow trout (Oncorhynchus mykiss) Sockeye salmon (Oncorhynchus nerka) Striped bass (Morone saxatilis) Marine fish Aholehole (Kuhlia sandvicensis) Albacore tuna (Thunnus alalunga) Cod (Gadus morhua)
Plaice (Pleuronectes platessa) Skipjack tuna (Katsuwonus pelamis) Striped mullet (Mugil cephalus) Turbot (Scophthalmus maximus) Crustaceans American lobster (Homarus americanus)
Blue crab (Callinectes sapidus) Brine shrimp (Artemia salina) Norway lobster (Nephrops norvegicus) Shrimp (Penaeusjaponicus) Spiny lobster (Panulirusjaponicus)
F F S S
36.9 50.4 11.7 1.87
0.004-0.05 0.004-0.05 0.6-12,300 20-200 0.0077 40-210 3-18 26-350
Brett and Glass, 1973 Klyashtorin and Yarzhombek, 1975 Kruger and Brocksen, 1978
Jobling, 1982 Graham and Laurs, 1982 Marais, 1978 Brown et al., 1984 Logan and Epifanio, 1978 Logan and Epifanio, 1978 McLeese, 1964 Laird and Haefner, 1976 Decleir et al., 1980 Bridges and Brand, 1980 Egusa, 1961 Nimura and Inoue, 1969
TABLE 2.8 (continued) Species
Form a
Variable range temp. (~
Molluscs American oyster b (Crassostrea salinity -- 32 g/kg salinity -- 14 g/kg salinity = 7 g/kg Clam b (Arctica islandica)
Reference wt. (g)
virginica)
Cuttlefish (Sepia officinalis) Dogwhelk b (Thais lapillus) Mussel b (Mytilus californianus) Pacific oyster b (Crassostrea gigas) a R = routine; S = standard; F = fed. b Based on dry weight excluding shell.
Shumway and Koehn, 1982 15.3 2.77 11.9 [374 mg/h [317 mg/h [196 mg/h 19.6 69.1 [922 mg/h
0.908 1.781 1.291 per kg] per kg] per kg] 0.596 0.583 per kg]
-0.490 -0.553 -0.615 -0.399 -0.578 -0.090 -0.400 -0.352 -0.230
10-30 10-30 10-30 10 10 17 5-20 13-26 20
0.03-0.7 0.03-0.7 0.03-0.7 0.03-1.0 2.9-16 0.1-1500 0.5-5.0 0.0-1.7
Taylor and Brand, 1975 Johansen et al., 1982 Stickle and Bayne, 1982 Bayne et al., 1976 Gerdes, 1983
34 In salmon and trout production systems, OFRs ranging from 0.20 to 0.22 kg oxygen/kg wet feed have been reported (Willoughby, 1968; Westers, 1981). In commercial high density warm-water fish culture, a value of OFR = 1.00 lb oxygen/lb wet feed is commonly used. Limited data are available for OFRs in recycle systems. The oxygen demand from bacterial oxidation of organic compounds, ammonia, and solids strongly depends on the unit processes and their operation. The upper bound for OFR equals the ultimate biochemical oxygen demand (BOD) of the feed, which for channel catfish feed is equal to 1.1 lb O2/lb dry feed (Harris, 1971). Careful feeding and rapid removal of solids from the system can significantly reduce the OFR. The feeding rate is commonly reported in terms the amount of actual feed fed (wet feed or on a 'as fed' basis). The feeding rate can also be reported in terms of amount of dry feed. Many commercial dry diets contain only 5-8% moisture, so the difference between the wet and dry values are small. This is not the case for 'semi-moist' diets (i.e., Oregon moist pellets) or diets that are made from unprocessed fish products. In these cases, the moisture content can vary from 30 to 90%. It is likely that the feeding rate used in Eq. 2.21 should be based on a dry fed basis for semi-moist and moist diets, although specific data are lacking. On a daily basis, the maximum oxygen consumption occurs 4 to 6 h following feeding in a flow-through system. The peaking factor can be reduced by increasing the number of feedings per day. Westers (1981) suggested a peaking factor of 1.44 to account for the maximum daily oxygen consumption rate: Maximum daily oxygen demand (kg/day) = 1.44 x OFR x R
(2.22)
Working with freshwater fish, Piper et al. (1982) popularized an approach to estimating stocking density as a function of animal size. This approach is based on a density index (DI) which is equal to: DI --
Biomass (lb) Rearing volume (ft 3) • Total length (inch)
(2.23)
The units of the DI are lb/(ft 3 • inch). Eq. 2.23 can be rearranged to: DI =
Density (lb/ft 3) Total length (inch)
(2.24)
or
Density = DI • Total length
(2.25)
For domestic rainbow trout, a DI = 0.50 is commonly used; for anadromous salmon, DI values in the range of 0.08 to 0.15 are used. For a DI = 0.50, 2-inch fish could be held at 1 lb/cf while 8-inch fish could be held at 4 lb/cf. For many hatchery fish, the behavioral impacts of density are not a significant problem. The density computed from Eq. 2.25 accounts for the impact of size on metabolic activities that would occur when the water flow over the production cycle is relatively constant. The intrinsic impact of density is much more important for crustaceans and many marine fish, especially for small juveniles. Regardless, of these impacts, Eq. 2.25 offers a simple way to evaluate the impact of animal size on metabolic carrying capacity. Water reconditioning and reuse internal to a flow-through system will raise the carrying capacity. This assumes
35
Fig. 2.4. Mass balance relationship for flow-through systems.
that the water parameters are the limiting factor, which is often but not always the case. It is also critical to know the specific limiting water quality parameter, because improving a nonlimiting parameter will not have much effect. Predicting the carrying capacity of a system with considerable water reuse approaching a closed system is complex and beyond the scope of this book (see Appendix C). In a flow-through system, flow is needed to supply oxygen and remove ammonia, carbon dioxide, soluble organic compounds, uneaten feed, and fecal matter. Typically, the most limiting water quality parameters are dissolved oxygen, un-ionized ammonia, and carbon dioxide. The flow needed to maintain a given water quality criterion can be developed from mass-balance considerations. For a given control volume (Fig. 2.4), the mass-balance on a single parameter under steady-state conditions (concentration is not changing) is simply: in § generation - decay -- out
(2.26)
The mass of compound 'x' leaving the control volume is equal to the mass entering, plus any generation within the control volume, minus any decay within the control volume. The ability to estimate mass (or concentrations) using the mass balance approach depends strongly on the complexity of the system and how well the biological and chemical processes are understood. Fig. 2.4 has generation of ammonia within the control volume but no decay. This is appropriate for a flow-through system where detention times are in the range of 20 to 60 min, but would be totally invalid in pond systems. The following equations are based on typical flow-through systems where gas transfer across the air-water or water-bottom surfaces is commonly small and may be neglected and the only metabolic loads are from the culture animals. These equations are not valid for pond systems because a significant part of the pond's metabolic activity may be from algae or bacteria and the gas transfer across the air-water and water-soil interface can not be neglected. Modeling of these types of systems is much more complex.
36 Application of the mass-balance equation to the system presented in Fig. 2.4 and neglecting mass transfer across the air-water and water-substrate interfaces results in the following two equations: Koxygen • R Qoxygen - - DO(in) - DO(out)
Qammonia - -
- o t Kammoni a x R
NH3-N(in) - NH3-N(out)
(2.27)
(2.28)
where Q is flow required to maintain a given oxygen (Qoxygen) and un-ionized ammonia criterion (Qammonia), K is production rate per unit of feed for oxygen (Koxygen) and un-ionized ammonia (Kammonia), R is total ration, DO is influent (DO(in)) and effluent dissolved oxygen concentrations (DO(out)), NH3-N is influent (NH3-N(in)) and effluent un-ionized ammonia concentrations (NH3-N(out)), el is mole fraction of un-ionized ammonia. The values of DO(out) and NH3-N(out) are set to the water quality criteria for the specific species under consideration. Values of Koxygen and Kammoni a are typically in the range of 200 and 30 g/kg of feed, respectively. Typically, the influent concentrations of ammonia in nearshore waters are small and can be neglected. Based on the above assumptions and unit selections of lpm for Q, kg/day for R, and rag/1 for all concentrations, Eqs. 2.27 and 2.28 can be written as: Qoxyge. =
Qammonia =
0.694 x SFoxygen • Koxygen • R DO(in) - DO(out) 0.694 x SFammonia • o/K~mmo,i~ x R NH3-N(out)
(2.29)
(2.30)
A safety factor (SF) term has been added to each equation. The K values are based on daily averages. This safety factor is used to adjust the flow requirement for periods of higher than average metabolic activity. The instantaneous K values can range from 0.5 to 3.0 (or larger) depending on feeding or other activities. The maximum K values for active fish such as tuna or striped bass may range as high as 10 times the average value. For typical fish, a SF of 2 is probably adequate. For less active crustaceans or mollusks, a value of 1.25 is suggested. For large systems, this safety factor can represent a significant cost and pilot-scale determination may be prudent. A similar equation could be written for carbon dioxide. When oxygen is not limiting, the consumption of 1 mole of oxygen produces approximately 1 mole of carbon dioxide (1.375 mg carbon dioxide per mg oxygen). Under normal surface water carbon dioxide concentrations (< 1 mg/1) and common loadings, carbon dioxide is rarely limiting. This may not be true if deep ground waters and/or pure oxygen aeration are used. Carbon dioxide does have a significant impact on flow requirements due to its impact on pH and un-ionized ammonia concentrations. Eqs. 2.29 and 2.30 can be used to estimate water requirements to maintain oxygen and un-ionized ammonia concentrations for a specific species under conditions typical of common flow-through marine culture systems. The water flow estimates will depend strongly on the water criteria values selected. The actual design requirement will be the largest of the two flows.
37
O
pH
=
7.9
.llnl
I n I I
8.0
I | l I l
8.1
,
1I l |
8.2 RR=0 ~
[
I
I
I
I
I
I
!
0
5
10
15
20
25
30
Temperature
I I
35
(C)
Fig. 2.5. Reuse ratio (RRoxygen/ammonia) as function of pH and temperature. Based on Koxygen = 200 g/kg, Kammonia = 30 g/kg, NH3-Nout -- 10 Ixg/1, DOin -- saturation, DOout -- 6 mg/1 and salinity of 35 g/kg. Note that between 32 ~ and 33~ the reuse ratio is equal to zero because DOin < 6.00 mg/1.
For general design considerations, it is also useful to consider the reuse ratio: Qoxygen RRammonia/oxygen - - Oammonia
(2.31)
If the reuse ratio is below one (Eq. 2.31), the maximum allowable ammonia concentration has already been reached and aeration will not help. The reuse ratio for typically marine conditions (salinity of 35 g/kg) is presented in Fig. 2.5 as a function of pH and temperature. The reuse ratio decreases as the pH increases due to the increase in the mole fraction of un-ionized ammonia. At a given pH, the reuse ratio first decreases as temperature is increased, then increases as the temperature increases above 20~ Over typical pHs and temperatures, the reuse ratio varies from 2 to 3, so aeration can often increase the carrying capacity. Above 25~ the reuse ratio rapidly increases, but the actual carrying capacity is decreasing because the available DO is rapidly decreasing. Above 32~ the reuse ratio is zero because influent DO is less than the DO criteria. In general, Eq. 2.30 will over-estimate the flows needed to maintain the required un-ionized ammonia concentration. This is due to chemical reactions of metabolic carbon dioxide in
38 water and the impacts of metabolic carbon dioxide on pH and un-ionized ammonia. The mole fraction of ammonia (c~ in Eq. 2.30) depends strongly on pH. If the metabolic carbon dioxide excreted by the animal reduces the pH, Eq. 2.30 will significantly over-estimate the value of Qammonia.The impact of these two factors on the required flows depends strongly on the amount of carbon dioxide retained in the water (under normal pHs, negligible ammonia (NH3) is lost to the atmosphere). Compared to oxygen, carbon dioxide is a very soluble gas and is much more difficult to remove by aeration. To accurately predict the impact of metabolic carbon dioxide on water requirements, it is necessary to estimate carbon dioxide losses to the atmosphere across the water surface (due to aeration or other processes). Limited information is available on the transfer of carbon dioxide to the atmosphere under marine culturing conditions. As a result, this book will neglect all reactions of carbon dioxide in water. This is equivalent to assuming that pH is constant and carbon dioxide gas is an inert gas. For more information on the impact of metabolic carbon on water chemistry and water flow, see Colt and Orwicz (1991 a). While the reuse ratio increases at high temperatures, the actual carrying capacity decreases. This is better demonstrated by looking at the total amount of dissolved oxygen that can be used before the un-ionized ammonia criterion is exceeded. Assuming that the influent water is saturated and the effluent DO concentration is a minimum of 6.00 mg/1, the maximum available oxygen to the animals ranges from 4.107 mg//1 at 5~ to 0.236 mg/1 at 30~ (Fig. 2.6). Even though the reuse ratio is higher at 30~ than at 5~ the potential available dissolved oxygen is less. If the oxygen consumption rate of the culture animal is known, the loading rate can be computed. The loading rate will depend on dissolved oxygen and un-ionized ammonia criteria, temperature, and size of animals. Guidelines for maximum loadings are presented for holding (Fig. 2.7A), production (Fig. 2.7B), and research (Fig. 2.7C) conditions. For holding conditions, aeration can increase the loading rate. For production conditions, it has been assumed that aeration will be used. The maximum loading for unaerated production conditions is typically 50 to 60% of the aerated values but falls off significantly above about 25~ For research conditions, maximum loading is controlled by the ammonia criteria and aeration will have no effect on loading. Note that the recommended maximum loading is higher for large animals and at low temperature. In the absence of more specific loading criteria, this information can be used in system design. While these recommended loadings should, in most circumstances, be conservative, they should still be used with caution. The loading rates in Fig. 2.7 are significantly less than under freshwater conditions, primarily due to the high pH of seawater increasing the toxicity of un-ionized ammonia. Depending on the pH, temperature, and water quality criteria, un-ionized ammonia is often the most limiting parameter and when this limit is reached aeration cannot be used to increase carrying capacity. In fact, above 20~ only 1-2 mg/1 of dissolved oxygen can be used before the un-ionized ammonia criterion is exceeded (Fig. 2.6). Increasing the loading above these somewhat arbitrary numbers results in rapidly increasing risk. The given values assume good conditions but no internal water reconditioning and may represent the carrying capacity of even a reuse system, whose water reconditioning has either mechanically or biologically failed. Earlier life stages are usually much more demanding on water quality and quantity then juveniles and adults and this must be considered when using data such as Fig. 2.7 in hatchery applications (see Example 2.9 for production and Example 2.10 for holding). Except as an
39 Available DO = 0
l I I I I I l i i I l l l I I I I I i g | I I I
o~
Influent pH
<
m |
Maximum available D O / for single pass system
I
I
I
I
I
I
I
0
5
10
15
20
25
30
35
Temperature (C) Fig. 2.6. Potential available dissolved oxygen. Based on same conditions and assumptions as in Fig. 2.5.
emergency measure, it would also be unwise to initially plan to use a seawater system at its maximum capacity until experience is acquired with the system. A great deal remains to be learned about water quality and water reconditioning. Success with heavily loaded systems is in many ways an art. The best policy for high water reuse or recycling systems is not to exceed very conservative limits, unless one is very experienced, is researching such questions or enjoys high risk. In those cases where little is known about the effects of ammonia on growth or mortality, chronic bioassays may be needed to develop design loading information (Meade, 1988). These tests are expensive but will allow optimum design and should be cost effective in the long-run, especially for large commercially oriented culture systems.
2.9 Design requirements Design is based on the loading criteria, density criteria, and production objectives. The most important design parameters to be computed in the design phase are the water flow rate, rearing volume (or area) for each step, and the size and number of individual rearing units.
40
Temperature (F) 35
40
100
45 I
50
55
60
I
65
I
70
I
75
I
80
I
85
I
90
I
I
~5 :700
70
A) HoldingConditions
50
"500 300
30 "200 20 -100 10
"" ",,
70 "50
..... :, ...... : ~ ....... ! ' %
30
~
"20
E
,, ] -
,:,..
",~.
"',,, i "',... " ~
~
looog
10
E m~
7 =
0.7 ~ 9
0.5
5
%%%
"- 9
"',,
9 9
10g
o
O
0.3
%
0.2
9
%%
",
= or
9
9
9 1 4 9 1 4 ~9
0.1 g 1
0.1 "
0.1 g
"
9149g 9
0.07
td000 g
~~ 9
t
0.7 0.5
0.05
9
.......
~t 9
Unaerated
0.03
I| 1
99
I
0.3
|1 It !
0.2
| | It I
0.1
'
Aerated
0.02
t t
0.01
I
0
5
10
i
I
15
20
;,
3'0
35
Temperature (C) Fig. 2.7. Maximum loading rate guidelines for a pH of 8.2 and a salinity of 35 g/kg. The loading rates are significantly less than for freshwater due primarily to the high pH and toxicity of un-ionized ammonia. These loadings will be even more conservative at lower salinities except at high elevations. Guidelines for maximum loadings for holding (A), production (B) and research (C) conditions, as a function of temperature and animal size. Dissolved oxygen for inflow assumed at saturation and discharge at 6.0 mg/1. Oxygen consumption is assumed equal to that for a fed rainbow trout for production and research and an unfed trout for holding. Ammonia criterion for holding is 40 Ixg/1, for production 10 Ixg/1, and for research 2 Ixg/1. The dashed lines are used in Examples 2.9 and 2.10. These guidelines should be used with caution and only in the absence of data more relevant to the species or conditions.
41
Temperature (F) 35
40
45
50
55
60
65
70
75
80
85
90
95
100
700
70
500
B) Production Conditions
50
300 30 200 20 100 10
70
7
50
5 30 3 20 2
gl. 10
l'
7
0.7
,I~ ~0
5
oJ,~
3
,..]
0.5 0.3 1000 g
0.2
1
0.1
10g
0.7
0.07
0.5
0.05 0.1g
Aerated
0.03
0.3 0.2
0.02 0.1 0.01 0
5
10
1
Temperature
20
25
35
(C)
Fig. 7 (continued).
2.9.1 Water Flow Given the loading requirements and number of animals at the start of each culture step, the water flow rates for each culture step can be computed. If some of the tanks are to be filled, it is important to compute the maximum water requirement that may occur when everything is 'on' and cleaning and tank filling is also in progress.
42
Temperature 35
40
I
100
45
I
50
I
55
I
60
I
I
70
(F)
65
70
I
75
I
80
I
85
I
90
I
I
700 500
C) Research Conditions
50
300 30 200 20 100
10
I
70
7
50
5 I
I
I
~
30
3 20 gl,
2
%9
I I
]
% %9
9 9149
Q~
@
10 7
9
0.7 ~
0.5
9 99
%% % %%
9
0.3
,.~
%
I
9
%%
%% 9%
%%
%%
0.2 9% 9
%
0.1-
9
3
@
%
1 %%
9 9
0.07
oil
%% %%
%~%
5
%%
%
0.7
%%
%%
0.5
0.05
,, 9
9 9 9 % 9 % 9
0.03
% 9
0.3
9 9%
Unaerated
0.02
1000 g
9
" 9.
% 9
9" , 9 0.01
I
I
0.2
lO'g"
0.1
g
I"
35
0
Temperature
(C)
Fig. 7 ( c o n t i n u e d ) .
2.9.2 Rearing Volume/Area Based on the density criteria and the total number of animals at the start of each culture step, the total volume (or total area) is computed for that culture step. If the animals spawn only once a year or fry are available only during a limited period, it may be prudent to size equipment and capacity based on the needs of this surge period.
43
Example 2.9. Carrying capacity m production One thousand pounds of fish are kept in a flow-through system at 12~ pH of 8.2, salinity of 35 g/kg (ppt) and a maximum ammonia criterion of 10 Ixg/1NH3-N. Compute the required water flow needed for 10 and 1000 g animals. How much more water is needed if the temperature increases to 25~ (A) Compute the water flow requirement at 12~ From Fig. 2.7B, loading = 1.0 kg/lpm for 10 g fish and 2.7 kg/lpm for 1000 g fish. Q =
454 kg
= 454 lpm for 10 g fish
1.0 kg/lpm
454 kg Q = 2.7 kg/lpm = 168 lpm for 1000 g fish (B) Compute the water flow requirement at 25~ From Fig. 2.7B, loading = 0.20 kg/lpm for 10 g fish and 0.51 kg/lpm for 1000 g fish. Q = 2270 lpm for 10 g fish Q -- 890 lpm for 1000 g fish
10 g fish Increase in flow =
2270 lpm 454 lpm
= 5.0 times or 500%
1000 g fish Increase in flow --
890 lpm
168 lpm
= 5.3 times or 530%
2.9.3 Rearing Container Size and Number While the volume or area needed can be directly computed, the size of the individual rearing unit can vary widely. A small number of large units are often cheaper to purchase than a large number of small units of the same total capacity. On the other hand, smaller units are easier to monitor and manage. In many systems, the number of individual units will be determined by the intended use or application. Often individual unit size will be determined by the optimum batch size as determined by production objectives and timing. 2.10 Constraints
While the bioengineer's solution to providing a successful low risk system is excellent input water quality and quantity coupled with very conservative density and loading of the system, there are many realistic constraints to this approach. The influent water quality parameters may not always be within the desired limits and the water may have to be treated before use. This creates costs which are proportional to the flow rate, degree of treatment specified, and risk of equipment failures. The limiting environmental parameters that can be controlled to increase system performance or reduce risk may vary depending on season and culture conditions. The
44
Example 2.10. Minimum required flow m holding
It is desired to hold 2560 one-pound lobsters at 10~ without aeration for several weeks. What is the minimum required flow rate in gpm? This is a holding situation. Therefore, Fig. 2.7A applies. From the figure, one pound (454 g) animals held without aeration at the stated temperature have a maximum recommended loading of 3.0 kg/lpm. Biomass of animals = (2560 animals)(0.454 kg/lb) = 1162 kg Flow rate x Loading rate -- Biomass of animals -- 1162 kg Flow rate =
1162 = 387 lpm, 3.0
387 lpm = 102 gpm 3.78 1/gal
The conversion factor is from Appendix Table A-1. This is the minimum flow rate and should be used with caution as there are many inherent assumptions in Fig. 2.7 and in the problem statement. Using flow rates in excess of the minimum will often reduce risks.
problem is most acute where there are economic limits on initial cost or profitability. The problem then becomes a tradeoff between treatment level, with associated cost and equipment reliability, and reduced growth and increased stress (risk) to the culture organisms without treatment. While retaining conservative loading is rarely a problem in educational or research systems, loading is often a critical factor in commercial systems. Overly conservative values may make the system economically unattractive. There are strong economic incentives in commercial systems to minimizing the water treatment quantity and quality (front end capital) and maximizing the loading of the system. In the inevitable absence of sufficient directly relevant hard data, the resulting evaluation and tradeoff of anticipated system performance versus risks is difficult and usually highly subjective. What is often forgotten, or at least minimized, is that most system parameters are interrelated and that pushing the limits on more than one variable at a time generally produces unfavorable interactions resulting in multiple sources of increased stress on the organisms. Unfortunately, quantitative cost factors usually dominate over subjective consideration of risk. If the decisions reached are overly optimistic, the resulting high risk operations often exhibit reduced growth rate, susceptibility to disease, increased cannibalism and frequent mass mortalities. Under most situations, input water parameters are very unlikely to be at optimum values at all times. A major problem is in determining the costs versus benefits of regulating these parameters and compensating for these naturally occurring deficiencies. Seawater processing equipment can be expensive to buy and maintain and quantitative requirements in these areas need careful evaluation. While the same types of equipment might also be useful for internal water reconditioning, the two functions should, at least initially, be considered separately. Additional information, including considerations and more specific tradeoffs are presented later. Since seawater processing equipment is expensive, a c o m m o n approach is to initially install only the m i n i m u m necessary to meet specific short-term needs. However, the initial design can be made sufficiently flexible to enable the easy retrofit of new types, higher performance, or larger-scale equipment to meet future needs or changing priorities. Without this early foresight, later retrofit may be very expensive or even prohibitive. This added
45 flexibility is not necessarily very expensive and in most cases requires only the crudest idea of future requirements. However, providing flexibility and preserving future options does involve costs and can be over done. The major considerations are easy access to seawater lines in appropriate places, spare floor space, floor drains and access to power. Additional flexibility can be acquired by not burying pipes, and by using threaded or flanged connections in piping, facilitating future reconfiguration of the piping system. In summary, considerable care should be exercised in the quantitative statement of 'requirements' for seawater systems involving living organisms. In particular, careful evaluations are needed of cost/benefit/risk tradeoffs for each specific application. Special care should be taken in differentiating between short-term and long-term requirements. In many applications, short-term requirements are readily quantifiable but are usually subject to frequent change and are often of peripheral importance. Long-term requirements are often more difficult to define but are usually of a more fundamental nature.
This Page Intentionally Left Blank
47
Chapter 3
Site Considerations
3.1 Marine conditions Ideally, the site should be selected by means of a thorough site reconnaissance and selection process (Webber, 1971; New, 1975). Some of the important selection parameters are listed in Table 3.1. In many private and public projects the site selection process is very abbreviated because the site options are either few or the site is already determined at the start. In public sector projects, it may be necessary to complete a formal selection process to justify the site picked, but in reality, the site has already been selected. This site may already be owned by the project owners (or good old Uncle Fred), or may be dictated by legal, political or economic reasons. Trying to design a system for a site chosen for reasons independent of seawater system requirements may prove to be very expensive or even impossible. While there is never an ideal site, a bad site can easily doom a project. Prior to the start of the design phase, basic information is needed on the marine conditions at the site. Seawater systems design requires detailed information on the tidal variations, including both high and low tidal information. The astronomical tides are due to the rotation of the earth, moon, and sun. These tides can be accurately predicted and detailed tidal predictions are available for major ports. Tides for other sites close to the reference station can be estimated by published tidal differences in elevation and time. The tables are only good for the published times. Typically, there are two high tides and two low tides per day. The height of the two daily tides may be equal or unequal. Some areas only have a single tidal cycle per day (one high and one low tide per day). The average interval between two successive high tides or between two successive low tides is 12 h and 25 min. In working with tides, it is important to clearly define the datum with which all tidal elevations are referenced. At least three datums are commonly used in tidal information: mean sea level (MSL), National Geodetic Vertical Datum (NGVD) and the chart datum (see Fig. 3.1). In general, computation of mean sea level, chart datum, and other tidal parameters requires long-term tidal data, commonly 19 years. Some of the most important tidal datums and parameters are defined below.
Mean sea level (MSL). This is the average hourly tidal heights over a 19-year period. On a global basis, sea level is rising approximately 0.50 ft/century. National Geodetic Vertical Datum (NGVD). This is a fixed surface whose elevation does not vary with time. Due to elevation changes in sea level and land surface, differences between MSL and NGVD in the U.S. range from - 1 . 9 0 to +2.33 ft (Harris, 1981a). Elevations on land are usually based on NGVD. Chart datum. Predicted tidal heights and chart soundings are referred to the chart datum. The chart datum is selected so that most tidal elevations are positive. In the U.S., either MLW
48 TABLE 3.1 Important site factors
Meteorological factors Winds - - prevailing directions, velocities, seasonal variations, storm intensity and frequency. L i g h t - total annual solar energy impingement, intensity, quality, photoperiod, diurnal cycle. Air temperature and variations. Relative humidity or dew point and variations. Precipitation - - amount, annual distribution, storm maximums and frequency.
Locational factors Watershed characteristics - - area gradients (elevations and distances), ground cover, runoff, up-gradient activities. Ground water supply - - aquifers, water table depth, quality. Tides w ranges, rates, seasonal and storm variations, oscillations. Waves - - amplitude, wave length, direction, seasonal and storm variations, storm frequency. Coastal currents - - magnitude, direction and seasonal variations. Existing facilities and characteristics. Accessibility of site. History of site - - prior uses and experiences. Soil factors Soil type, profile, subsoil characteristics. Percolation rate m coefficient of hydraulic permeability. Topography and distribution of soil types. Particle size and shape. Angle of repose - - wet, dry. Fertility. Microbiological population. Leachable toxins - - pesticides, heavy metals, other chemicals.
Biological environment Primary productivity - - photosynthetic activity. Local ecology - - number of trophic levels, dominant species. Wild populations of desired species - - adults, sources of seed stocks. Presence and concentrations of predators - - land, water, airborne. Endemic diseases and parasites. This listing includes only those factors most important to the technical aspects of the system. It does not include the many social, political, legal, and economic aspects that can be equally important or even critical in the selection of a site. (Modified from Webber, 1971.)
or MLLW (see below) are commonly used as chart datum, although different datums may be used for the Gulf Coast and Great Lakes (Harris, 198 l a). The difference in elevation between the chart datum and mean sea level is usually given in the tide tables and charts. Mean low water (MLW). Average height of the tide at low water over a 19-year period. Mean lower low water (MLLW). Average height of the tide at the lowest tide of the day over a 19-year period. Extreme lowest water recorded. Compiled as the lowest low water for each month. Mean high water (MHW). Average height of the tide at high water over a 19-year period. Mean higher high water (MHHW). Average height of the tide at the highest high tide of the day over a 19-year period. Extreme highest water recorded. Compiled as the highest high water for each month.
49 I~~Ext
S
rem~-'e'Highest Water Recorded
I~'--'~M HHW / (MeanHigher High Water) ~/MHW (Mean High Water)
~
I~--~/~xtreme
~
/ I~/M
SL (Mean Sea Level) ( NGVD) National Geodetic Vertical Datum
, ~ d
'~ ,
eon.o. Wo,erl
.Wor
MLLW often used MLLW ( Mean Lower Low Water )__.,Jas Chart Data
LowestWater Recorded
Fig. 3.1. Diagram of tidal datums.
In the U.S., tidal data are collected and tabulated by the National Ocean Survey (NOS). This agency maintains a running summary of the monthly mean and extreme high and low water of the month, and many other tidal statistics for approximately 50 reference stations (Harris, 1981 a) (see Table 3.2). This information is available at a nominal charge. A wide variety of tidal information may be needed in the design process. Depending on the type of intake and pumping cycle, detailed information may be needed on both the high and low tides at the site. The design of the intake, pumping system, and vertical location of the pumps will be strongly influenced by the tidal conditions. Ideally, the detailed information on the distribution of the high and low tides should be used to define the tide in a probabilistic manner. Detailed probability density distribution tables have been developed for seven tidal parameters for 50 sites in the U.S. (Harris, 1981 a) and include the following parameters: (1) (2) (3) (4) (5) (6) (7)
extreme monthly high water; extreme monthly low water; highest high water; high water; hourly water levels; low water; lower low water.
The tidal height corresponding to a desired probability can be simply interpolated from the appropriate table. Once the design tidal height is determined at the reference station, the design tidal height at the closest secondary station (listed in the tide tables) can be adjusted by use of the published tidal height difference for the station. The accuracy of this adjusted value to the actual site under consideration may depend strongly on the water depth and configuration of the coastline. For design of the pumping and intake system, a 1% probability level might be used. If the system was designed to be operable down to the 1% low tide, in an area with two
50 TABLE 3.2
Tidal data for selected stations Station
Standard
MHHW
MHW
MLW
MLLW
deviation Eastport, ME Portland, ME Boston, MA Newport, RI New London, CT Sandy Hook, NJ Atlantic City, NJ Hampton Roads, VA Wilmington, NC Charleston, SC Savannah River Entr., GA Mayport, FL
Miami Harbor Entr., FL Key West, FL St. Petersburg, FL
Pensacola, FL Mobile, AL Galveston Ship Chan., TX San Juan, PR San Diego, CA Los Angles Outer Harb., CA San Francisco Gold.Gate, CA Humboldt, CA Astoria, OR Aberdeen, WA Seattle, WA Ketchikan, AK Juneau, AK
Anchorage, AK Dutch Harbor, AK Honolulu, HI
6.32 3.24 3.40 1.33 0.94 1.70 1.52 0.92 1.51 1.88 2.52 1.66 0.95 0.58 0.71 0.54 0.58 0.53 0.54 1.81 1.66 1.75 1.93 2.53 2.99 3.50 4.91 5.31 9.01 1.24 0.60
9.32 4.87 5.16 2.18 1.48 2.66 2.41 1.41 2.26 2.87 3.77 2.49 1.33 0.92 1.04 0.67 0.73 0.57 0.87 2.90 2.63 2.59 2.97 3.98 4.54 4.83 7.36 7.82 12.73 1.52 1.08
8.88 4.45 4.72 1.93 1.22 2.33 2.01 1.22 2.02 2.50 3.38 2.20 1.26 0.63 0.72 0.61 0.65 0.47 0.57 2.11 1.91 2.04 2.26 3.28 3.74 3.94 6.46 6.93 11.99 1.21 0.58
-9.01 -4.46 -4.86 -1.69 - 1.34 -2.34 -2.07 - 1.22 -2.24 -2.67 -3.56 -2.27 -1.26 -0.64 -0.70 -0.57 -0.62 -0.44 -0.58 -2.09 -1.87 -1.93 -2.24 -3.19 -4.03 -3.75 -6.47 -7.10 -13.90 -1.07 -0.65
-9.41 -4.80 -5.19 -1.75 - 1.45 -2.47 -2.18 - 1.26 -2.33 -2.81 -3.70 -2.38 -1.40 -0.88 -1.14 -0.63 -0.70 -0.85 -0.79 -3.06 -2.82 -3.14 -3.44 -4.38 -5.35 -6.48 -8.02 -8.72 -16.18 -2.17 -0.81
Extreme values high
low
13.8 9.4 9.0 11.9 9.3 8.0 7.4 7.2 6.3 8.0 7.5 5.1 5.1 3.2 4.1 8.3 8.2 10.6 1.8 5.3 5.0 5.6 6.1 7.8 9.4 8.2 13.0 14.6 18.7 4.4 2.7
-13.6 -8.2 -8.7 -4.5 -4.8 -6.7 -6.0 -4.4 -3.6 -6.0 -8.0 -5.5 -2.9 -2.2 -3.7 -2.8 -3.8 -6.1 -1.7 -5.8 -5.4 -5.5 -6.4 -7.1 -8.4 - 11.3 - 13.2 - 13.8 -23.3 -4.9 -2.5
Computed datums, extremes, and standard deviations referenced to mean sea level. Extreme values are highest and lowest ever recorded. U.S. data from Harris (1981 a). All values are in feet.
daily tides this would correspond to approximately 7 tidal cycles/year when the system would not be operable. In choosing the design probability level, decreasing these periods of inoperability may, depending on conditions, result in significantly increased construction costs. For example, costs of providing storage or curtailing operations for the 7 tides/year when the intake system will not operate may be cheaper than designing the system to operate for the extra 7 tidal cycles/year. Under other conditions, it may be just as easy to design the pump station to withstand the 0.01% tidal level (one down time every 27 years). These considerations will tend to be more critical for areas with high tidal ranges. In most locations, detailed probability information on tides will not be available. Tidal data do not form a normal distribution and interpolations of data can be difficult and misleading.
51 It is still n e c e s s a r y for design and operations to develop m a x i m u m and m i n i m u m tidal elevations. In the absence of g o o d tidal data, the following p r o c e d u r e is r e c o m m e n d e d : D e s i g n low tide = M L L W -
( m e a n tidal r a n g e ) / C
D e s i g n high tide = M H H W + ( m e a n tidal r a n g e ) / C
(3.1) (3.2)
A value of C = 4 is r e c o m m e n d e d for intakes and a C = 2 for m o r e critical elevations such as for p u m p stations, electrical c o m p o n e n t s and rearing units. For m o s t conditions, this procedure will yield conservative results. However, it does not a c c o u n t for rare e x t r e m e events such as typhoons, hurricanes, and t s u n a m i s (see E x a m p l e 3.1 for a n u m e r i c a l example). W h e r e very expensive or critical facilities are involved, m o r e detailed site specific analysis m a y be prudent. T h e effects of b a r o m e t r i c pressure and w i n d are m u c h m o r e difficult to precisely predict. T h e s e two c o m p o n e n t s m u s t be added to the a s t r o n o m i c a l p r o d u c e d tides. T h e b a r o m e t r i c c o m p o n e n t m a y exhibit a seasonal pattern w h i c h is i n c l u d e d in tidal data, but these two factors are m u c h m o r e significant in relationship to m a j o r storms. Tides c a u s e d by m a j o r infrequent storms can be critical. E x t r e m e high water levels c o m m o n l y result w h e n (1) high a s t r o n o m i c a l tides and strong inshore winds occur simultaneously, or (2) m a j o r tropical w e a t h e r events occur. T h e s e events occur r a n d o m l y and the fact that a severe tidal event has not o c c u r r e d in the last 20, 30, or 40 years does not p r e c l u d e two serious floods occurring in the next year.
Example 3.1. Determination of design tide It is desired to build a culturing facility on the seashore near Atlantic City, NJ. What are the design tide elevations for normal operations for the intakes and facility components nearest to the water edge? Atlantic City tidal data can be acquired from Table 3.2 and design tide values calculated from Eqs. 3.1 and 3.2. MHHW = +2.41 ft (from Table 3.2) MLLW = -2.18 ft (from Table 3.2) Mean tidal range -- 2.41 + 2.18 = 4.59 ft (A) Using Eqs. 3.1 and 3.2 for the intakes: Design low tide = -2.18 - 4.59/4 -- -3.33 ft Design high tide = +2.41 + 4.59/4 = +3.56 ft (B) Using Eqs. 3.1 and 3.2 for the facility: Design low tide = -2.18 - 4.59/2 = -4.48 ft Design high tide -- +2.41 + 4.59/2 = +4.71 ft These values are all relative to mean sea level and tend to be conservative. A more detailed analysis using the probability functions of 1% and 99% yields -3.25 and +3.31 ft for the intakes and -4.12 ft and +3.45 ft for the facility. Note that the extremes of record at the site are -6.0 ft and 7.4 ft. This analysis accounts for seasonal weather factors but n o t rare extreme events, such as hurricanes, which must also be considered in placing components.
52
.;-.:9 ::.: ..-:-: ...i.:..! :::.:.:::...:....
...:!. _ .. :,.,, ...... !:. 'i:iii-
WinterFetch N W-'I'-- E S
~176 i.~
Fig. 3.2. Ocean exposure example. This example is of a relatively well sheltered site. Waves generated by prevailing winds in summer and winter will be relatively modest in size due to the short distances (fetch) the winds have available for generating waves. However, major storms coming up from the south may generate impressive waves coming in from the exposed south and southeast. These waves and their effects will be maximized if the site is actually hit by the northeast (for example on the east coast of the U.S.) quadrant of the storm due to its forward wind speed combined with the storm speed and general counter-clockwise rotation.
For extreme storm events, it is necessary to be able to predict both the peak surges and the wave run-up (how far up the beach the waves will climb). Wave run-up can result in physical damage to buildings or more commonly erosion of the foundation and total building collapse. The surge and run-up can be computed from the storm track, intensity, size, and forward speed (U.S. Army Corps of Engineers, 1975; Taylor, 1980). In many coastal areas, 100-year flood maps have been prepared by the National Flood Insurance Program (Fry and Rhodes, 1985). The prevailing seasonal wind directions and speeds should be obtained from local weather stations or published data and plotted on a hydrographic chart of the area. The greatest length of unobstructed length of sea over which the wind can blow is called the fetch (Fig. 3.2). The hydrological and meteorological conditions will determine the height and frequency of normal seasonal wind generated waves. Storm directions and associated wind velocities can be very different from prevailing seasonal wind directions and velocities. Since wave forces and wave effects associated with rare storms are usually the biggest threat to intakes, pump stations and other shore-side facilities, it is important to quantify these waves and their effects (see Section 5.3). Information on the beach profile is needed for specifying the burial depths for piping and locating the intakes (see Section 5.3). Beach profiles should be collected at least four times
53 Berm
~fCrest of Berm
~" ~' ~ ' - ~ -,, '--'~ \ ~ "--..~. "Winter" Profile-~ ~ ~
" Summer" Profile ~
~,,fPlunge Point ~,.~-Low Tide Terrace
0
I
0
High Tide
Low Tide
50m I
lOOft
lOOm I
20Oft
I
I
:50Oft
Fig. 3.3. Typical beach profile characteristics. Beach profiles can vary considerably. This figure shows some of the common features that may exist and possible seasonal changes for a typical 6 ft (2 m) tide. Additional features might include a scarp (sharp drop) in the beach above the normal high tide, an alongshore trough below low tide or an alongshore bar further out. Single events, such as major storms, can substantially alter beach profiles.
over a year, to get seasonal variations caused by changes in weather patterns and seasonal storms (Fig. 3.3). A longer period of observation would be even better. Beach profiles can be variable and depend on soil characteristics and oceanographic conditions and are dynamic in nature (Shepard, 1963). The profile should be extended to at least 10 ft or 3 m below the extreme low tide elevation, preferably deeper. The average slope of the off-shore bottom should be estimated from published charts. The salinity and temperature of the ocean water should be determined at least four times over the year. Other parameters that may also be required are presented in Chapter 2. For sites on coastal rivers or estuaries, it may be necessary to sample over the full tidal cycle as both temperature and salinity may vary widely. If the facility is near any rivers, special care must be directed towards determination of the salinity during the rainy season or during periods of maximum run-off. River effects may vary considerably from year to year, so long-term data are very desirable. Rainy season salinities may be so low or uncertain as to make the site unacceptable. If the site is located close to present or prior industrial and agricultural activities, detailed chemical analysis of soil and groundwater may be required. If a site has previously been used in agriculture, possible residues from past use of herbicides, insecticides and other chemicals could be important, especially if there are to be earthen ponds. In cases of questionable water quality, it may be necessary to test the water by bioassaying with target species or determine residues in native animals or plants. During site survey, information on quality and quantities of freshwater available near the site must also be collected. This can become critical if freshwater is required to control salinity. Evaporation during dry seasons or larval culture needs may require continual freshwater input to reduce salinity to acceptable levels. Some freshwater will also be needed for washing, laboratory use, or drinking. Hauling of freshwater or production by reverse osmosis is both expensive and troublesome. Freshwater requirements of 'marine' systems have often resulted in the elimination of otherwise attractive sites.
54
3.2 Terrestrial conditions Prior to the start of any serious design work, the property should be surveyed and all corners and sidelines clearly marked. It may also be prudent to establish permanent comer markers with concrete and steel. Unless the site is clearly within the property boundaries, it may be necessary to use registered surveyors so that there is a legal basis for any property disputes. It is also desirable to bring a vertical datum from the nearest benchmark. In sites where it is prohibitively expensive to survey in a vertical datum, comparison of observed and predicted tides may allow a rough estimate of mean sea level. A permanent vertical benchmark should be established nearby. A topographic survey of the potential site must also be completed as part of the property survey. In areas with low bearing strength soils, detailed soil testing may be required. Having to provide extensive filling and compaction can significantly increase the cost of site development. Soil permeabilities can be important factors for seawater or freshwater wells and if natural soils are to be used to seal ponds or raceways.
3.3 Permitting Major commitment of time and money may be required to obtain all the necessary permits for a marine or estuarine project. Permitting problems have increased substantially in the past decade. The actual cost of the permits themselves is typically low. The major costs are associated with the man hours that must be directed towards obtaining the permits, and possible delays to the project as a result of permitting complications. Permitting can easily be a long lead time item, requiting submissions very early in the design process. It is possible to spend years and thousand of dollars on permits, only to have the last agency refuse to grant approval. The number of permits required will depend on (1) the country, state, and county where the project is located, (2) the type of agency proposing the project (government, company, private individual), (3) the year the process was started, (4) the ownership of the site, (5) the organism to be maintained or cultured, and (6) the project characteristics. The construction and operation of a seawater system must be consistent with local county or city zoning. A specific seawater system may be classed as agriculture, fisheries, education, or some other industrial operation. An unfavorable ruling on zoning can eliminate many potential sites. Very often, the acquisition and use of land for a seawater facility is a new use not contemplated by zoning authorities and not designated under the guidelines of a local master plan (Wypyszinski, 1984). Therefore, it may be necessary to obtain a variance or an amendment to the zoning code, which can be difficult and time consuming. In many coastal areas of the U.S., Flood Insurance Rate Maps (FIRMs) have been developed by the National Flood Insurance Program (Fry and Rhodes, 1985). Areas covered by FIRMs are classified from high to minimal flood hazard. Construction in high hazard areas will be restricted or limited to structures built above the flood line of the 100-year storm. In the United States, the Federal Government passed the Coastal Zone Management Act (CZMA) in 1972 and established a system of federal grants as incentives for individual states to develop enforceable programs for land and water use planning in the coastal zone. The coastal zone includes just about all possible sites that may need a seawater system. Installation
55 of new facilities or construction in the coastal zone will require a water development permit and require detailed engineering drawings. Permit applications may require a formal environmental impact statement. An archeological survey may also be required. Evidence of old buildings or native American activities may seriously constrain facility layout on a site. In developed countries, other than the U.S., the details will vary but the nature of the problems is very similar. Even in third world countries, considerable attention will be focused on possible environmental impacts. The days when large coastal areas could be bulldozed into ponds and mangrove swamps destroyed without any questions are over. Generally, anyone seeking to raise or hold freshwater, marine, or anadromous animals must apply for an aquaculture permit from a state fish and game agency for a nominal fee. The species to be cultured must be listed on the permit. As long as commonly reared indigenous species are cultured, this type of permit is sometimes simple and straightforward to obtain. Exceptions can exist when the culture species is a 'game' animal and possession of under-sized specimens or holding 'out of season' is illegal. If game and conservation laws are interpreted to apply to culturists, this can doom projects. Problems are also aggravated when dealing with exotic species, which have to be brought in from somewhere else. Acquiring permits to import, even between adjacent counties and states, can be difficult. Possible ecological impacts, escapes, and disease and parasite transmission to indigenous species may become significant issues. The discharge of waters from aquaculture projects are regulated by the Environmental Protection Agency through the National Pollutant Discharge Elimination System (NPDES) and generally administrated by the states. An application for a discharge permit must be filed before the pollutants are discharged. Certain types of aquaculture facilities may be exempted, but most agencies will make a case-by-case ruling. Exemptions are much less likely today than they were in the past. The following factors may be considered in the designation of an exempted facility. (!) (2) (3) (4)
The location and quality of the receiving waters. The holding, feeding, and production capacities of the facility. The quantity and nature of the pollutants from the facility. Other factors relating to the significance of the pollution problems in the area.
NPDES permit applications may require a definition of a proposed facilities effluent waste stream. Common parameters that may need to be specified include BOD, solids (settleable and suspended), nitrogen and phosphorus loading, and residual chlorine (if applicable). Necessary input data include design biomass, anticipated feed type and feeding schedule (% body weight per day) and species-specific waste generation numbers. Unfortunately, waste generation data on many aquatic species are not presently available. Numbers can be extrapolated from trout/salmon (Liao, 1970; Clark et al., 1985; Seymour and Bergheim, 1991) or catfish data (Chen, 1998). Trout and salmon could be considered representative of high animal protein users, while catfish would be more typical of low protein users. Additional data exist for seed clams (Pfeiffer et al., 1999) and adult clams (Zhu et al., 1999). Another possibility is the use of 'rules of thumb', such as one pound of feed generates 0.03 pounds of ammonia, 0.5 pounds of fecal matter, and 0.4 pounds of BOD. For some applications waste generation data from terrestrial animals may be useable. There are a lot of data on farm animals. However, it
56 may be surprising that there is information on the waste generation of Bengal tigers but none currently available for seals. In aquaculture facilities, simple removal of settleable solids may be the only type of treatment required to achieve discharge standards (Federal Register, 1978; Harris, 1981b). This is equivalent to the 'primary treatment' process in municipal wastewater treatment. Information on the types and quantities of drugs and chemicals to be used in the facility may also be requested. The U.S. Environmental Protection Agency is currently reviewing NPDES regulations for aquaculture facilities. While the final regulations have not been drafted, possible changes to the regulations may include use of best available technology for treatment of discharges and regulation of pathogens in discharges. Construction in navigable water requires a permit from the U.S. Corps of Engineers. The definition of 'navigable waters' includes the ocean, tidal rivers, and just about any water connected to or within 20 miles of the ocean. Regulated activities include construction of intake and discharge lines, pilings, piers or dikes. A permit submitted to the Corps of Engineers may be reviewed by the U.S. Fish and Wildlife Service, state fish and game agency, National Park Service, National Marine Fisheries Service, Coast Guard, coastal zone management agency, and the EPA. If one of these agencies objects, the Corps will seek project modification from the applicant. The Corps requires that all other permits must be obtained before the Corps will process an application (Bowden, 1981). This makes the Corps of Engineers the last step in the permitting process in marine or estuarine areas. When permits are finally issued, they may contain specific restrictions that may have major impacts on design, construction, and/or operation of aquaculture facilities. These restrictions could be on effluent parameters, pipe intake/discharge placement or the timing of activities. In some areas, in-water construction is only allowed when it is difficult to impossible to carry out such work, such as during the winter. This restriction may be due to a possible negative impact on migration or spawning of aquatic or terrestrial species that occur during specific time periods. Many of the permit fees and monitoring requirements are based at least in part on effluent flow. A relatively small laboratory or commercial operation can have a flow similar to a small town. An effluent flow of 500 gpm (1900 lpm) corresponds to the wastewater from a town of about 7000 people. Municipalities, however, have a considerably greater ability to pay for permits, monitoring, and reporting activities than aquaculture and research projects. The costs of water quality monitoring can be very significant, especially if nutrient analysis is required. The mandated administrative aspects of keeping record and reporting can also become burdensome. These problems are increasing with time. 3.4 Site selection
Many considerations have been mentioned as being important in evaluating sites. However, all factors are not of the same relative importance. No site is ever perfect in all regards. So how does one select one site among a selection of useable sites. It has to be assumed that all the choices are 'possible' and without any major 'red flags' (problems of sufficient severity that, if not resolved, can doom a project). Picking a site with 'red flag' items is a very fast, and unfortunately not uncommon, fatal mistake. Schedule or funding considerations or wild optimism can sometimes force such a serious premature decision. A common variation on
57 this problem is that there is only one site being considered and it has serious 'red flag' items. Going ahead, while assuming that these problems will be resolved in time or go away, is often fatal to the project. Site selection is a critical decision and, unfortunately, often has a high subjective content. One approach that can be useful in clarifying choices and in decision making is shown in E x a m p l e 3.2. In this example, important factors are listed. S o m e are positive factors (i.e., good water quality) and some are good in the negative (i.e., lack of archaeological sites). Relative subjective weighing factors can be assumed based on the project's objectives, proposed scale of application and technical approach. This evaluation method, and a n u m b e r of similar variations, is helpful in eliminating obviously less desirable sites but should be used with considerable caution if the numbers are close. As in this example, water quality and quantity tend to be the key considerations. This is not u n c o m m o n , especially in predominately flow-through systems. If, as sometimes happens, there are no suitable sites, this requires a basic rethinking about project objectives, assumptions and approaches. With basic changes, there may be a suitable site. Site problems involving suitable water quantity and quality and sometimes coupled with area limitations, have been a big factor in the increasing interest in reuse systems.
Example 3.2. Site evaluation and selection Four sites (numbered 1 to 4) are to be evaluated for shrimp culturing using pond grow-out. Each site is graded on a 1-5 scale (5 is best) for 13 considerations. The importance of each consideration is adjusted by the use of a weighing factor (1-10). The overall score for each factor is equal to the weighing factor times the 1-5 grade. The total score is equal to the sum of the overall scores of the 13 factors for each site. The site with the highest total score is the most desirable and the relative values an indication of site differences. (A) Determine the relative importance of the 13 considerations by choosing weighing factors (1 to 10) for each of the factors in column 2 of the table below. On-site evaluation of each of the sites has resulted in the given 1-5 grades indicated in the table. Considerations
Water supply Water quality Terrain/topography/soils Physical access Purchase availability Proximity to seed stocks Future land use/expansion Human comfort/amenities Past agric, spray/chem. Flooding and storm potential Archaeological sites Availability of utilities Permitting Total
Weighting factor
Sites
1
2
3
4
4 2 3 4 5 4 3 3 4 3 4 4 4
3 3 4 5 4 4 4 4 2 3 5 5 4
3 3 4 5 5 5 4 4 3 2 5 4 3
5 5 5 5 3 2 3 2 5 5 2 4 3
47
50
50
49
58
E x a m p l e 3.2. (continued) (B) With the data above and the assumed weighing factors given below, complete the numerical evaluation and determine the 'best' site. The presented weighing factors are subjective and are presented as only one possible set of values. Values in columns 4 through 7 below are intermediate 'solutions' to be calculated by the reader. Considerations
Water supply Water quality Terrain/topography/soils Physical access Purchase availability Proximity to seed stocks Future land use/expansion Human comfort/amenities Past agric, spray/chem. Flooding and storm potential Archaeological sites Availability of utilities Permitting Total
Weighting factor 10 10 9 5 6 4 6 4 9 9 5 7 7
Maximum score
Sites 2
1
3
4
50 50 45 25 30 20 30 20 45 45 25 35 35
40 20 27 20 30 16 18 12 36 27 20 28 28
30 30 36 25 24 16 24 16 18 27 25 35 28
30 30 36 25 30 20 24 16 27 18 25 28 21
50 50 45 25 18 8 18 8 45 45 10 28 21
455
322
334
330
371
Using this methodology, site 4 is the superior site. By changing the weighting factors, it is generally possible to 'select' a given site.
59
Chapter 4
Seawater Sources
4.1 Options and considerations If a seawater supply is needed for the system, there are only three possible sources. The three sources are artificial seawater, a seawater well, and an ocean or estuarine intake. This supply may be needed continuously, for just a part of each day, only during certain seasons, or in batches at intervals. Artificial seawater is used mostly for small aquariums, research projects that need a highly repeatable and predictable seawater source, or inland systems where there is little choice. If large quantities of seawater are required at an inland site, a closed system will be needed. The advantages of artificial seawater are a defined composition with a reduced risk of chemical and biological contamination, but this risk is not zero. Artificial seawater systems often work quite well, especially if operated very conservatively. However, for some specialized applications, such as the hatchery phases of delicate organisms, there may be some problems due to the complex interactions of seawater and marine organisms which are not presently completely understood. Seawater wells are another option (Clark and Eisler, 1964; Cransdale, 1981). Their advantage is that the water is already filtered, reducing or eliminating the introduction of disease microorganisms, parasites and predators. In addition, diurnal and seasonal water temperature variations are damped out resulting in much greater temperature stability and predictability (Cransdale, 1981). In-ground temperatures down to 50 ft below the surface are usually fairly constant and very close to the yearly mean average air temperature for the site. If seawater is withdrawn from this zone, it will be at ground temperature. Depending on the biological requirements, this may greatly reduce system complexity and cost by reducing needed water processing equipment, such as filters, disinfection and heating equipment. The big problem with seawater wells is that the site conditions and geology are usually unfavorable. In addition, high flow requirements can prove difficult and expensive, even where the geology is favorable. By far the most common source of seawater is from shore side or offshore intakes. The water may be drawn from a sump constructed on a very protected shore or canal, or from behind a dike or berm with a large-diameter pipe connecting it to the sea. Intakes can be suspended from a dock or pier with the pumps immediately above on the pier. Where there is a suitable dock and conditions permit, this is a very desirable approach. The seawater lines can be run to shore along the side or even under the dock. The seaward section of the seawater system tends to be relatively inexpensive and everything including the intakes are easily accessible for maintenance. However, the intake pipes hanging down from the pier may be subject to ice and wave damage and the presence of treated wooden pilings and boats may create potential water quality problems. A variation is intakes suspended from a floating
60 platform which also holds the pumps. The platform is often moored to a pier providing both access and power. This approach may be particularly attractive for seasonal systems or if there is a large tidal range, which otherwise might seriously complicate pump placement, selection and operations due to excessive suction lift. Like the pumps on the pier, this approach tends to have easy access for maintenance and to be relatively inexpensive. However, such floating platforms can be very susceptible to ice damage, boat collisions, and waves and currents from major storms. Alternatively the water may be drawn from a pipe leading to an offshore submerged intake. Submerged intakes, while the most common type, are often very expensive to install and maintain, and are still somewhat vulnerable to damage by major storms, boat traffic and fishing gear. While there are many complex considerations (see Section 4.4), there is also considerable prior experience with such intakes in culturing systems. In all cases, the input water quality will be subject to all the diurnal and seasonal variations of the natural environment. Drawing seawater in batches and trucking it to a distant seawater system is sometimes used as a cheaper substitute for artificial seawater. The intake, in this case, may be simply dropping an intake hose from a tank truck over the side of a dock. This approach can work but if, as commonly occurs, the water quantity requirements prove greater than originally planned, it can be expensive, annoying and time consuming. Some variation of this approach is often used for systems that bioassay industrial waste effluents, drugs or chemicals on indicator marine organisms. Due to potential and often unknown toxicity, these systems are required to be completely disconnected from any seawater system. Post-use seawater processing, handling and disposal can get complicated. 4.2 Artificial seawater
There are a large number of published recipes for the preparation of artificial seawater (Bidwell and Spotte, 1985). This comprehensive citation lists 169 seawater and enrichment formulations for culturing everything from marine yeasts, fungus, and up to vertebrates. Enrichment formulations contain trace elements and compounds that may not be present in artificial seawater formulations or may be depleted in culture systems, but are incapable of supporting marine organisms by themselves. Many artificial seawater formulations have been developed and used in specialized applications involving single species and for only parts of the life cycle. In addition, there exist commercial mixtures that have broader uses. An artificial seawater should have some specific characteristics (see Table 4.1). Many of the published mixtures do not meet these requirements. Some have compositions far removed
TABLE 4.1 Characteristics of good artificial seawater mixtures (derived from Spotte, 1979) (A) (B) (C) (D)
It is simple but contains in reasonable concentrations elements known to be essential to animals and plants. It is relatively easy to measure and mix. Major ions are present in concentrations and ratios approximating natural seawater. Components consist of salts that do not easily precipitate out of solution.
61 TABLE 4.2 Some applications of different synthetic seawater formulations in the holding or cultivation of marine animals Name of mixture
Purpose
Reference
Many different species and culture purposes
Bidwell and Spotte, 1985
Culture of oyster larvae, Ostrea edulis Bioassays with mussel embryos, Mytilus edulis Culture of Pacific oyster larvae Culture of colonial hydroid, Bouganvillia sp.
Helm et al., 1973 Courtright et al., 1971 Zaroogian et al., 1969 Tusov and Davis in Kinne, 1976
Instant Ocean Instant Ocean Instant Ocean
Nurse sharks, Ginglymostoma cirratum Culture of lobster juveniles Culture of prawn larvae, Macrobrachium
Honn and Chavin, 1976 Gallagher and Brown, 1976 Maddox and Manzi, 1976
Instant Ocean and Utility Seven Seas Marine Mix
Culture of crab larvae, Rhithropanopeus harrisii
Sulkin and Minasian, 1973
Holding lobsters and depuration of oysters/clams
Wood and Ayers, 1977
Review articles
Summary of 169 mixes Custom formulations
Summary of 169 mixes Summary of 169 mixes Summary of 169 mixes Tusov and Davis Commercial formulations
rosenbergii
Simplified recipes
Instant Ocean and Utility Seven Seas Marine Mix
Modified from Wickins and Helm (1981). Complete citations in Reference Chapter.
in content and concentrations from natural seawater. Others have very elaborate preparation processes involving up to four steps. Very few can simply be dumped into the right amount of freshwater and used soon thereafter, except in small quantities. Preparation of large quantities of artificial seawater requires use of specialized dissolving equipment (see Spotte, 1979). Some formulations require considerable time for components to dissolve and reach chemical equilibrium. Artificial seawater mixtures that are useable with animals (see summary Table 4.2) may not be suitable for plants. Even for only marine animals, a given mixture may not be suitable for a particular species or life stage. Using a mixture for a new application will involve some risk. Unless there is specific information that a mixture is suitable for a given application, the risk can be minimized by using a commercial preparation that has been demonstrated to have broad applications. One such commercial mixture, trade-marked 'Instant Ocean', was developed based on earlier research (Segedi and Kelly, 1964) and is listed in Table 4.2. For routine use, commercial mixtures may be a better choice than custom mixing of components because of better quality control of individual components and the mixing process. Trace contamination of specific components can be a serious problem in critical applications. For critical applications, enough chemicals (or commercial mixture) should be purchased for the entire experiment and preliminary bioassay should be conducted to ensure that the resulting seawater is acceptable. Commercial seawater mixtures, however, neglect the needs of marine plants (Spotte, 1979). Mixtures suitable for marine plants have been reviewed by Droop (1969), McLachlan (1973) and Kinne (1976).
62 One of the major problems with artificial seawater in closed systems is that if something goes wrong, it may be impossible to determine the precise cause. It could be the original mixture or a whole host of other problems involved with closed systems, such as build-up or depletion of critical substances. The precise requirements of many species are not well documented, especially in the area of trace components. Since all mixtures are a simplification of natural seawater, the small missing components may prove critical in a given situation. 4.3 Seawater wells The relationships between seawater and freshwater ground waters near the shoreline can be quite complex and variable depending on specific site conditions. Fig. 4.1 shows two simple possibilities. Even in the case of a narrow sand spit surrounded by seawater, there can be significant freshwater present. The higher elevation of the freshwater is due to the lower density of freshwater and static equilibrium conditions. The second case is also typical, with alternating permeable and impermeable layers. It demonstrates how it is possible to get freshwater offshore and just below the bottom. Since the layering can be complex, discontinuous, folded and spotty in distribution, the number of variations is immense. Therefore, it is very important to have a clear understanding of the local geology of the site before one can seriously discuss the possibility of developing seawater wells. For any type of seawater well the soil permeability will be critical. Permeability is measured by hydraulic conductivity, in older texts also called the coefficient of permeability, which may have a number of units (ft/day, cm/s). Gravel and clean sand, preferable coarse and large grained, might be suitable for a seawater well. Clays and soils with high organic
~
/1 Mean
~a itwaii~\ \ Freshwate~~ ~'~S~'\~iw~ el
B --- FreshwaterMReUan noff Freshwater_i.~---..."c~... SeaLevel Freshwater rnn..... . .... FOlW __~T~.__~b~ . Freshwaatslta,wPari2g
Fig. 4.1. Simplifiedexamples of near-shore geology.(A) Sand spit or peninsula. (B) More commoncoastal geology with alternating permeable and impermeable layers.
63
Example 4.1. Determination of hydraulic conductivity A l0 cm diameter cylinder of undisturbed soil (as close to in situ condition as possible) is tested for hydraulic conductivity with a constant head permeameter. The difference in the water elevations in the two standpipes connected to the soil sample at a vertical separation of 20 cm is 127 cm. The flow rate through the column is found, with a graduate tube and stop watch at the bottom, to correspond to 1 1 in 2.88 m. What is the value of the hydraulic conductivity for this sample and is there any possibility of developing seawater wells in this soil material? Cross-sectional area of cylinder -- 0.00785 m 2
Overflow
Area (A)
Soil Column
Q = 1/2.88 = 0.347 1/min = 500 1/day = 0.5 m 3/day L =0.2m h = 1.27 m (0.5)(0.2) 0 = 10.0 m / d a y (1.27)(0.00785) This is an attractive value characteristic of coarse sand, and seawater well possibilities should be further examined. K = QL/hA
--
content, unfortunately very common in estuarine and sheltered waters, are clearly unsuitable. While soils vary widely and generalities without knowing the specific requirements of a given situation are hazardous, some 'rules of thumb' can be presented. Soils with hydraulic conductivities less than about 10 ft/day or 3.0 m/day are poor prospects. The higher the value the better. Hydraulic conductivity of coarse-grained soils is usually determined by a constant head permeability test (Lowe, 1969). In this test, water is supplied from an elevated constant head tank (see Section 9.2) to an undisturbed vertical column of soil. There are two standpipes connected to the column with a fixed difference in elevation on the column, one near the top of the sample and one near the bottom. The differences in elevation of the tops of the two static water columns in the two standpipes is measured as is the flow rate through the column. The hydraulic conductivity is then calculated from the equation below, while being careful with dimensions (see Example 4.1). K --
QL hA
(4.1)
where K is hydraulic conductivity, Q is flow rate through sample, L is sample length in flow direction (vertical separation between standpipe taps), h is difference in head (elevation) across sample as measured by water levels in standpipes (hydraulic gradient), A is cross-sectional area perpendicular to flow. Many times it is impossible to obtain a truly representative and undisturbed soil sample and it may be necessary to drill a test well to determine hydraulic conductivity and well yield. This approach has the additional advantage that the water quality can be tested. There are several approaches to seawater wells. One is to lay a gravel-packed crib just below the bottom (see Fig. 4.2A). This in effect uses the overlying coarse sand as a sand filter (see Section 10.6). The cap is needed to prevent short-circuiting directly above the crib
64 Seawater
Cove.-u nClean ~ d ba or~Gravei Bottom , ~
. . . . . . . . . . . . . . . . . .
SolidCap
"~
,/
~ ['I'~'s r,~_-I FilteredSeawater__~
Highly Perforated Box or Cylinder
Gravel Pack
Seawater
. . . . . Side
B
View Top View
\ () / I~ I I~
Clean Sand
or~f~,-~
\()/ \ Q) / Filtered Seawater---~ ~ 12x4./Perforated I~t l j~ ~--~176 'Laterals{ [=~ / l Filtered Seawater ---I~
Seawater
~
Clean Sand or Gravel Bottom
c
/
~ I "~--_~
~
Packing Well
Pain! . . . . .
Slots Typically 1.5-2.5mm
Fig. 4.2. Basic approaches to seawater wells. (A) Offshore gravel-filled crib with internal header pipe. (B) Offshore header system, side and top views, length and diameter of laterals to suit. (C) Conventional well point, penetration may be shallow or deep, for shallow penetration may be packed with gravel or coarse sand, system may have more than one such well point.
and in the event of backflushing. There are a number of variations of this approach and in a variety of sizes. Placement can be intertidal, which lowers the pumping opportunities but makes installation easier, or preferably for operational flexibility, sub-tidally. Large flows are produced by thin layers of highly permeable sands. Even in geographic areas composed primarily of coarse sand, the bottom's surface layers just offshore are likely to be more impermeable and to have quantities of organic and inorganic fines to clog the system. In addition, too thin a cover could be removed by currents, wave action or backflushing of the filter. If there is little likelihood of strong currents or waves even during storms, the cover could be as small as about a half foot (15 cm). Since breaking waves, even small ones, can cut terraces in near-shore bottoms, a much safer value for most sheltered sites would be about 1.5 ft (0.5 m). If large ocean waves can be encountered during major storms, considerably more may be needed. The flow possible from such a crib will be highly variable but a 'rule of thumb', assuming favorable conditions, is 1 m 3/h per 3 m 2 of perforated crib surface area (0.14 gpm/ft 2) (Cransdale, 1981). Generally, several smaller cribs can produce more water
65
than a large one. If more than one is to be used, they should be separated by at least 20 (6 m) and preferably about 30 ft (9 m). Another way is to lay an offshore leaching field of perforated piping a few feet below the bottom and just below or above the low tide line (see Fig. 4.2B). One can specify the size of the trench, the quantity of coarse material packing around the well and the depth of uniform cover based on the same considerations as for a crib or sand filter (see Example 4.2). However, properly installing such a system sub-tidally, in an area with any currents or wave action, range from difficult to impossible. Intertidal placement reduces pumping opportunities and the
Example 4.2. In situ sand filter A 1-ft-diameter slotted or screened pipe (or a smaller pipe with a packing) is buried with its center-line 2 ft below the surface of a submerged sand bottom. The sand is the same as in Example 4.1 (K = 10 m/day) and the sand is barely covered by seawater at low tide. (A) If the maximum acceptable head loss across the sand filter is 3 ft of water, roughly estimate the seawater flow rate in gpm per linear foot of filter pipe. Using Eq. 4.1 with K = 10 m / d a y = 32.8 ft/day, L = 2 ft (average), h = 3 ft Pipe surface area/unit length = red = 3.1 = 3.14 ft2/ft K -
Q• h x A
Qx2
-
= 32.8
3 x 3.14
Q = 155 ft3/day per ft of pipe = 0.81 gpm/ft of pipe (B) What is the minimum length of sand filter pipe to be buried underwater to provide a 100 gpm system? Qtotal --- QL or 100 = 0.81L L -- 123 ft, to allow some safety margin specify minimum of 150 ft (C) At 100 gpm, what is the equivalent loss coefficient for this sand filter? Eq. 6.6 h -- KvZ/2g, h -- 3 ft Q = 100 gpm = 0.222 ft 3/s
V = Q/A = 0.222/(3.14 x 150) = 0.000471 ft/s (water moves very slowly through soils) 3=
K x 0.0004712 2 • 32.2
K = 8.7 x 108 (a huge number compared to loss coefficients for pipe fittings given in Table 6.4) (D) If the pump (just before it fails due to NPSH limitations, see Section 7.3) can create a maximum partial vacuum in the well equivalent to an extra 10 ft of head (which is a lot), what is the maximum possible flow rate Q'? You can assume that the K in part C is constant (see Section 6.5). 8.7 x 108 x V '2 13=
V' = 0.00098 fl/s
2 x 32.2
Q,
Q
!
0.00098 ft/s = - - = A 471
Q ' = 0.45 ft3/s (208 gpm)
66 hydrostatic head available for filtering. Even under the best of conditions and using the same 'rule of thumb' provided for the crib, the size of the required leaching field may be impressive. The laterals must be sufficiently separated, at least 10 ft (3 m), preferably more. There is always the possibility of storm waves carving a terrace fight through the system, burying it much deeper than intended or exposing parts, thereby short-circuiting the filtering action of the bottom. If the design is overly optimistic or the flow requirements increase, the pumps will pull an increased partial vacuum in the well system. There is a high probability that this deterioration in pump suction side conditions will lead to major pump problems (see Chapter 7). The crib system also shares some of these problems. The leaching field approach will often prove to be both expensive and risky. The crib system is usually easier and cheaper to install and is somewhat less vulnerable to storm damage. If working properly and conditions are good, neither should have any fouling problems nor loss in performance with time. If shortcircuiting and resulting biofouling occur, or fines clog the intake, backflushing may be helpful. The third approach uses more conventional well technologies (Driscoll, 1986) with the hope of finding seawater which has percolated in from the nearby ocean (see Fig. 4.2C). These well points may be shallow, in the order of 15 ft (5 m) or much deeper. If shallow, they can be 'jetted' in and the oversized hole packed with gravel or very course sand. What one may find instead of seawater is freshwater, even at the shoreline, or fossilized seawater. Coastal areas, even sand barrier beaches, often have sub-layers of impermeable clay or organic materials, such as peat or old mangrove swamps. Even if seawater is found, it may be of poor quality. It may be low in dissolved oxygen, even anaerobic, and may contain dissolved hydrogen sulfide. With the exception of iron and manganese, the concentrations of heavy metals from seawater wells is generally lower than that in the open ocean (Scholes, 1980; Chen et al., 1985). In addition, the salinity and pH of well water also are usually lower than the sea. On aeration of anaerobic waters, iron and manganese may precipitate out. These solids can clog the gills of larval forms resulting in high mortalities. A quick test is to place a few young specimens of the culture organisms in the well water. If they quickly die, the water quality is clearly inadequate; on the other hand, their survival for more than a few hours would mean that the water should be thoroughly analyzed for more subtle factors. When the seawater quality is marginal, treatment may increase water quality. However, this negates most of the advantage of such systems relative to intakes and increases complexity, cost and risk. Even though the quality is good, it may be difficult to get the needed quantity, requiting a number of separated wells, increasing costs. If the conditions are favorable and the system is designed and constructed properly, a well point system will have the highest reliability and the most consistent water quality (see Example 4.3). 4.4 Marine intakes
Submerged marine intakes are the most common type of intake. There are a number of important interactions between the location of the intakes and the rest of the system. These are discussed in Section 5.3. This section will concentrate on the engineering considerations dealing with the intakes themselves. Marine intakes by necessity must protrude above the bottom. They are, therefore, subject to currents and wave forces. Wave effects associated with rare storms are usually the biggest threat to intakes, pump stations and other shore-side facilities. It is important to quantify
67
Example 4.3. Seawater well A four inch seawater well is to be placed on a sand spit. The static level of the seawater in the ground is at about mean sea level (MSL) and there is no freshwater present. The sand layer is believed to extend at least 50 m below M S L . A m a x i m u m draw down of 5 m, a radius of influence of 25 m and a tested soil hydraulic conductivity of 10 m / d a y (see Example 4.1) are believed to be reasonable for this site. Based on the equation given below for an unconfined aquifer, what is the m a x i m u m sustainable flow rate from this seawater well? Q = l ' 3 6 6 K ( H 2 - h2) (flow in m3/day)
K R r H h
= hydraulic conductivity (m 3 / m 2 per day) = 10 m / d a y -- radius of cone of depression = 25 m - radius of well - 2 inches - 0.0508 m --- static head from base of aquifer to static water level -- 50 m = static head from base of aquifer to depth of water in well = H - draw down = 50 - 5 = 45 m
Q =
1.366K(10)(502 - 452) log {
= 2410 m 3 / d a y = 0.279 m 3 / s
25 0.0508 }
Q = 0.0279 m 3/s = 0.985 ft 3/s = 443 gpm
these waves. Wave characteristics can be predicted from meteorological conditions such as wind speed, wind duration, direction, distance over which the wind blows, and depth of water relative to the waves (Pierson et al., 1955; U.S. Army Corps of Engineers, 1975). These methods are based on empirical data. Since our interests are centered on rare extreme storms, the accuracy of these methods is low. This is due to the rarity of data-taking opportunities and problems of taking accurate data under extreme conditions. Based on a cost-risk evaluation, a 'design' storm is chosen. For large permanent systems, this may be the 25, 50 or 100 year storm (the worst storm that on the average occurs with this frequency). For short-term systems it could be a lot lower, say the 5 to 20 year storm depending on the desired system lifetime and season of use. Since the winds create a whole series of different waves, the predictions are for the 'significant wave height'. The significant wave height is the average of the highest one-third of the waves present. Although it is not a normal distribution, the wave height is inversely proportional to the probability of occurrence. At very low probabilities, a very impressive wave might be seen during a not very rare seasonal storm. For design purposes a maximum wave height of twice the significant wave height of the design storm is conservative. There are no unique values for wave length or period for a given wave height. However, most probable values can be used. The characterization of the worst or 'design' wave includes the wave height and the most probable associated wave length and period. Wave forces at any depth can now be calculated on different shapes (U.S. Army Corps of Engineers, 1975). Wave forces decay hyperbolically with depth, at a depth of one-half wave length wave forces can be neglected. As waves approach shore, their heights will increase and their wave lengths decrease. Given the near-shore bottom topography, wave characteristics and possible changes
68
4
Potential Surf Zone
~f'Seaward Extremity of Potential [ Surf Zone under Design Low Tide ..[ and Storm Conditions Design Wave
/
[ / ~ e a .c h
Slope
"X~. ,%,.
"~~1~
" ~
~-2-~----..-.. Low Tide Wave Allowance 4 ~ v~
All~
,__.J L._, Ingestion Allowance I Intake I / , / /IStucJ u/re,,/ / / //~ ~, , / / / / , , , / / , / / ,-
Fig. 4.3. Intake elevations and placement. Near-shore topography, wave characteristics and required vertical
dimensions will dictate intake placement and lengths of intake lines. Efforts to reduce required intake water depths and resulting intake pipe lengths with their high associated construction costs may introduce substantial risks and losses in operational flexibility.
in direction due to refraction can be calculated as a function of position. At some point, the wave will break. Using the maximum and minimum tidal elevations that could be associated with the selected design storm and the offshore beach slope, it is important to determine the maximum and minimum distance from the shore that the storm waves will break (U.S. Army Corps of Engineers, 1975). The reason is that wave forces due to breaking waves can have peak values as much as 100 times higher than the already impressive numbers from a non-breaking wave with the same parameters. Intakes or other structures caught in the surf zone of a major storm have a high probability of being destroyed. Waves can be destructive even above the highest water elevation due to wave runup (U.S. Army Corps of Engineers, 1975). Storm waves and storm flooding are a serious threat to many coastal facilities. The intakes should be sufficiently seaward and deep enough to be outside of the area of breaking storm waves at an extreme low tide (see Fig. 4.3). There is also a minimum water depth which is required above vertically oriented open pipe intakes to prevent them from drawing air. At high levels of air ingestion the pumps may cavitate and self destruct. At lower levels of air entrainment, high levels of gas supersaturation may be produced (see Section 12.4), with possible resulting mass mortalities. The minimum water cover is a function of the average velocity in the intake pipe (see Fig. 4.4). This submergence is also helpful in avoiding collisions with small craft, which can be a serious hazard to intakes too close to the surface at low tide. The intake lines should be sufficiently buried to avoid exposure by storm induced beach profile changes (see Section 5.3). The intakes may be screened or unscreened. In areas with floating and drifting debris of natural or man-made origin, or ample marine life, there is no choice. In the vast majority of cases, the intakes have to be screened to prevent the possible ingestion and resulting damage from these causes. If there is little or no seaweed, debris or fish, even associated with storms, then the risk of not using a screen may be acceptable. The advantages of unscreened intakes are greater simplicity, reduced cost and lower suction-side frictional losses (see Chapter 6). Intake screens can be a complex subject whose basic characteristics and tradeoffs are often not understood even by longtime users (Huguenin and Huguenin, 1984).
69 (m/s) |( ~ r
I i
10
2 i
3 I
,3
i,.
E =~E E ._c .
5-
E
_
0
0 5 t0 Average Fluid Velocity in Pipe
(ft/s)
Fig. 4.4. Minimum intake submergence to preclude air ingestion. Vortexing of a submerged intake is a very complicated subject without any easy answers. It can sometimes occur even with considerable submergence, especially at high pipe flow velocities. The presented guidelines are believed to be conservative under most conditions and assume a vertically oriented straight pipe intake (the worst case situation). A velocity cap or horizontal plate above the intake may dramatically reduce the minimum required submergence.
There are a number of important points with screens. The mesh openings should be as large as possible and still protect the pumps. The pump manufacturer will usually state the size of the largest solid object that a given pump can accommodate without damage. Some pumps, such as 'trash' pumps, allow ready access to the pump impeller without disconnecting the piping, resulting in easy removal of any pump-clogging debris. Small screen holes foul much faster than big ones. The usual range would be with hole dimensions of about 0.5 to 1.5 in. (13-38 cm), preferably on the large side. Fish with lengths as much as five times the hole size can get through, sometimes unhurt although most will get hashed by the pumps. The screen area should be as large as possible so that the through screen velocity (flow rate/screen open area) is very low, in the order of 0.1 ft/s (3 cm/s) (see Example 4.4). A low velocity reduces both the capture effect of the screen on nearby drifting marine life and debris and the frictional head loss across the screen. A properly designed screen in clean condition will have a negligible head loss. However, blockage from drifting objects and biofouling can dramatically increase the head loss, if the screens are not periodically cleaned and maintained. A vacuum gage on the suction side of the pump should be used to monitor the fouling of the screens and intake line. Intake screens have to be regularly inspected and kept clean of any significant biofouling. Rapid biofouling of screens can be a serious problem (Huguenin and Ansuini, 1981) and a maintenance nightmare. With proper care, biofouling-resistant copper-nickel metal screens have been successfully used for marine laboratory seawater intakes. The screens, if made from thin mesh, should be internally reinforced to structurally withstand the pressure differential caused by even modest fouling. At high fouling levels, the pressure differential could approach one atmosphere, at which point pump system problems are certain. Most screens will collapse under these conditions, sometimes restoring flow but without providing any protection for the pumps. Another consideration is that the screen should be shaped to best shed drifting debris. A spherical shape is best but impractical. An uptight cylinder or cone is the next best. The intakes should not be placed where debris is likely to accumulate due to local flow patterns.
70
Example 4.4. Intake screen You have to design an intake screen to flow a maximum of 100 gpm. (A) You wish to keep the average through screen velocity to under 0.1 ft/s, what is the minimum open area on the screen? 100 gpm = 0.222 ft3/s Flow = area x velocity = 0.222 = area x 0.1 Area -- 2.22
ft 2
(this is the minimum required open area of the screen)
(B) You are considering an intake made of large-diameter plastic or fiberglass pipe drilled with 1-inch holes. What is the minimum required number of 1-inch holes? Area of 1 inch in diameter hole = 0.00545
ft 2
Number of holes = required area/area per hole = 2.22/0.00545 = 408 holes (C) 1-inch holes on 2-inch centers with staggered rows gives an effective open area of only 20% of the screen area (prove this to be true). If you use only the sides of the cylinder and a 1-ft-diameter pipe, what is the minimum height of the intake screen? Screen face area = 2.22/0.2 = 11.1 ft 2
=
7r(d)(height) = (3.14)(L)(height)
Minimum screen height = 3.5 ft (D) If you use strong expanded metal formed into a 1-ft-diameter cylinder with an effective open area of 60% instead of the pipe above, what is the minimum height of the screen? Screen face area = 2.22/0.6 = 3.7 ft 2 = 7r(d)(height) = (3.14)(L)(height) Minimum screen height -- 1.2 ft
A v e l o c i t y cap is a h o r i z o n t a l plate a b o v e an intake pipe w h i c h c h a n g e s the intake flow f r o m the vertical d i r e c t i o n in the pipe to h o r i z o n t a l l y at the point of intake. T h e cap s h o u l d be p o s i t i o n e d a b o u t a h a l f d i a m e t e r or m o r e a b o v e the intake pipe and should have a s o m e w h a t larger d i a m e t e r than the pipe, in the order of one and a h a l f diameters. Studies have s h o w n that s m a l l s w i m m i n g m a r i n e o r g a n i s m s are m o r e resistant to capture by the intake if flow is h o r i z o n t a l rather than vertical. A cap can also greatly r e d u c e the r e q u i r e d intake s u b m e r g e n c e d e p t h s h o w n in Fig. 4.4, e s p e c i a l l y at the h i g h e r fluid velocities, but adds surface area for b i o f o u l i n g and for s t o r m w a v e s to act upon. A v e l o c i t y cap can be used alone or built into a screen structure as a solid h o r i z o n t a l top surface (see Fig. 4.5). T h e screen top m a y be h i n g e d to allow access to the interior w i t h o u t h a v i n g to r e m o v e the entire screen. This can be helpful in r e c o v e r i n g pipe c l e a n i n g 'pigs' f r o m inside the intake. M a n y different types of intake structures have b e e n u s e d but m o s t have b e e n s o m e variation of the t w o a p p r o a c h e s s h o w n in Fig. 4.5. B o t h have flanges, a l l o w i n g t h e m to be d i s c o n n e c t e d f r o m the intake piping, facilitating initial a s s e m b l y and s u b s e q u e n t r e p l a c e m e n t in the e v e n t of d a m a g e by storms or fishing gear. A p p r o a c h 'A' uses a s m a l l light block, w h i c h is buried, and it can be u s e d w i t h or w i t h o u t a screen. A p p r o a c h ' B ' uses a m o r e m a s s i v e block, w h i c h is o n l y partially buried. It is u s u a l l y u s e d with a screen w h i c h is attached directly to the
71 Approach A Small Light Block
Approach B Large Heavy Block Upward Extension of Pilings Can Protect Intake from Fishing char r
Fig. 4.5. Two conceptual approaches to intake design. Both approaches should use as dense a concrete as possible. Approach 'A' can be used with or without a screen. If the block in approach 'B' is heavy and dense enough and the area is not subject to strong currents or major storm effects, it may not need further anchoring. block. A variation of Type B is to have a truncated pyramid with a solid top and mesh on its four sides. The advantage is that the shape more readily sheds fishing gear and boat anchors. The disadvantage is that the base is larger and heavier, increasing handling and placement problems. All types are often secured to the bottom with pilings, screw augers or other means, especially if placed on the surface. The pilings can be steel pipes or concrete-filled plastic pipes with or without steel reinforcement bars. Any appreciable penetration of the intake structure into the bottom will substantially reduce the need for piling to resist lateral forces. In benign areas with no appreciable currents or waves, even during storms, mass of concrete alone may suffice for anchorage. On a flat surface, a general guide for clump mass anchors is that they can withstand a horizontal force of about one-half their in-water weight (Myers et al., 1968). There is an advantage in using denser concrete. Obviously the dimensions and specifications of these systems will depend on individual conditions and site characteristics. Another variation is building the intakes on a sled (Bouck, 1981), which can be towed into position and secured. Storm forces being what they are, even a good intake design may not compensate for a poor intake location. Another type of submerged intake sometimes encountered in sheltered areas involves an above-bottom variation of the crib shown as A in Fig. 4.2. In this case, no fine filtering with its attendant advantages occurs. However, the gravel can provide coarse filtering, acting in the manner of a screen. However, with this type it is harder to remove biofouling than from a screen, since much of it will be on the gravel internally. Frictional head losses through the gravel are likely to be much higher, especially once biofouled. It is usually much bigger and heavier, but possibly cheaper if made of available materials, than comparable screen intakes of the types shown in Fig. 4.5. Since larger, it will also experience greater current and wave forces and will have to be better secured to prevent movement. Due to its mass, it may provide considerably greater protection to the intake from boat and fishing gear collisions.
This Page Intentionally Left Blank
73
Chapter 5
System Planviews and Elevations
5.1 Generic system Assuming that the general site and desired system flow rate are known, it is necessary to consider piping layout options and considerations. The physical dimensions, soil types, elevations, and orientations of the site should be precisely determined. It is particularly important that the elevations of the seawater source be precisely known under all possible conditions relative to the site. This includes normal as well as unusually high and low tides associated with extremely rare storms (see Section 3.1). Errors of only a few feet in elevation could have serious consequences to the operation of the seawater system. From a strictly seawater system perspective, the best location is to have all the components close together and as close to the seawater source as topographical constraints and storm-tide elevations and wave run-up will allow. Unfortunately, the specific locations are often fixed by previous facilities, practical considerations not directly related to the seawater system and site-specific factors. In short, economic, political and regulatory factors, rather than technical considerations, often dominate. A simplified diagram of a generic seawater system is given in Fig. 5.1. It does not show any redundancy, parallel operations, or cross-connections that may be required. The water processing equipment may be at positions noted as #1, #2, both, or neither locations. If all the water to be used in the system is to receive the same level of treatment, the water processing equipment will usually be at location #1. The advantage is that the pumping equipment can then be selected to compensate for whatever frictional head losses are created by the equipment. If only part of the water needs treatment or some part of it needs more extensive treatment than the rest, treatment equipment is likely to be located at position #2 Short Circuit to Drain
Intake(s)
Pump(s)
Pro_ cessmg 1
Tank(s)
g2
Unit A Culture Unit B
Water Culture Pro- .. F - - ~
cessing~'l [-~ NaterUni Ct :~econ:litioning
Fig. 5.1. Generic seawater system.
74
.,__Two Screened 8" Intake Pipes
I Pumps J
[~-----8" Pipe
[Sand Filterl i -,----8" Pipe I Head Box] Seawater in ~ ~ PipeTransition 8"~6"
Max Flow Rate 872~pm (23tgpm) ~
4" Pipe Maturation 198,000~pd Max 1384~pm
~-.--6"Pipe
1.5" Pipe Spawning 6,020 J~pd Max 50.2~pm
Hatching 2,400 ~pd Max 20~pm
Artemia 6,0 O0 ,Ppd 1 l ,, Max 502pm 1.5"Pipe ~ 2 Pipe
Larval 240,0002pd Max 480J)pm
Intensive Mass Algal Algal 3,0002pd tO,O00Ppd Max 50 ~pm Max8:3.3 2 pm
Fig. 5.2. Crustacean hatchery seawater distribution example (Colt and Huguenin, 1992).
and only on those parallel lines where specifically required. For multiple small-scale culture units with diverse requirements, such as is typical in a marine laboratory context, location #2 is often at the point of use immediately adjacent to the culture unit. The nature of the processing equipment at any position #2 will vary according to the requirements of that culture unit. While location at position #2 allows use of smaller-sized equipment, it can also have undesirable effects on flow stability and regulation downstream. This problem is most serious if any of the treatment units has significant head losses (see Sections 5.3 and 9.1). To correctly place, size, and specify the processing equipment, it is very important to know the specific seawater quality and quantity requirements for the different parts of the system. All the uses may have the same requirements or they may be quite different. Furthermore, experience has shown that off-line operational needs, such as tank filling, rinsing and cleaning, can increase the maximum flow requirement by as much as 20%. It is necessary to determine both the total daily water requirement as well as the maximum flow rate. Fig. 5.2, as an example, shows the internal water distribution in a crustacean hatchery and placement of processing equipment (Colt and Huguenin, 1992). In this case, algal culture needed finer filtration than the other functions. A diatomaceous earth filter with an integral booster pump was used. Determining the scale for each activity is not as straightforward as it may appear, if the activities are interrelated. There are inherent assumptions both present and future as
75 to loading, growth, survival ratios, and efficiency in each activity and these are linked to the activity-specific water treatment specifications. Under-capacity in one part of the system can limit the entire system, while over-capacity is irrelevant and expensive (unless it can be translated into extra production at some intermediate stage, such as post-larvae, that can be sold to other culturists). 5.2 Elevations and head tanks
Head tanks, head boxes, or head ponds are needed to minimize the pumping steps in the system by providing the elevation required for gravity flow. The importance of water elevations in head tanks has already been stated. Even so, it can get complicated. Fig. 5.3 shows the elevations and relationships of components in one marine research facility. In this case, elevations were very precisely determined because there are many steps in the gravity flow sequence for different process waters. A head tank is a common component of most seawater systems and can serve a number of other functions (Table 5.1). It provides a buffer to smooth-out transients between supply and demand caused by irregularities in demand and variations in supply. Irregular demand can be created by batch processes and the cleaning or refilling of tanks. Variations in supply can be caused by tidal effects, switching of pumps or changes in water processing. By necessity the average supply must be greater than the demand. If the supply is adequate to maintain the head box elevation at overflow and the piping between the head box to points of usage are large enough to have negligible frictional losses, the head box can be used to provide a constant flow rate at any desired point. This control through the use of fixed orifices (see Section 9.1) or valves can be quite accurate and stable, sometimes eliminating the need for expensive flow control devices. A major function of head boxes is to provide water for a period of time after a seawater supply failure. This provides time to troubleshoot the failure
NUTRIENT _.~OOVERFLOW RETURN HEAD BOX
! SEAWATER .
. P MP
.
-- v ; ~9. ~ _ . ~ , z ~
.
r STANDI
~TOeAG~
/
~JP,PE I' t
~
;~;E ~
~No
I..~ ~,-,~,.,~
~
IND
R
STAND | PUMPS"'" ,~.M - - , , . P - , . ~ v , , , . ~ v n l PIPES t ~
i
.
.....
9
I
S E . w A , ER SuPPL,r
HEADCHANNEL
INTAKES
Fig. 5.3. Woods Hole Oceanographic Institution's Environmental Systems Laboratory seawater system elevations. From the head boxes the fluids flow by gravity from one level to another. The nutrient distribution system shown is actually a double system, since two different fluids can be fed simultaneously to the ponds and raceways from the three large storage tanks. The seawater inflow is about 600 gpm with one pump on (Huguenin, 1974).
76 TABLE 5.1 Headtank functions 9Buffer transients between supply and demand 9Storage for aging, treatment or part time pumping 9Provide time after supply failure 9Flow control (constant head) 9Sedimentation tank 9Distribution system (large header)
and/or make repairs without loss of seawater supply to the culture units. The time available is a function of system design and can vary from minutes to days. The time period available for critical uses can usually be increased by stopping or cutting back the flow to lower priority uses. The size of the head box is usually determined by the time estimated to be needed after failures under the range of likely present and future operating conditions. Cost is also a factor, but in the range of flows being considered the head boxes are still usually rather modest in dimensions, and the costs are a small part of the total system cost and can be further reduced by clever design. Another service performed by head boxes is to act as sedimentation chambers (see Section 10.8). This may be especially valuable during storms, due to the large increases in suspended materials often encountered in shallow coastal areas. Performance in this capacity is dependent on several factors, including the size and specific gravity of the suspended particles. Generally, residence times in the head box of greater than 1 h can significantly reduce turbidity. During severe storms that produce high concentrations of suspended solids, even a few minutes of settling may produce dramatic improvements. On the other hand, suspended colloidal-clay particles may never have sufficient time to settle. Provisions must be made to periodically remove the settled solids. The quantity and weight involved, even for just one short storm, can be impressive. It is worth remembering that sediments not removed in the head box may be deposited in other places downstream within the system wherever the flow velocities are reduced. Head boxes can affect water parameters other than turbidity. They often develop surprising amounts of resident life forms, which can affect water properties. Surprisingly, while water temperatures may be much different from ambient temperatures around the head box, water temperatures do not change significantly from that in the incoming water. This is due to the high specific heat capacity of the water, the limited residence time in the head box, and the insulative properties of the tank materials. However, indoor head boxes can under humid conditions condense impressive amounts of freshwater out of the air on their cold sides. Over-head seawater pipes can also 'sweat'. This sweating can be reduced by insulation. Electrical and corrosion hazards from the condensed water can be reduced by careful layout.
5.3 Intake and pump house considerations While water quality should have been one of the primary considerations in site selection, there are often intake location options on a given site which will maximize the available water quality. The intake should be placed far away and upstream from any possible source of
77 water contamination. In addition to other discharges, these include inlets, streams or brooks, boat anchorages, industrial or municipal outfalls and storm drainage. Some of these possible contaminant sources are readily visible, others may be less obvious. Sources of drainage may only be obvious at certain seasons or after major storms. Sewage or industrial outfalls may not be shown on hydrographic charts, even when completely legal. Illegal or quasi-legal outfalls can sometimes be spotted, especially from the air, at extremely low tides or by chemical, physical, or biological differences at slack water, when the discharge water may rise to the surface. For examples, seagulls may congregate over sewage outfalls to feed or the water color may be significantly different. Another way of maximizing water quality is to draw water from an area where its properties show the highest consistency and least variation. This usually means as deep and as far from shore as possible. This will also minimize wave damage and possible boat damage to the intake structure but increases the installation cost and probability of damage from fishing gear. If they exist, there is a strong temptation to use sheltered shallow water areas for the intake. This minimizes the probability of storm damage, simplifies the system and decreases costs. On the other hand, shallow areas often have reduced circulation, higher probability of pollution incidents from other users or boat traffic, variable water temperatures and may be subject to rapid salinity drops after heavy rains. The probability of short-circuiting the drain may also be increased. The best intake location is strongly a function of the specific site conditions, evaluation of risks, and the available budget. Tidal water elevations and their variations are very critical to the design of the seawater system. There are three tide components: astronomical, barometric and wind (storm) tide. The astronomical tide, being due to the rotation of the earth, moon and sun, is the only completely predictable component. The barometric component may exhibit seasonal patterns, which can be factored in on a probabilistic basis. On top of these values it is important to get an appreciation of the extreme highs and lows that could be associated with really rare events, such as the 25, 50 or 100 year storms (see Section 3.1). Once the water elevations can be quantified in a probabilistic manner, there are a series of decisions to be made relative to the intake system and pumps. If the range of elevation values from maximum high to minimum low are not great, there is no problem with continuous operations throughout the tidal range. If the tidal range is high, say greater than 18 ft or 6 m, then continuous operations may add cost to the system. Pump selection and elevation becomes critical (see Chapter 7) and pumps can provide serious design problems at both high and low tides. At this point it is worth considering pumping only during some portion of the tide, usually the high side. For a given daily flow, this will require bigger and more expensive pumps and place some restrictions on the operators, but will reduce the excavation depth of the pump house with possible major cost savings. Obviously, this is an area of complex tradeoffs. Placing the pumps on a floating platform will solve the high tidal range pump problem but may create a number of others. The criterion with respect to major storms is usually one of survival and damage minimization. One of the important decisions is defining the storm which the system is designed to survive. This is a cost and risk assessment tradeoff. The more extreme the storm, the higher the system's cost. Luck also helps, as one could get hit with the 100 year storm the very first year. Assuming survival, some parts of the seawater system, but not necessarily the intakes and main pumps, may have to remain functional during the storm, such as for providing life
78 '" Ill Note-C ~.~~EL:+t7
EL =
550' (Pipe is Longer) 500' (Pipe is Longer)
Note-A ~-- ' - ' ~ . . ~ . [-,-,EL=+6' A ~ . ~ ~ ~,. Prefabricated ~ Fiberglass Pumping Station
L~
"J,,~
EL=+IO' High-High W a t e r ~ All ~
Elevations Relative to
Mean Low Water
A "~
Two Polyethylene Pipes L a y e d - ~ . _ _ ~ Side by'Side in s Trench ~ , ~
~
EL:-2L-~Low-Low Water,-----. e - D
, Two Intakes w'th Screens
-"
EL=-6'
Fig. 5.4. Seawater intake profile example. There are several potential trouble points in a marine intake. This example is typical of benign or sheltered sites, where the probabilities of large waves and major storms effects are small but not zero. Note A. Pipes in this area near the pump station should be straight and especially well supported from below before backfilling. All turns should be as gradual as possible to prevent flat spots and stress concentrations on the pipe, which might lead to pipe collapse under partial vacuum. Note B. The points where the beach changes its slope is where the cover over the pipes is likely to be at a minimum. One such point generally occurs at or just below the normal low-tide elevation. Care must be taken during installation to assure the minimum cover over the pipes at this vulnerable point. Note C. If direct wave action is a possibility during rare storms, some measures to protect the investment in the pump house are likely. The hatch on the access trunk would be elevated above grade and water tight. The area around the hatch should have a sloped concrete collar to protect the access trunk. The base of the collar should extend below grade to counter scouring. To prevent the lateral movement of the collar and subsequent shearing of the access trunk, there should be at least four steel pilings or concrete-filled plastic pipe, say about 6 ft or 2 m long, acting as pilings and rigidly cast into the collar. The pump house should have a strong steel snorkel of adequate height to provide ventilation, an automatic sump pump and an external means of turning off power. Note D. The cast concrete intake structures should be buried with the tops as close to grade as possible. The actual opening of the intake pipe should be at least 1 ft above grade to prevent the ingestion of bottom-drifting sediments and debris. The intake structures must be well secured to the bottom with small pilings or screw-auger anchors, as forces from breaking waves under some conditions could be substantial. Under extreme conditions, destruction of screens and riser pipe are highly probable and spares should be available for replacement.
suppo rt to b r o o d stocks or critical life-stages. H o w e v e r , for the duration of the storm, w a t e r quality r e q u i r e m e n t s can u s u a l l y be greatly r e d u c e d with little or no l o n g - t e r m c o n s e q u e n c e s . E m e r g e n c y survival r e q u i r e m e n t s h a v e to be c o n s i d e r e d during the d e s i g n phase. B e a c h profiles are also i m p o r t a n t in s y s t e m s layout. D a t a on b e a c h profiles s h o u l d be a c q u i r e d at least four t i m e s over a year, to get s e a s o n a l variations (see Fig. 3.3). A l o n g e r p e r i o d of o b s e r v a t i o n o v e r several years w o u l d be e v e n better. T h e profile s h o u l d be e x t e n d e d to at least 10 ft or 3 m b e l o w the e x t r e m e l o w - t i d e elevation, p r e f e r a b l y deeper. B e a c h profile i n f o r m a t i o n is n e c e s s a r y for s p e c i f y i n g burial depths for p i p i n g and p o s i t i o n i n g intakes. T h e intake lines h a v e o t h e r potential h o u s e s h o u l d be as c l o s e to the m i n i m i z e suction side frictional e x c a v a t i o n r e q u i r e d to install the
p r o b l e m s , s o m e of w h i c h are s h o w n in Fig. 5.4. T h e p u m p intakes as tides, w a v e s and flood z o n e r e g u l a t i o n s allow to losses (see C h a p t e r 6). This also g e n e r a l l y m i n i m i z e s the p u m p h o u s e (see C h a p t e r 7). H o w e v e r , p u m p h o u s e s p l a c e d
very c l o s e to the w a t e r m a y i n c r e a s e the p r o b a b i l i t y of s t o r m d a m a g e and p l a c e m e n t on the flood plain (flood z o n e defined by 100 y e a r flood in coast al z o n e regulations) can t ri gger a n u m b e r of r e g u l a t o r y issues. T h e worst case is an e x p o s e d shore w h e r e a piping t r e n c h c a n n o t be e x c a v a t e d or r e a d i l y blasted. Blasting, in any case, is very u n p o p u l a r with r e g u l a t o r y
79 agencies and in some area may not be allowed. Fortunately for rocky shores, it is often possible to find at least a partially sheltered intake location for protecting and securing the pipes and intakes. For unfavorable site conditions, even the best engineering and construction may result in a system with very high risk of damage to intakes and piping during storms. The best answer may be to have backup equipment that can be quickly deployed after damage from a major storm. A major new and rapidly expanding problem with intakes, pump houses and discharges concerns regulatory restrictions. Coastal Zone Management regulations in the USA make it very difficult to place pump houses in the 'flood plain'; in short, on a spot with a ground elevation below the 100 year storm surge for that area. This results in longer suction side pipe lengths and deeper pump house placements, with increased costs. The deeply buried pump houses activate 'confined space access' regulations, which make routine access legally very difficult without elaborate and costly procedures. The increased costs and operational restrictions often result in serious compromises in the design with subsequent increased risks of system failure. Another area of regulatory restriction is on when and how to construct and emplace intakes and discharges. In some areas, in-water construction is only permitted in the winter, due to possible construction impacts on 'migration' and 'spawning' of fish and birds. Winter weather can make actual construction very difficult or even impossible. The costs in winter are very much higher and also impossible to predict with any confidence ahead of time due to uncertainties about the weather. In addition, use of sediment fences along in-water pipe excavations may be required, even in winter. In-place casting of concrete may not be allowed due to restrictions permitting only fully cured concrete. Due to serious weather and time constraints, the resulting construction may have important and serious deviations from specifications and design criteria. It is imperative for the user to inspect all in-water construction as soon as possible after emplacement and certainly before any legal 'sign-off'. If serious deviations are found, what can realistically be done to 'fix' the situation may be very limited. All this impacts available design options and may result in serious compromises with good design practices. Regulatory construction restrictions are highly site-specific. Construction in some areas is getting much harder to accomplish. It may not be long before otherwise excellent sites are, for practical purposes, no longer useable due to regulatory restrictions. It is unfortunate that many of the restrictions appear illogical and based more on ideology than scientific facts.
5.4 Discharge considerations The discharge is an often neglected aspect of seawater systems. Since metabolic wastes, chemicals, disease organisms, and decay products will be in the drain water, it is essential that the drain be situated so as to prevent discharge water from reentering the intake (short-circuiting). This is easy to state but sometimes hard to do. There are several basic possibilities, which will be determined by the site configuration and conditions (see Fig. 5.5). The best situation is to have intakes on one side of a sand spit and the discharge on the other with no nearby inlet (Fig. 5.5A). If the site has a consistent alongshore current, the discharge can be placed downstream and as far from the intakes as practical (Fig. 5.5B). An alongshore current under some conditions can switch direction and develop countercurrents or large
80
// Barrier 1 Beach or / Peninsula
A
/
1--I Inflow----I~
I I Discharge j v
/
Intake
/
Facility
/
/S~horeli ne f
... Disc arge
B Inflow Intake ~Consistant jl AlongshoreI
Facility
JJCurrent
C
i;orelin e
Inflow L "J Inshore"~. Intake I Discharge 0
r---] l !
Facility
Alternating ~"j Alongshore r~. Currents I.,/ Fig. 5.5. Discharge configurations.
eddies, resulting in short-circuiting of water flow. If the alongshore currents are unpredictable or switch direction with the tide, which is a common situation, the intake can be taken from as far out as practical while the discharge is dropped along the same line but as close to shore as possible (Fig. 5.5C). The discharge could even be on the beach or buried in a rock jetty. Factors of aesthetics and environmental regulations must also be considered. Short-circuiting under various environmental conditions can be checked with fluorescent dyes. Another major consideration is that drain lines should be greatly oversized. They cannot be sized for the normal flow but must be capable of handling much larger transients. These high transient flows can occur when someone pulls the plug on a tank for cleaning, recycling or repair. If the drains cannot handle the flow, it will be quickly apparent. The consequences run from annoying to serious, depending on the circumstances. Subsequent tank draining procedures will then require much more time and attention to avoid overflow. In addition, drains have a great tendency to fill with sediments, wastes, debris, and assorted garbage, further reducing flow capacity. Any sumps or other chambers in the drain system will be particularly susceptible to clogging or fouling due to biological growth. Most gravity-flow drain lines do not have sufficient flow velocity to be self-cleaning. Drains must be frequently inspected and maintained and can require considerable time and effort. The extra costs of oversizing drain lines can usually be easily recovered in reduced problems and reduced servicing.
81
Chapter 6
Piping Design and Calculations
6.1 Major tradeoffs There are basically two ways to transport water: open channels and pipes. Open channels require strict control of elevations over their entire length but usually have very low frictional losses compared to pipes. At flow rates one or more orders of magnitude above our area of interest, open channels can be cheaper than pipes if the soil types, required length, and topography are favorable. For our flow rates and typical conditions, channels are clearly not competitive, with one possible exception. This is for drain lines. Even in this case, a prepared channel will most likely be more expensive than pipes. However, the cost difference may be compensated for by the improved access to channels for inspection and cleaning (see Section 6.6). Given a desired flow rate, one apparent choice is between a small pipe with high frictional losses or a large pipe with low frictional losses. Frictional losses in pipes or equipment can be measured in pressure units or elevation (head) units. These are interchangeable by knowing the specific weight of the fluid, i.e., seawater (see Table A-3). Considerations include the extra cost of large diameter pipe balanced by the increased pumping costs of small pipes due to higher frictional losses resulting in more powerful pumps and increased energy costs. Smaller pipes have higher velocities, which make them more susceptible to water hammer (see Section 6.3). As will be seen, predicting frictional losses is difficult, imprecise, and may vary with time and conditions. Frictional losses are a function of the fluid properties, pipe velocity, pipe material, pipe condition, and pipe diameter. Additional frictional loses also occur wherever piping changes direction, such as at an elbow, a T, or where the pipes change diameter, such as at a reducing fitting or the inlet/outlet of a tank. If flow is controlled by gravity rather than pumps, the acceptable frictional losses for the desired flow rate may be severely constrained by the available elevation differences and the piping system will have to be carefully designed to minimize frictional losses. For preliminary design purposes, pipes can be sized by using a velocity criterion, such as 1.5 m/s (5 ft/s). This is a suitable compromise for many conditions, although gravity flow situations and long pipe runs may require even lower values. In calculating average pipe velocities it should be noted that the nominal pipe sizes are not exactly the same as the inside diameter dimensions. However, for rough calculations the errors will usually be small. An exception is polyethylene pipe on the suction side of a pump, where concern for possible pipe collapse under partial vacuum requires thick walls. As an example, 16 inch nominal polyethylene SDR 11 pipe can have an inside diameter of only 12.9 inches (an error of about 35%). Since pipes come in discrete sizes and not all sizes are readily available or provide the same selection of fittings, the velocity criteria will not necessarily provide a clear decision on the best pipe size.
82 A fact not fully appreciated by many, is that the specifics of the piping, processing system and the pumps are highly interdependent in seawater systems. If you alter the specifications or operating conditions of the piping or other system components, you will alter the operation and performance of the pumps, and vice versa. How a seawater system will perform is dependent not only on the installed equipment but also on the precise operating conditions at any given time, especially in areas with high tidal variations. If the conditions exceed the operating envelope of the system, problems and possible failure will follow. For this reason determining system head losses and pump selection are very complex and receive considerable attention.
6.2 Biofouling control One of the most important factors which can affect the performance of a seawater system over a period of time is due to marine biofouling. Its control is also one of the most troublesome factors associated with the operation and maintenance of seawater systems. The extent of the problems is highly dependent on season and location. Different sites can have high variations in biofouling rates, even over relatively short distances. Biofouling roughens the surfaces of pipes and equipment, thereby increasing frictional losses. If allowed to continue, it will effectively reduce the flow areas, dramatically increasing pipe velocity and frictional losses while reducing flow capacity. Over a period of 12-16 months, a heavy growth of mussels can turn an 8 inch pipe into a 3 inch pipe. High frictional losses can lead not only to greatly reduced system performance but also to total system failures. As a consequence, considerable efforts are often devoted to biofouling control. Biofouling control will be much easier if the various control options are considered during the design phase, rather than only after problems become evident during operations, as is common. If biofouling is allowed to proceed until flow rates are significantly reduced, it may be necessary to turn the system off for a prolonged time period, days, weeks, or even months, to fix the problem. The methods needed to clean heavily fouled lines may seriously damage plastic or fiberglass pipes and fittings, especially in turns. In some cases, it may be quicker and cheaper to completely replace heavily fouled pipes and fittings. Many methods have been used to control biofouling in seawater systems and all have their advantages and disadvantages (Table 6.1). In a complex system, many of them may be used in different parts of the system. It is general practice to have at least two options for every part of the system. There can be major cost and reliability considerations involved with these decisions and they will affect system design. A common approach in many research oriented seawater systems is to have two complete parallel piping systems, suitably cross-connected for maximum flexibility. This allows the water in the non-operating system to go anaerobic, thus killing all the fouling organisms, while the other system is in use. These two systems are alternated in use on a fixed schedule, usually about every 2 weeks. This approach is very reliable and works well but has the increased cost associated with a double system. In the special case of a short term or seasonal system in a low biofouling area, the same control method might be used with a single system. Another approach is to design the system such that the water velocities in the pipes are sufficiently high to preclude biofouling. Minimum water velocities in the range of 8-10 ft/s (about 3 m/s) are required. These values are valid only under conditions which preclude
83 Small flow of high pressure water needed to move pig into the pipe being cleaned
,/
Blind flange or threaded cap
Valve section one size larger ~ --'- Pipe diameter than pipe being cleaned
(3
Expansion fitting Pig Types:
II
I Y Fitting
Fig. 6.1. Pig launcher schematic and pig configurations. photosynthesis, such as opaque pipe, since some attached algae can tolerate much higher velocities. This approach can be reliable, works well and is best suited to long straight pipe runs, especially for supply lines. If operational conditions change and the velocities drop below the minimum, the lines may foul. Intakes, discharge areas, and lateral branches, where velocities could be lower, may be particularly susceptible to biofouling. If the velocities are in the region of 15 ft/s (4 m/s) or above and there are entrained abrasive particles, such as sand, the life time of plastic pipe may be seriously reduced by erosion, especially in turns. In areas of substantial tide, the flow velocities in the main supply lines may vary by 1.5- to 3-fold over a tidal cycle. Therefore, if the minimum flow velocity is high enough to prevent biofouling, excessive velocities may be produced at other times. In addition, high water velocity coupled with long pipe runs increase the hazard from water hammer (see Section 6.3). In spite of its potential problems, this approach is sometimes attractive because it is simple and tends to minimize front end capital expenses. For long pipe runs, it is often combined with a 'pigging' option (see below). Mechanical methods are very reliable and can be practical, if provisions for their use have been anticipated by allowing for easy access. Often these methods are the method of last resort because all others have failed to restore system capacity. Few of the other methods actually remove attached biofouling, sediments or debris. If biofouling is allowed to get out of hand, these other methods may kill the organisms without restoring adequate flow capacity. 'Snakes' are the most effective for short runs while polyurethane or plastic 'pigs' work well on long runs. These 'pigs' are bullet- or spherically shaped, flexible and contractible (see Fig. 6.1). They are of somewhat greater diameter than the inside diameter of the pipe to be cleaned. They are pushed through the piping under pressure, cleaning the pipes by friction as they proceed. They are available for various types of biofouling, from just surface films to major encrustation. They can make 90 degree turns and get through partially closed gate valves. The increased pressure normally does not exceed 100 psi and is often considerably
4~
TABLE 6.1 Biofouling control methods used in seawater systems Method
Advantages
Disadvantages
Comments
Double system, half anaerobic at a time
Very effective Built in flow growth potential No need for prior water processing
Very expensive
Commonly used for main lines at research labs
Desiccation
Cheap Must assure complete drainage
Long downtime Does not remove dead organisms
Effective, especially for larger systems
High flow velocity
Cheap and with care effective Can be backed up by mechanical methods No need for prior water processing
If velocity too low, get biofouling If velocity too high, can get excessive pipe erosion Very susceptible to water hammer No flow rate growth potential Problems at slow spots and laterals High energy losses due to friction
Most likely in large systems with long pipe runs where cost is a major factor
Hot water pulse
Eliminates need for redundant system May exploit existing capabilities
Plastic pipes susceptible to high water temperatures Does not remove dead organisms Downtime during treatment
Most likely in closed temperature-controlled loops
Chlorination
With care, usually effective
Problems with chlorine residuals Possible environmental objections Possibly high operating costs
Mostly limited to discharges and batch processes
Freshwater pulse
Periodic use, usually effective Usually cheap
At large scales, freshwater supply may be limiting Does not remove dead organisms Downtime during treatment
Most applicable at small scales
Mechanical devices
'Snakes' effective for short lengths 'Pigs' effective in long straight lines Removes fouling, restoring flow capacity 'Pigs' have little downtime and are cheap
Need discipline to carry out periodic maintenance before problems arise 'Pigs' need access to both ends
Usually last resort; more effective if considered during design
TABLE 6.1 (continued) Method
Advantages
Disadvantages
Comments
Toxic surfaces
Effective for small surfaces where fouling consequences most severe Environmentally acceptable if used within limitations
Limited applicability Must be conscious of surface area, leaching rate, flow rate relationships to avoid problems
Used in limited high value applications
Filtration
Can greatly reduce fouling problems if equipment needed for other reasons
Can be expensive at high flow rates Failures or short circuits can nullify effectiveness with no warning
For best effect need fine filtration
UV treatment
Can greatly reduce fouling problems if equipment needed for other reasons
Can be expensive at high flow rates Performance must be monitored
Commonly used in lab and hatchery systems
Ozone treatment
Can greatly reduce fouling problems if equipment needed for other reasons
Potential toxicity problems from residue oxidation products, especially with seawater
Very effective in reuse systems
86 less and suitable for plastic pipes. They take only a few minutes of system downtime for use but require piping access for the launch and recovery of the 'pigs'. 'Pigs' can be launched from a pressurized cleanout in the line or a specialized 'pig' launcher. A 'pig' launcher for 'bullet-shaped' pigs can be made with a 'Y' fitting (see Fig. 6.1), with the branch having an expansion fitting connected to a section of pipe one size larger in diameter and with a length about twice that of the pipe to be cleaned. The end of the branch should be closed with a removable cap to allow insertion of the 'pig'. The end of the branch requires a means of adding a launching pressure to the back of the 'pig'. This can be a small auxiliary pump or small diameter line from the discharge side of a pump. 'Pigs' can be recovered from cleanouts, rock traps, wetwells, intake screens or tank water surfaces (most types float). This method is cheap, quick, efficient and reliable but does require the discipline to carry out the scheduled cleaning. The time interval is dependent on conditions, but cleaning intervals of about 2 weeks during the fouling season should be adequate under most conditions. Many of the other biofouling control methods depend on the availability of specialized processing equipment already there for other purposes. These include filters, sterilizers, and heat exchangers. They can all be very effective in reducing biofouling problems. Other options, such as chlorination and biofouling resistant surfaces, may be acceptable and attractive under some conditions. Chlorination is most likely to be used in a flowing system for treating the drain lines. An application of 1-1.5 mg/1 chlorine for 24-48 h every 2-4 weeks, if allowed by environmental regulations, should suffice to keep drain lines free of biofouling. However, this will have no effect on clogging problems. 6.3 Water hammer
Water hammer is a transient pressure phenomenon caused by the rapid stopping of flow. The rapid deceleration causes a pressure wave to propagate upstream, where it is reflected and returned creating an increased pressure pulse at the slowing point. This pressure pulse causes a loud 'bang' to be heard followed by others at regular intervals. The intervals are determined by the sound velocity in seawater (5000 ft/s or 1500 m/s) and twice the upstream distance to the point of reflection. The pressure pulse can have a value several orders of magnitude above the normal operating pressures. Due to the widespread use of relatively low pressure rated synthetic pipe materials in seawater systems, the transient pressures produced by water hammer can easily rupture pipes or blow fittings apart. The magnitude of the pressure pulse depends on the average velocity in the pipe, pipe length and rate of valve closure. High velocities and long pipe runs are the worst combination. Pipe diameter or flow rate, by themselves, are not important. The most common situations producing water hammer are the rapid opening of a line resulting in a high velocity flow proceeding down an open pipe and suddenly encountering an elbow or almost closed valve, or the closing of a valve in a flowing line too rapidly. The quarter turn full-open to full-close ball or butterfly valves commonly found in seawater system are particularly dangerous in this regard, since they can be fully cycled with a flick of the wrist. If these types of valves cannot be avoided, such valves in critical places should be red flagged, have their handles removed, or be locked. Fig. 6.2 is a nomograph for calculating water hammer with an example to demonstrate the use of the figure. In order to further illustrate the danger, if a typical main supply line from a pump house had a velocity of 10 ft/s, a length of 700 ft from the pumps to the valve and
87 D 2,000
4~ t 30
B
- t,000
=~opoo ==5poo
2o
-
_;.00
i
-
=100
- .6 .8
6
---
5-
50
800
600
500 400
Ol o
_o >
-
o
-
(D
-
(1)
-2
r
3 V-
"o 2 -
0
L
--
-5 -6 -
~_ o m
=5
a
1-
-40
., _~
-i-.
-1
J:
_-0.5 _
~
-0.1
>
-50
I
Ol
LL
60
=10
~
200
- 100 - 80
4-
e~
<
-
3OO --.4
8 _---.5j
~
-
~.
- -
10
C
J
,.i,,,0
>
a.
-i.E~ r" a) _J (I) (3. o~ n
-
30
-
20
-10
Fig. 6.2. Water hammer nomograph. The pressure pulse due to water hammer can be found by knowing the average velocity of the fluid in the pipe, the pipe length, and the valve closing time. The fluid velocity is found on the left side of scale (A) and the pipe length on scale (D). These two points should then be connected by a straight line. The intersection of this line with scale (C) is noted and connected with a second line to the appropriate value for the closure time on the right side of scale (A). The pressure pulse in psi is then read from the intersection of the second line with scale (B). The pressure pulse must be added to the existing line pressure to find the m a x i m u m pressure needed for selecting pipe type and wall thickness. The example shown is for a pipe length of 200 ft, fluid velocity of 20 ft/s and a valve closure time of 1 s. The resulting transient pressure pulse has a value of 270 psi.
the same closure time of 1.0 s, it would produce a pressure pulse of about 500 psi above normal pressure. Most synthetic pipes have pressure ratings of only a few 100 psi at best. It may require a closure time in the order of 15-20 s to avoid unacceptable pressures. While this is a potential problem with a straightforward solution, operating personnel may not have the patience to close valves this slowly. Example 6.1 is an example of a real water hammer problem with a real system. Its pipe velocity with one pump operating was just sufficient to preclude biofouling for decades until pump wear significantly reduced the flow velocity. This site has a negligible tide. 6.4 Frictional losses in pipes Frictional losses are usually measured in elevation or head units, which have the dimensions of length (ft or m). Sometimes frictional losses are given in pressure losses between two points.
88
Example 6.1. Main seawater line water hammer The system shown in Table 5.3 was designed with a single 1000 ft long 6 inch (actual ID = 5.72") polyethylene main intake line between the pump house and the facility. This was done to save the cost of a double line and the pipe was sized to provide sufficient flow velocity to prevent biofouling in the line (see Section 6.2). The system was designed to provide 600 gpm with one pump and 1000 gpm with both main pumps on. The pressure rating for normal usage of the main pipe is 100 psi. (A) What is the minimum closure time for a valve at the end of this 1000 ft supply line to not exceed the pressure rating of the pipe due to water hammer for the two design flow rates? Since the transient pressure must be added to the normal operating pressure in the line, there is in fact less than a 100 psi margin. In the context of this example, the normal operating pressure is unknown but is usually half or less of the rating. The rating is usually determined by the shut-off head of the pump (or the other way around). The ratings are also usually conservative, especially for very short duration transients. Realizing that it may be a bit optimistic, we will assume that the full 100 psi is available. 600 gpm = 1.33 fi3/s,
V = Q/A
=
1000 gpm = 2.22 ft3/s,
V = Q/A
= 2.22/0.1785 = 12.4 ft/s
1.33/0.1785 = 7.5 ft/s
Using Fig. 6.2 with L = 1000 ft and pressure increase = 100 psi For V = 7.5 fl/s (600 gpm),
Tminimum :
4.5 S
For V = 12.4 fl/s (1000 gpm),
Tminimum :
7.0 S
(B) What are the minimum closure times if the normal operating pressure at the valve is around 50 psi? Using Fig. 6.2 with the same length and velocities but with an allowable pressure increase of 50 psi instead of 100 psi With 600 gpm, Tminimum-- 8.5 s With 1000 gpm,
Tminimu m :
11.0 s
(C) The main seawater valve at the end of the line in this system is in fact a quarter turn full-open to full-close valve. What is the transient pressure pulse if the valve is inadvertently closed in one second? Using Fig. 6.2, closure time = 1 s, length = 1000 It, and stated velocities At 600 gpm get 500 psi pressure increase At 1000 gpm get 800 psi pressure increase (D) Is there a significant threat of system's damage? Given the existing system, discuss your options. Yes, serious threat Options in increasing order of intensity: 9 Place red streamer on valve with written warning 9 Remove the valve handle 9 Lock valve 9 All of the above
The conversions from one unit to another are from the equation below. h =
Pl - P2
(6.1) V where pl - p2 is pressure drop, lb/fl 2 (Pa), h is head loss, fl (m), and ~, is specific weight of fluid, lb/fl 3 ( N / m 3)
89 The value of specific weight varies slightly with temperature and salinity (Table A-3). At 35 g/kg salinity and 25~ 1 atm (14.7 psi) is equal to 10.10 m or 33.12 ft of head. In the English system of units, it is important to note the units of pressure are expressed as lb/ft 2 rather than the common units of lb/in. 2. There are many factors which effect the frictional head loss in pipes. All of the following discussion relates only to pipes running full. Pipes with a free surface are open channels (see Section 6.6). While the frictional losses will depend on both fluid temperature and salinity, over the range of 1-35 g/kg salinity and 0-40~ temperature, these effects are relatively small due to the small changes in the physical properties of water (see Table A-3). Other major factors affecting frictional head losses are the specific characteristics of the pipe. This includes not only the pipe material but also the existing and future condition of the pipe. The condition of the pipe depends on age, velocity and sand load, formation of scale, deposition of sediment, and biofouling. There are a number of analytical methods for estimating frictional losses in pipes (Davis and Sorensen, 1969). They all use a 'flow coefficient', which has various definitions depending on the method. All these coefficients are estimated for 'average' or 'normal' conditions, primarily based on freshwater applications where biofouling is not generally as severe a problem as in seawater. Therefore, the choice of a coefficient value is at best an educated guess as to the current or projected situation and is time dependent. Usually, one will carry out the calculations for a 'used' pipe with minor biofouling. The head losses for a new pipe will be much lower than for a 'used' pipe, but once in operation a new pipe will very soon become a used pipe. The inherent assumption is that the lines will be maintained and that biofouling will be controlled. If the lines are not maintained, the biofouling will reduce the pipe diameter, increase the pipe velocity and dramatically increase the frictional losses, and reduce the flow rate. Many pipe manufacturers and engineering texts will present frictional head losses in pipes in tabular form (see Table 6.2). The input variables are nominal pipe diameter (which may be quite different from the actual inside diameter) and the flow rate. The outputs are the average pipe velocity and the frictional head loss per unit length. The total frictional head loss is then calculated by multiplying the table value by the number of unit lengths in that pipe run. These tables are fine if the assumptions and conditions of the table and the resulting errors are acceptable. Most of the available tables are for freshwater (Table 6.2 is for seawater), a fixed temperature and assumed pipe type and conditions. Since most tables do not consider biofouling, such values should be used conservatively. Depending on conditions, errors from use of such tables may or may not be significant. They are useful for preliminary calculations but should be checked by more rigorous methods during the detail design phase. Because of potential biofouling problems, these tables should be used very conservatively. There are a number of methods to calculate estimated frictional losses in pipes. Probably the most common method in fluid mechanics texts (Granet, 1989; Roberson and Crowe, 1990; Munson et al., 1994) is based on the Darcy-Weisbach equation (6.2). A major advantage is that it is readily useable in both metric and English units. h =
flV 2 2gd
(6.2)
where h is frictional head loss in ft (m), f is resistance coefficient or flow coefficient (non-dimensional), I is length of pipe run in ft (m), d is pipe inside diameter in ft (m), V is
TABLE 6.2 Frictional losses in plastic Schedule 80 pipes Flow GPM
Flow LPM
Nominal pipe sizes (inches) 1
10 20 30 50 70 100 150 200 250 300 350 400 500 600 700 800 900 1000 1200
0.63 1.26 1.89 3.15 4.42 6.31 9.46 12.62 15.77 18.93 22.08 25.24 31.55 37.85 44.16 50.47 56.78 63.09 75.71
1.5
2
3
4
vel.
loss
vel.
loss
vel.
loss
vel.
loss
4.4 8.9 13.3
8.8 32.0 72.1
1.8 3.6 5.5 9.1 12.7 18.2
1.1 3.7 7.9 20.0 37.1 71.7
1.1 2.2 3.2 5.4 7.6 10.8 16.2
0.31 1.1 2.3 5.6 10.7 20.2 42.8
0.5 1.0 1.5 2.4 3.4 4.8 7.3 9.7 12.1 14.5 17.0 19.4
0.04 0.16 0.32 0.81 1.5 2.9 5.9 10.2 15.5 21.7 29.5 37.4
6
vel.
loss
0.6 0.8 1.4 2.0 2.8 4.2 5.6 7.0 8.4 9.8 11.2 13.9 16.7 19.5
0.04 0.09 0.21 0.39 0.74 1.5 2.6 4.0 5.5 7.4 9.7 14.5 21.5 26.9
8
12
vel.
loss
vel.
loss
vel.
loss
0.6 0.9 1.2 1.9 2.5 3.1 3.7 4.3 4.9 6.2 7.4 8.6 9.9 11.1 12.3 14.8
0.03 0.05 0.10 0.21 0.36 0.54 0.74 0.98 1.25 1.9 2.7 3.6 4.6 5.7 6.9 9.7
0.7 1.1 1.4 1.8 2.1 2.5 2.8 3.5 4.2 4.9 5.6 6.3 7.0 8.4
0.03 0.06 0.09 0.14 0.19 0.26 0.32 0.49 0.68 0.90 1.16 1.4 1.8 2.4
0.5 0.6 0.8 1.0 1.1 1.3 1.6 1.9 2.2 2.5 2.8 3.2 3.8
0.008 0.014 0.020 0.028 0.036 0.047 0.070 0.097 0.127 0.166 0.202 0.242 0.336
These values should be typical of clean used plastic pipe and seawater. Note assumptions inherent in calculations. They do not include allowance for any significant biofouling, which can create much higher losses. Since Schedule 80 pipe has relatively small inside diameters, these values should be conservative for Schedule 40 or other thinner-walled pipes. Velocities in ft/s and losses in ft of head per 100 ft of pipe. Assumptions: seawater, 70~ (21~ kinematic viscosity 1.1 x 10 -5 ftZ/s; equivalent sand roughness 4.2 x 10 -5 ft; PVC Schedule 80 pipe inside diameters, Darcy-Weisbach equation.
91 TABLE 6.3 Equivalent sand roughness for internal pipe surfaces Pipe material
Feet
Glass, plastics, fiberglass, copper, brass, drawn tubing Steel, wrought iron Asphalted cast iron Cast iron Riveted steel Concrete
4.2 1.5 4 x 8.5 3 x 1 x
Meters x 10 -5 x 10 -4 10 -4 z 10 -4 10 -3 t o 3 x 10 -2 10 -3 to 1 x 10 -2
1.3 4.6 1.2 2.6 9 x 3 x
x 10 -5 x 10 -5 x 10 -4 x 10 -4 10 -4 t o 9 • 10 -3 10 -4 to 3 • 10 -3
Approximate values under 'normal' conditions for used pipe. Pipe materials are those that might be used for supply or drainage in a seawater system. Values do n o t have allowance for significant biofouling.
average pipe velocity, isvolumetric flow/pipe cross-sectional flow area, in ft/s (m/s), and g is gravitational constant, is 32.2 ft/s 2 (9.81 m/s2). While the equation is straightforward, this is not true for getting the proper values for f . A parameter called equivalent sand roughness (Ks) must be developed. This is an estimate of the roughness of the pipe's interior surface due to basic material, service use and cleanliness. Table 6.3 presents values for average conditions. These values are not specifically for seawater uses and do not allow for any significant biofouling. If a plastic line has ever been heavily biofouled, its values are likely to be in the range of steel to concrete pipe values even after rigorous cleaning due to unremovable calcareous deposits. Pipes with significant biofouling in effect have reduced diameters. Losses for biofouled pipes can be calculated using the estimated effective or available unfouled pipe diameter and high Ks values in the order of 0.01 ft or 0.003 m. The actual value of equivalent sand roughness to be used in design calculations is a function of where the specific length of pipe is situated in the system, the natural biofouling characteristics of the water expected to go through the line and the degree and quality of the expected maintenance on this section. In short, no simple answer and it is best to be conservative. Another needed parameter must be derived from the equivalent sand roughness, and this is the relative roughness. Relative roughness is defined as Ks/d and is dimensionless. Another non-dimensional factor to be defined is called Reynold's number (Re). Reynold's number is used to characterize and compare fluid flow. Vd Re = (6.3) 13 where Re is Reynold's number (non-dimensional), V is average pipe velocity in ft/s (m/s), d is pipe inside diameter in ft (m), and v is kinematic viscosity, Table A-3, ft2/s (m2/s). The relative roughness and Reynold's number are used to estimate the resistance coefficient ( f ) by use of the Moody diagram (see Fig. 6.3). The relative roughness value is found at the fight-hand margin and followed to the left and slightly upward until it intersects a vertical coming from the calculated Reynold's number on the bottom margin. The output is the resistance coefficient (f), also non-dimensional, which is read by going directly to the left-hand margin from this intersection. Most practical applications will fall in the fight half and, especially if biofouled, towards the top of the Moody diagram. The read value of f can now be inserted into the Darcy-Weisbach equation (6.2) to determine the line's frictional head loss.
92 .oi .09 .08 .07
.05
.06 N,,.-
~-
- 0, (.)
.05 -
.04
"01
0 . ~( j .~
o
-.,7_.. "E
LL
8 (I.) 0
0
.025
~ .02 n,- .015
.01 .009 .008
E
.005
DO ~
i"II
I 10 3
z 34
.001 .0005 .0001 .000,05
.ooo,o~ 681
10 4
105
106
1Or
-.000,001
Reynold's Number, Re = V..~D V
Fig. 6.3. Moody diagram: resistance coefficient vs Re. Modified from Moody (1944) courtesy ASME.
If the flow goes through several different pipe diameters or if the pipe materials or conditions change along the line, each section must be calculated separately with the procedures above and the results added to get the total pipe losses. Another method for estimating frictional losses in pipes involves the Hazen-Williams coefficient (C) and equation. There is a metric (6.4) and an English (6.5) form for this equation listed below. h -
10.7Q]-85/ C1.85d4.87
(6.4)
where Q is flow rate in m3/s, l is pipe length in m, C is the Hazen-Williams coefficient (dimensionless), d is pipe diameter in m. 4.72Q1.85/ h -
C1.85d4.87
(6.5)
where Q is flow rate in ft3/s, l is pipe length in ft, C is the Hazen-Williams coefficient (dimensionless), and d is pipe diameter in ft. One advantage of the Hazen-Williams equations is that the coefficients are independent of flow rate and velocity. Useable values for 'used' plastic pipe is C -- 100 and for new conditions C - 130. 'Used' concrete pipe is about C -- 60 and new around C -- 100. As with the Moody approach, these values do n o t allow for significant biofouling, assuming that the pipes will be well maintained.
93
6.5 Frictional losses in fittings If a section of pipe has any fittings, equipment or transitions (changes in diameter), these items will have additional frictional losses themselves. It must be remembered that both a pipe exiting a tank and a pipe flowing into a tank are pipe diameter transitions with associated frictional losses. These losses must be calculated separately and added to the pipe losses to get the total losses between any two points. The frictional loss will depend on the type of fitting or transition, the abruptness of the flow change, the material and the present condition of the internal surfaces. Usually only the type of fitting and the abruptness are considered. For some fittings, such as valves, there can be considerable variation in losses from one manufacturer to another due to internal dimensional differences. Even for more standardized fittings, the variations can be as much as 50%. The frictional head loss for any given fitting or transition is calculated from the equation below. KV 2
h =
2g
(6.6)
where h is head loss in ft (m), K is loss coefficient (non-dimensional), V is average velocity in adjoining pipe in ft/s (m/s), and g is gravitational constant, 32.2 ft/s 2 (9.81 m/s2). The loss coefficient (K) is a function of the type of fitting and conditions (see Table 6.4). Note that losses are keyed to the velocity in the adjoining pipe and that for transitions it is either the upstream or downstream velocities depending on conditions. Head losses for some inline components, such as heat exchangers and UV systems, may be given by the manufacturer for only the rated flow rate. If the diameters of the inlet and outlet pipe are known, a loss coefficient value can be computed. This will allow calculation of head losses at other flow rates (see Example 6.2). The engineering assumption is that the K value is constant and independent of the flow rate or velocity (if the internal configuration is not altered). Head losses and K values will be very manufacturer- and model-specific. A length of pipe may contain a large number of fittings and pieces of equipment. Many of these fittings may be identical. If the pipe diameter is constant over this length and there is no flow branching, the velocity will also be constant. In this case, it is not necessary to calculate the head loss for each fitting individually but to take the sum of the K values. With the sum of all the contained K values, the head loss from all the fittings can be acquired with one calculation and combined with the pipe loss (see Example 6.3). If there is more than one pipe diameter or pipe material, the sum of the K values have to be taken separately for the fittings associated with each type pipe. Another method that might be encountered for finding head losses in pipe fittings involves the concept of equivalent length. In this method, the head losses for the fitting are expressed in terms of the length of straight pipe with the same head losses. Tabulated data on fittings are presented as a factor which is multiplied by the associated pipe diameter to produce an equivalent length of straight pipe.
6.6 Open channel flow Seawater flow in open conduits or channels is more common at flow rates much higher than those of interest to us. However, it may still occasionally be encountered, especially in
94 TABLE 6.4 Loss coefficients for welded or flanged fittings K as a function of fitting size:
Fittings
Couplings and unions 45 ~ elbows 90 ~ elbows 180 ~ return bend T (line flow) T (branch flow - - tee used as elbow) Ball valve, open Gate valve, open Globe valve, open Butterfly valve, open Basket strainer Foot valve Swing type check valve (fully open) Disk type check valve (fully open) Ball type check valve (fully open)
2 inches
6 inches
20 inches
0.08 0.20 0.38 0.35 0.20 0.8 0.15 0.4 9 0.57 1.5 0.8 2 10 70
0.06 0.17 0.28 0.26 0.12 0.6 0.04 0.1 6 0.15 0.9 0.8 2 10 70
0.04 0.14 0.21 0.20 0.07 0.4 0.02 0.03 5 0.05 0.45 0.8 2 10 70
Transition
Loss coefficient (K) Contraction
"~ , 71 " - - - " / ~ D1
,
D2 ,
I
{
~
72
v for Computations = V2 Expansion
D1
v I ---~')
,
i
D2
I
/
._.~ V2
! v for Computations = V 1
D2/D1 0 (from large tank) 0.2 0.4 0.6
0.50 0.49 0.42 0.32
0.8
O. 18
D1/D2 0 (into large tank)
1.0
o2
o92
0.8
O. 16
0.4 0.6
0.72 0.42
These are approximate loss coefficients for standard fittings and transitions (adapted from Hydraulic Institute, 1979 and other sources). There is no allowance for biofouling. Threaded fittings will have higher values than flanged fittings or solvent welded fittings. Check valves that are not flowing sufficiently to fully open will have much higher losses. The loss coefficients are dimensionless. Actual values depend strongly on type of material, specific brand, and installation; the typical range of variation may be as large as 4-25 to 50%.
drains. The advantage of drains is easy access for cleaning and amen
