1. Coldwater Biofilter Design Examples
M.B. Timmons, Ph.D.
Biological & Environmental Engineering
Cornell University
Ithaca, NY

2. Coldwater Design Example
Production Goal: 1.0 million lb/yr (454 mton/yr)
Arctic char

3. Large Operations Dominate Commercial Trout & Salmon Culture
Both culture technologies face tough environmental challenges.
6 m3/s flows to some farms
1,000-20,000 m3 per cage
There are few large water resources available for aquaculture development.

4. Large Production Systems are More Cost Effective
Economies of Scale
Reduce fixed costs per MTON produced
Reduce variable costs per MTON produced

5. Design Assumptions Assuming for the growout system:
Mean feeding rate: F = 1.2% BW/day;
Feed conversion rate: FCR = 1.3 kg feed/kg fish produced;
(these rates are an average over entire year)

6. System Biomass Estimation
Estimate of system’s average feeding biomass :

7. Oxygen Requirements
Estimate the oxygen demand of system’s feeding fish:
where:
RDO = average DO consumption rate
= kg DO consumed by fish per day (about 0.4)
aDO = average DO consumption proportionality constant
= kg DO consumed per 100 kg feed

8. Oxygen Requirements Estimate the mass and volume of oxygen required:
Account for oxygen transfer efficiency
In coldwater applications, a oxygen transfer efficiency of > 70% would be easily achieved in an LHO or oxygen cone with an outlet DO of 16 mg/L.

9. Flow Requirements
Estimate water flow (Q) required to meet fish O2 demand:
Assuming culture tank:
DOinlet = 16 mg/L
DOeffluent= 9 mg/L steady state)
DOsaturation = 10 mg/L
The change in DO across the culture tank is 7 mg/L in this example, which is conservative for a well designed recirc system for coldwater fish. A more aggressive, but usually achievable, inlet DO would be mg/L, which would produce an available DO across the culture tank of about 13 mg/L. In practice, I consider this to be about the maximum DO loading that can be safely managed with coldwater systems. First, 22 mg/L is near the effective limit of LHO technology to add DO in coldwater applications. Also, consider the later discussions on waste production to realize the CO2, TSS, and TAN are added within the culture tank at a DO consumption of 13 mg/L.

10. Flow Requirement traditional trout culture rule of thumb
50 lb/yr production in 1 gpm of water flow (correct water temp.)
76,000 L/min for 454 MTON/yr production
20,000 gal/min for 1 million lb (500 TON) annual production
Traditional trout culture requires more water flow to obtain a given production because the cumulative dissolved oxygen consumption is about lower than the DO consumption in this example.

11. Tank Volume Requirements
Assume an average fish density across all culture tanks in the system:
culture density = 60 kg fish/m3
A culture density of 60 kg/m3 is considered the average density found within the entire system. The maximum density in a culture tank might be 100 kg/m3 while other tanks in the same facility might be as low as 30 kg/m3. In reality, unless batch production is used in the entire facility, the average culture density found in facility will be considerably less than the maximum density that could be supported in a given culture tank. Also, because densities can vary between tanks, some tanks would receive more water than others to supply the extra oxygen needed to support these fish.

12. Culture Tank Exchange Rate
At a Q of 61.7 m3/min, the culture tank volume of 2160 m3 would be exchanged on average every 35 minutes .
Assuming ideal tank mixing.
In general, a culture tank exchange every minutes provides good flushing of waste metabolites while maintaining hydraulics within circular culture tanks (when the tank inlet and outlet structures are designed properly.

13. Tank Requirements Assuming 30 ft dia tanks Assuming 50 ft dia tanks
water depth
2.3 m
7.5 ft
culture volume per tank
150 m3
40,000 gal
14-15 culture tanks required
Assuming 50 ft dia tanks
water depth
3.7 m
12 ft
culture volume per tank
670 m3
177,000 gal
3-4 culture tanks required
If future expansion of the farm is expected to achieve 2-4 times more production, then maybe choose 50 ft dia tanks. Otherwise, choosing 30 ft tanks would be a okay choice to achieve 1 million lb maximum production.
To use tanks as large as 50 ft diameter requires knowledge of tank inlet and outlet structure designs to ensure that good mixing is achieved and that safe rotational velocities can be maintained for fish health and for flushing settleable solids. If water must rotate about the axis of a culture tank once in seconds to achieve good solids flushing, then the fish swimming in larger (e.g., 50 ft tanks) must be capable of swimming at these speeds. Water velocities are greatest near the tank’s outside wall where the perimeter distance is D. Therefore, in a 50 ft tank fish swimming with the current near the tank’s outside wall would be swimming between ft/s. Maximum safe swimming velocities for salmonids are considered to be between 1-2 body length per second. However, in ‘Cornell-type’ dual-drain tanks the water velocity decreases as the fish swim closer to the center of the tank, which allows fish to select what velocity they want to swim against by moving to different locations in the culture tank.

14. Ammonia Production Estimate
Calculate TAN production in system
where:
RTAN = TAN production rate
= kg TAN produced by fish per day
aTAN = TAN production proportionality constant
= kg TAN produced per 100 kg feed

15. Assume a Fully-Recirculating System (no water exchange)
Size biofilter to remove all of daily TAN production
Example 1: Fluidized-bed biofilters with fine sand, i.e., D10 = m.

16. Biofilter Sizing
The volume of static sand required to remove the PTAN can be estimated using either volumetric or areal TAN removal rates:
0.7 kg TAN removed per day per m3 static sand volume

17. Biofilter Sizing
The volume of static sand required to remove the PTAN can be estimated using either volumetric or areal TAN removal rates:
0.06 g TAN removed per day per m2 bed surface area (Sb) and Sb=11,500 m2/m3

18. Selecting a Sand for FSB
Select a fine graded filter sand that expands % at a velocity of cm/s (10-15 gpm/ft2).
a sand with D10=0.23 mm and a uniformity coefficient of would expand about 50% at v = 1.0 cm/s.

19. Biofilter Sizing
Biofilter cross-sectional area can be calculated from the required flow rate (Q) and water velocity (v):
Twelve biofilters that are each 11 ft dia
(or other geometries could be used)

20. Static Sand Depth
Static sand depth can be calculated from the biofilter cross-sectional area (Q) and sand volume requirement:

21. Assume a Fully-Recirculating System (no water exchange)
Example 2: Trickling Filter
Size biofilter to remove all of daily TAN production

22. Trickling Filter Sizing
The volume of packing required to remove the PTAN can be estimated using an areal TAN removal rate.
TAN removal rate, g/d/m2
(Nitrification data at 15°C from Bovendeur )

23. Trickling Filter Sizing
The volume of packing required to remove the PTAN can be estimated using 0.25 g TAN removed per day per m2 bed surface area (Sb); Sb=200 m2/m3
(approximately $170,000 of ACCUPAC structured packing)

24. Trickling Filter
Biofilter cross-sectional area can be calculated from the required flow rate (Q) and hydraulic loading rate (HLR=300 m3/day per m2):
Six biofilters that are each 7.0 m x 7.0 m (23 ft x 23 ft) square
(or other geometries could be used)

25. Trickling Filter
Packing depth can be calculated from the biofilter cross-sectional area (Abiof) and packing volume (Vpacking) requirement:

26. Trickling Filter Must also design:
flow distribution manifold above packing
packing support structure
sump basin below packing to provide cleanouts and overflow back to pump sump
air inlet and outlet structures
Select air handler/fan to provide G:L = 5:1 (vol:vol)

27. Stripping Column Design
Design criteria used for the forced-ventilation cascade column:
hydraulic fall of about m
hydraulic loading of m3/min per m2
Six stripping columns each with diameter = 3.0 m = 10 ft

28. Stripping Column Design
Design criteria used for the forced-ventilation cascade column:
volumetric G:L of 5:1 to 10:1
Each stripping columns will ventilate 3,630 scfm

29. Ozone Requirements
Estimate the ozone requirement of system’s feeding fish:
where:
aozone = kg ozone added per 100 kg feed

30. Overall Conclusions Use appropriate level of intensification.
Risk of failure higher for commercial reuse systems.
Trends towards larger and more intensive reuse systems for smolts and coldwater food-fish production:
reduced capital costs per MTon produced
reduced variable costs per MTon produced
especially labor and electric cost savings.
Technologies must scale functionally and cost effectively:
certain technologies are better suited than others at large scales