EnviroQuip International, Cincinnati, OH provides aeration in each BioSump with a supply of 1.4 m
3
min
− 1
of air. A 2.0 kW regenerative blower Sweetwater Model S-45, Aquatic Eco-Systems, Inc. provides the air to the aeration arrays in both
BioSumps at a combined rate of 2.8 m
3
min
− 1
at 76 cm H
2
O pressure, to liberate carbon dioxide and add dissolved oxygen. Water is then pumped from the bottom
of each BioSump with two-2 kW centrifugal pumps G Sweetwater, Aquatic Eco-Systems, Apopka, FL at a rate of 415 l min
− 1
each through oxygen injection components to each tank Fig. 6. For each tank, two downflow oxygen contactors
H Oxygen Saturator Model OY110, Aquatic Eco-Systems, Apopka, FL are used. Water flows into the top of the downflow oxygen contactors, is mixed with
gaseous oxygen, and exits the bottom in a pressurized 0.5 – 1.0 bar flow stream for delivery to the culture tank. The oxygenated water re-enters the culture tank
through two vertical manifolds I ECO-FLOW Model 110, AquaOptima AS, Trondheim, Norway that allow for even distribution of the water from top to
bottom in the tank water column.
3. Water treatment system design and operational characteristics
The following is a description of the operational characteristics of the growout tank’s water treatment system described above.
Fig. 6. Fish culture tank, downflow oxygen contactor and BioSump biofilter after Hobbs et al. 1997.
Table 2 Design and operational characteristics of the CPLEPRI water treatment system
Average mea- Design rate
Units Category
sured rate New water used per unit of feed added
141 l d
− 1
kg per day 97
18.3 l min
− 1
kg day Recycle flow rate per unit of feed added
16.6 KW kg
− 1
per Pumping energy per unit of feed added
0.085 0.082
day Aerationdegassing energy per unit of feed
KW kg
− 1
per 0.030
0.031 day
added g TANm
− 2
per Biofilter nitrification
0.45 0.43
day
3
.
1
. Design characteristics The water treatment system for the two-tank growout system is designed to
remove or neutralize the waste created by approximately 100 kg of fish feed per day 38 protein by weight. The system was designed to have the capacity to treat,
renovate and recycle water back to the culture tanks at a rate of 1660 l min
− 1
. Much of the system component sizing is dictated by the daily feed amounts
required to grow the specified biomass of fish in each culture tank Losordo and Hobbs, 2000. Table 2 lists some important sizing and design characteristics per
unit of feed input to the growout system.
In general, the system was designed to use approx. 5 – 10 of the system volume per day in replacement water. The new water replaces that which is lost to
discharge at the drum screen filter, the sludge collectors, draining and refilling tanks during harvesting, and evaporation. In the growout system, with a tank volume of
120 m
3
and a BioSump water volume of 9.35 m
3
, a daily replacement volume of 7.5 is approximately 9.7 m
3
of water. With an estimated feed rate of 100 kg per day, the design rate for replacement water based on feed rate is 97 l kg
− 1
of feed per day 9,700 l100 kg feed per day Table 2.
The design flow rates of new water into the system and recycle water through the treatment system were based upon mass balance analysis and experience with
smaller systems of similar design. The design recycle flow rate, listed in Table 2, is 16.6 l min
− 1
of recycle flow per kg of feed per day 1660 l min
− 1
100 kg feed per day. Depending upon the specie of fish, this design procedure often results in a
flow rate that produces a hydraulic retention time of 55 – 60 min in the culture tank. In this case the design hydraulic retention time was 72 min. While this may not
maintain appropriate water quality for some of the more sensitive species, the flow rate has proven to be appropriate for tilapia Oreochromis niloticus culture in this
specific system design. Less retention time might be required for other species at similar stocking rates. Each centrifugal pump is specified by the manufacturer to
have a full-load amp FLA rating of 8.9 at 230 V 2.05 kW. As such, the total electrical load for the four pumps in the system would be estimated to be 8.2 kW.
Thus the design rate listed in Table 2 for energy used for recycled water pumping per unit of feed per day is estimated at 0.082 kW per kg of feed per day 8.2
kW100 kg feed day. Similarly, the energy used in aeration and degassing can be estimated by
summing the total FLA for the regenerative blower 10.4 amps and high volume blower 2.7 amps and multiplying by 230 volts. Therefore, the design rate listed in
Table 2 for energy used for aeration and degassing was 0.030 kW kg
− 1
feed per day 3.01 kW100 kg per day.
Previous studies at the North Carolina Fish Barn have indicated that an appropriate design nitrification rate Table 2 for the type of trickling filter which
was used in this study is 0.45 g TAN m
− 2
per day Twarowska et al., 1997.
3
.
2
. Preliminary operational characteristics Tank 1 of the two-tank system was stocked on 29 June 1998 with 6752 – 140.5 g
average weight tilapia fingerlings Oreochromis niloticus. Tank 2 of the same system was stocked on 5 August 1998 with 9653 – 95.1 g tilapia fingerlings. The
biofilters were allowed to populate naturally with nitrifying bacteria. Tank 1 harvests began on 5 October 1998 and were completed by 10 November 1998. A
total of 3813.5 kg was harvested from this tank. On 13 November 1998 Tank 1 was restocked with 4668 – 364 g tilapia 12 the population from Tank 2 weighing 1699.2
kg. As noted above, the Tank 2 population remained stable until it was divided and half was transferred into Tank 1.
As one can readily see, the populations and biomass in this two-tank system fluctuated considerably during this study period. This is typical of tank systems that
share water treatment components and of those that are harvested numerous times to satisfy local markets. Maximum biomass for the two-tank system occurred
around late October or early November with a total estimated fish biomass of 6860 kg 57.2 kg m
− 3
. The feed rates listed in the figures fluctuated according to the system biomass and fish appetite.
Although the following data do not represent steady-state conditions, they do provide a preliminary evaluation of the water treatment systems’ capabilities. Water
samples were immediately transported to the Water QualityWaste Management Laboratory at North Carolina State University. Samples were analyzed by auto-
mated analysis Technicon, Auto-analyzer Model II for total ammonia nitrogen TAN by the salicylate method, nitrite – nitrogen NO
2
– N by the cadmium reduction method, and nitrite – nitrogen plus nitrate – nitrogen NO
2
− N + NO
3
− N by the copper – cadmium reduction method EPA, 1979. Filterable suspended
solids FSS were analyzed according to Standard Methods APHA, 1989. Dissolved oxygen concentrations were measured and recorded twice daily with a
portable oxygen meter Yellow Springs Instruments, Model 55, Yellow Springs, OH. These readings indicate that the dissolved oxygen concentration ranged from
6 – 9 mg l
− 1
. Lowest oxygen concentrations were experienced in the late afternoon after a prolonged period of feeding. Depending upon feed delivery adjustments,
feed was supplied for 20 – 24 h per day, every 30 min.
The actual volume of new water used within the growout system was recorded daily and averaged 12.3 m
3
per day over the 12-week sampling period. With an average daily feed rate of 86.9 kg per day, the actual rate for replacement water
based on the feed input rate Table 2 was 141 l kg
− 1
of feed per day 12300 l86.9 kg feed per day. Similarly, the actual recycle flow rate for the 12 week sample
period was estimated at 18.3 l min
− 1
of recycle flow per kg of feed per day 1590 l min
− 1
86.9 kg feed per day. The water quality and associated feed rate data for the two-tank growout system
for the 12 week sampling period can be found in graphical form in Figs. 7 and 8. The TAN and NO
2
– N concentrations for 22 September 1998 and 6 October 1998 were extremely high. Prior to these dates the system was operating with one
centrifugal pump per culture tank 415 l min
− 1
recirculation flow rate per tank. With the second pump operating, the flow rate approximately doubled, however the
biofilter distribution nozzle was not adjusted properly. For a period of time, the water from the system was overshooting the biofilter media and much of the water
was running down the sides of the filter reactor. The situation was corrected on or around 22 September 1998 and, as can be seen by the data in Fig. 7, the TAN and
NO
2
– N declined to more appropriate levels. TAN concentration ranged from 0.87 to 1.83 mg l
− 1
with an average of 1.34 mg l
− 1
over the period between 14 October and 8 December 1998. The pH within the system ranged from 6.9 to 7.4.
Nitrite – nitrogen concentration over the same period varied from 0.83 to 3.98 mg l
− 1
with an average concentration of 1.83 mg l
− 1
. While these NO
2
– N concentra- tions were higher than desirable levels, they are typical of production systems not
Fig. 7. Measured total ammonia-nitrogen and nitrite-nitrogen concentration in the two-tank growout system during a 12 week period.
Fig. 8. Measured nitrate-nitrogen and filterable suspended solids FSS concentration in the two-tank growout system during a 12 week period.
yet in steady-state conditions e.g. mature biological filters. Chloride concentration of the system water was maintained in excess of 100 mg l
− 1
to ‘protect’ the fish from the harmful effects of nitrite toxicity.
Data in Fig. 8 show a characteristically high nitrate-nitrogen concentration that is expected within a recirculating system that replaces 10 or less of the system
volume per day operating without an active denitrification system. The filterable suspended solids concentration within the tanks averaged just over 32 mg l
− 1
. While 20 mg l
− 1
or lower is desirable, this level was acceptable given the fact that the foam fractionation system had not been put into operation during this initial
test period. Areal nitrification rates for the tricking filters in the system are shown graphed
versus growout tank TAN concentration in Fig. 9. These nitrification rates were estimated by multiplying the average flow rate l per
day to the filter by the difference between the biofilter inflow and outflow TAN concentration g l
− 1
, then dividing by the total area m
2
of the biological filter media l per day × g l
− 1
m
− 2
. The data indicate that for tank effluent TAN concentrations of between 1 and 1.5
mg l
− 1
, the average nitrification rate of the two filters was 0.43 g TAN m
− 2
per day. This compares well with previous data from similar systems Twarowska et al.,
1997, and is remarkably close to the design nitrification rate of 0.45 g TAN m
− 2
per day. Data from Greiner and Timmons 1998 suggest that the nitrification rate of the trickling filters in this study were limited by the flow rate.
Fig. 9. Areal nitrification rate for the trickling filters servicing the two-tank growout system.
4. Conclusions
