Submitted to: Aquacultural Engineering
Publication Type: Peer reviewed journal
Publication Acceptance Date: 4/29/2008
Publication Date: 8/1/2008
Citation: Davidson, J., Helwig, N., Summerfelt, S.T. 2008. Fluidized sand biofilters used to remove ammonia, biochemical oxygen demand, total coliform bacteria, and suspended solids from an intensive aquaculture effluent. Aquacultural Engineering. 39:6-15. Interpretive Summary: Intensive aquaculture facilities that utilize flow-through, serial reuse, or partial reuse systems generally produce two separate effluents: a high volume discharge from culture tanks or pump sumps, and a moderately small, solids-concentrated backwash flow usually flushed from filtration units. High volume effluents can appear pristine but typically contain dilute concentrations of carbonaceous biochemical oxygen demand (cBOD5), total-ammonia-nitrogen (TAN), total phosphorous, suspended solids (TSS), and coliform bacteria. However, the cumulative loading of dissolved wastes within high volume effluents (relative to the total daily discharge) can be significant. Environmental regulation of aquaculture effluents (this applies in other countries too) often governs both the mass loading and concentration of metabolite wastes. Therefore, effective technologies are needed to remove dissolved nutrients and metabolite wastes from high volume aquaculture effluents in order to meet environmental regulations and prevent pollution of receiving watersheds. A variety of technologies are available to treat aquaculture effluents, including: constructed wetlands, submerged biofilters, sedimentation basins, and lagoons. However, these are often ineffective at removing dissolved wastes and can require considerable space. Research funded by the U.S. Department of Agriculture at the Conservation Fund's Freshwater Institute, demonstrates that fluidized sand biofilters (FSBs) can be highly effective at removing cBOD5, TAN, and coliform bacteria from high volume intensive aquaculture effluents. Three full-scale CycloBio™ FSBs (one 0.92 m inside diameter [i.d.] x 2.77 m tall, and two 0.62 m i.d. x 2.43 m tall) were used to treat a dilute effluent from a partial reuse and a flow-through system containing rainbow trout fry. In both cases the effluent was first passed through a microscreen drum filter. Two sizes of sand were evaluated, having effective sizes (D10) of 0.19-mm and 0.11-mm, which produced mean biofilter hydraulic retention times of 5 and 11 minutes, respectively. Additionally, two bed-management techniques were evaluated: 1) siphoning from the top portion of the bed and 2) a biofilm shearing method in which a submersible pump mounted below the FSB overflow was used to pump biofilm-coated sand back to the FSB inlet (Fig. 3), thus causing turbulence, shear, and the release of biofilm particles. The aerobic FSBs effectively removed 86-88% TAN, 66-82% cBOD5, and 94-98% total coliform bacteria from the high volume effluents from intensive aquaculture systems. The FSBs also removed small amounts of phosphorous and TSS during short-term events where TSS loading increased. The study also demonstrated that the smaller sand size (0.11 mm dia.) was better at removing TAN and cBOD5, than the larger (0.19 mm) sand, particularly during biofilm shearing trials. Using a submersible pump to shear excess biofilm from sand particles was more effective at controlling overexpansion of the sand beds than siphoning. The pump shearing method required substantially less labour, decreased sand loss, and enhanced both nitrification and removal of cBOD5. However, the relatively inexpensive centrifugal pumps we used to control bed growth were rapidly abraded and damaged. More robust diaphragm- or air-lift pumps could be better options for biofilm shearing applications. Aerobic FSBs offer significant advantages over other methods of treating the high volume effluents. The TAN and cBOD5 treatment efficiencies achieved by the FSBs at the Freshwater Institute are greater than those reported for sedimentation basins and submerged biofilters and slightly better than those reported for large wetland filters. FSBs occupy much less space than large submerged biofilters or wetlands that require large plots of land to treat similar size flows. The sand media used in FSBs has a
Technical Abstract: Effluents from aquaculture facilities must be effectively managed to remove dissolved wastes and suspended solids that can pollute receiving bodies of water. High volume, dilute flows leaving settling or filtration units can appear pristine, but still contain dissolved wastes. Effective technologies are needed to treat high volume effluents from intensive fish farms. The objective of this study was to evaluate fluidized sand biofilters as a treatment option for removing carbonaceous biochemical oxygen demand (cBOD5), total-ammonia-nitrogen (TAN), total phosphorous, total suspended solids (TSS), and total coliform bacteria from high volume intensive aquaculture effluents. Treatment across three full-scale CycloBio® fluidized sand biofilters was evaluated using two sand sizes, i.e., an effective size (D10) of 0.11-mm and 0.19-mm sand that were each expanded approximately 60% at a superficial velocity of 0.31 cm/sec and 0.64 cm/sec, respectively. Two bed management techniques were also evaluated: 1) siphoning from the top portion of the bed and 2) a biofilm shearing method in which a submersible pump was used to strip and release excess biofilm as it accumulated at the top of the expanded bed. Greater removal of cBOD5, TAN, and nitrite, and greater dissolved oxygen consumption across the biofilter correlated with the 0.11-mm sand. Additionally, improvements in removal of cBOD5, TAN, and nitrite, and greater dissolved oxygen consumption across the biofilter were measured when the biofilm shearing method was used to manage bed growth compared to siphoning. The biofilm shearing method was also more effective in controlling bed growth, resulted in less sand loss, and required substantially less labor than siphoning. which was a large contrast from the labor required using the bed siphoning and sand replacement technique. The fluidized-sand biofiltersremoved 66-82% of the cBOD5 each pass and 86-88% of the TAN when bed growth was controlled using biofilm shearing. Outlet cBOD5 and TAN concentrations were reduced to 1.7 ± 0.4 and 0.11 ± 0.04 mg/L, respectively and outlet nitrite was 0.10 ± 0.02 mg/L when using biofilm shearing. Total phosphorous removal efficiency was 15-41% across the biofilters, and TSS removal was inconsistent but was achieved at inlet concentrations above 10 mg/L for both bed management techniques. Results indicate that full-scale fluidized sand biofilters can effectively treat high volume, dilute aquaculture effluents.