Biological phosphate removal in a prototype recirculating aquaculture treatment system

Efforts to reduce phosphorus concentrations in aquaculture systems have mainly dealt with improving the bioavailability of phosphorus in fish feed. Once released into the culture water, phosphorus is generally left untreated and discharged with the effluent water. In the present study, results are presented on a prototype recirculating treatment system originally designed for removal of organic matter and inorganic nitrogen. Phosphorus determinations in the various compartments of the treatment system (a digestion basin, a denitrifying fluidized bed reactor and a nitrifying trickling filter) revealed that, after 210 days of operation, more than 90% of the added phosphorus was retained within the organic matter of the trickling filter. By means of batch experiments with bacterial consortia from the reactors and with denitrifying isolates, it was found that denitrifiers were capable of phosphate uptake in excess of their metabolic requirements. The phosphorus content of organic material in the fluidized bed reactor was as high as 11.8% (on a dry-mass basis) while it was much lower in the trickling filter (around 1.9%). Anoxic incubation of the trickling filter material in the presence of an external carbon donor resulted in considerable denitrification activity and phosphate uptake. This finding served as an additional indication for the fact that phosphate removal from the water in the system was mainly mediated by denitrifying organisms. Based on these findings, the feasibility of using denitrification to control phosphate levels in the culture and effluent water of recirculating aquaculture systems is discussed. (C) 2000 Elsevier Science B.V.