Anaerobic treatment of intensive fish culture effluents: digestion of fish feed and release of volatile fatty acids

Removal of organic matter and nitrate was studied in a laboratory-scale treatment system consisting of a digestion basin and a fluidized bed reactor. Fish feed was anaerobically degraded in the digestion basin and supernatant from the digestion basin, rich in dissolved organic degradation products, was used to fuel nitrate removal by denitrifying organisms in the fluidized bed reactor. Anaerobic digestion of the feed was determined in-situ using nylon-mesh bags. Feed degradation was described by considering the feed to consist of two fractions: a labile, rapidly degradable fraction and a recalcitrant, slowly degradable fraction. By using first-order kinetics, the degradation rate constants of each of these fractions were obtained allowing a quantitative prediction of sludge accumulation in the digestion basin. It was predicted that degradation rates and accumulation rates of sludge reached equilibrium after approximately 400 days of operation. The amount of sludge at equilibrium was approximately 23 times the weight of the feed which was added daily. The release of volatile fatty acids during fermentation of fish feed and sludge was determined as it is these organic compounds that mediate the denitrifying activity in the fluidized bed reactor. Predicted values for sludge accumulation and volatile fatty acid release were in agreement with measured values.