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-scal e 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 de gradation products, was used to fuel nitrate removal by denitrifying o rganisms in the fluidized bed reactor. Anaerobic digestion of the feed was determined in-situ using nylon-mesh bags. Feed degradation was de scribed by considering the feed to consist of two fractions: a labile, rapidly degradable fraction and a recalcitrant, slowly degradable fra ction. By using first-order kinetics, the degradation rate constants o f each of these fractions were obtained allowing a quantitative predic tion of sludge accumulation in the digestion basin. It was predicted t hat degradation rates and accumulation rates of sludge reached equilib rium after approximately 400 days of operation. The amount of sludge a t equilibrium was approximately 23 times the weight of the feed which was added daily. The release of volatile fatty acids during fermentati on of fish feed and sludge was determined as it is these organic compo unds that mediate the denitrifying activity in the fluidized bed react or. Predicted values for sludge accumulation and volatile fatty acid r elease were in agreement with measured values.