AEROBIC AND ANAEROBIC BIOFILTRATION IN AN AQUACULTURE UNIT NITRITE ACCUMULATION AS A RESULT OF NITRIFICATION AND DENITRIFICATION

Biological oxidation and reduction of inorganic nitrogen were studied in an aquaculture unit equipped with an aerobic trickling filter and two, anaerobic, fluidized bed columns. The rate of ammonia oxidation in the trickling filter was shown to be dependent on the ambient ammonia concentration in the unit, indicating that ammonia oxidation by the nitrifying bacteria was substrate-limited with respect to ammonia. The maximum removal rate of ammonia was 0.43 g NH4-N m-2 day-1. Nitrite removal by the trickling filter took place when ambient ammonia concentrations were lower than 1 mg liter-1 NH4-N, while at higher ambient ammonia concentrations nitrite accumulated. The fluidized bed columns, employed in the system, were fed with endogenous organic material from the pond. Under these conditions high specific nitrate removal rates were obtained at relatively short retention times. Nitrate removal, however, fluctuated sharply over a diurnal cycle and nitrite accumulation took place under all running conditions examined. Explanations for the nitrite accumulation observed in both filtration systems are presented and possible improvement of the treatment system is discussed.