Nitrate-rich wastewater is discharged from the processes of recovering precious metals from industrial wastes. This wastewater contains high concentration of acids and alkalis such as nitric acid and aqueous ammonia, and salts such as sodium chloride and sodium sulfate. Biological nitrogen removal from this wastewater was attempted by using a circulating bioreactor system equipped with an anaerobic packed bed and an aerobic threephase fluidized bed. In a bench-scale system, more than 90% of nitrate was removed in the anaerobic reactor when water temperature was over 15°C. By acclimating nitrifying and denitrifying bacteria with stepwise reduction of hydraulic residence time (HRT), the removal rate of total inorganic nitrogen (TIN) could be increased. In a laboratory-scale system, when inlet TIN concentration was gradually increased to 5000g/m3, TIN removal rate was increased to 0.32kg/m3/d. A nitrification batch test revealed that nitrification activity was quite sensitive to the concentrations of inorganic ions. In contrast, a denitrification batch test confirmed that the sludge of the anaerobic reactor of the benchscale system had been acclimated to denitrify even at hyper-saline condition where NaCl concentration was over 8.0%.
