Biological nitrification and denitrification in a modified activated sludge process

A 40 L/day continuous reactor, consisting of 5 basins arranged in an alternating anoxic-aerobic sequence ("modified Bardenpho process") was operated at low temperatures on municipal effluent to determine the rate and efficiency of nitrogen transformations and removal. Total nitrogen removal ranged from 95% at 18°C to 79% at 6°C. The average BOD and total phosphorus removals remained in excess of 91% and 81% respectively. The maximum unit nitrification rates were 1.76, 1.44, 0.38 and 0.42 mg oxidized N/gm MLSS/hr at 18°C, 14°C, 10°C and 6°C respectively. The maximum unit rates for endogenous denitrification and with waste-water substrate were 0.95, 0.86, 0.60, 0.31 and 1.52, 0.88, 0.71, 0.36 mg oxidized N/gm MLSS/hr at 18°C, 14°C, 10°C and 6°C. For the system studied, rates and performance appear significantly influenced by BOD, C:N ratio, sludge age, substrate concentration and possibly pH and toxicity. The necessary consensus among investigators on standardized techniques for reporting viable biomass and nitrogen concentration remains to be attained.