Nitrification at low oxygen concentration in biofilm reactor

A nitrification process under low dissolved oxygen (DO) concentration is proposed in a completely stirred biofilm reactor. The reactor was fed with a synthetic wastewater containing 250 mg NH4-N/L. A stable nitrite accumulation in the effluent was obtained during >110 days' operation; NO2-N:(NO2-N + NO3-N) in the effluent reached >90% under 0.5 mg DO/L. Ammonium was completely converted and NH4-N in the outlet was as low as 5 mg/L. A transient increase of the DO concentration in the reactor induced a complete conversion of ammonia and nitrite to nitrate after only 2 days. A return to a low DO concentration again induced nitrite accumulation. These results show that the nitrite oxidizers were always present in the reactor but were outcompeted at low DO concentration, due to their lower affinity for oxygen, compared with ammonia oxidizers. Nitrite accumulation could also be favored by free nitrous acid accumulation inside the biofilm.