Nitrogen removal from synthetic wastewater by simultaneous nitrification and denitrification (SND) via nitrite in an intermittently-aerated reactor

An intermittently aerated and decanted single-reactor process was proposed and some key control parameters investigated for nitrogen removal from wastewater by simultaneous nitrification and denitrification (SND) via nitrite. Two types of synthetic wastewater with acetate as the main carbon source, and chemical oxygen demand (COD):nitrogen (N) ratios of approximately 5:1 and 10:1 were used. For both types of wastewater the average COD removal efficiency reached above 95%, and under optimal conditions nitrogen removal efficiency also reached above 90%. This process consisted of 72 min aeration, 48 min settling and 24 min effluent decanting. Influent wastewater was fed from the bottom of the reactor, and did not require a separate mixing phase. In this process, nitritation (1st step of nitrification) was induced but nitratation (2nd step of nitrification) was effectively suppressed and denitrification was carried out using nitrite. Most important parameters for effective SND via nitrite in the proposed process were: (1) the increase rate and the minimum and maximum dissolved oxygen (DO) concentration during the aeration period, (2) length of one cycle, and of aeration per cycle, taking advantage of the lag-time of nitrite-oxidizers behind ammonia-oxidizers when adjusting from anoxic/anaerobic to aerobic condition, and (3) close contact between the mixed liquor suspended solids (MLSS) in the reactor and the influent wastewater under anoxic/anaerobic condition. During the aeration period, DO concentration dropped initially reaching the minimum value after several minutes, and then rose in 2nd-order fashion. The optimal maximum DO concentration (at the end of the aeration period) for nitrogen removal was determined to be around 2.0–2.5 mg/l in this proposed process.