In this work, four batch tests were conducted to comprehensively explore the effects of free nitrous acid (FNA) accumulation on nitrous oxide (N2O) emissions in a novel energy-saving and N2O-reducing static/oxic/anoxic (SOA) process. With the accumulation of FNA, the N2O emission factor increased from 1.51% to 4.32%, and the N2O emission ratio contributed by ammonia-oxidizing bacteria (AOB) increased from 74.0% to 78.6%, accordingly. Mechanism studies show that produced FNA and weakened aerobic metabolism bring synergy to competition between reductases. Aeration conditions and FNA cytotoxicity exert a greater impact on nitrite-oxidizing bacteria than on AOB, thus enhancing the potential for nitrite accumulation. Considering the removal of nitrogen and phosphorus and the reduction of N2O emissions in the SOA process, it is feasible to keep the average dissolved oxygen above 2.0 mg/L under the premise of nitrite accumulation.
Keywords: Free nitrous acid; Nitrite accumulation; Nitrous oxide; Static/oxic/anoxic process.
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