Evaluation of nitrous oxide emission during ammonia retention from simulated industrial wastewater by microaerobic activated sludge process

Considering the reciprocating processes of nitrogen gas (N2) fixation to ammonia (NH4-N) and NH4-N removal to N2 through nitrification and denitrification during wastewater treatment, a microaerobic activated sludge process (MAS) is proposed in this study as a pretreatment to retain NH4-N from high-strength nitrogenous wastewater for further NH4-N recovery through membrane technology, that is, inhibit nitrification, with sufficient removal of total organic carbon (TOC). With DO and pH control, the 3-reactor bench-scale MAS systems successfully realized an NH4-N retention rate of over 80 %, with TOC removal rates of over 90 %. In addition, the emissions of carbon dioxide (CO2) and nitrous oxide (N2O) during MAS were evaluated. The total N2O emissions were 407 and 475 mg-N/day when pH was controlled at 6.2 (S1) and 6.8 (S2), respectively, with average emission factors to total nitrogen load over 2 % in both systems. Also, the global warming potential of N2O is one order of magnitude larger than that of CO2, indicating the significance of N2O in the MAS process. Therefore, the mechanisms of N2O emission from each reactor were investigated. The first reactor, where most of the TOC was adsorbed, emitted only 1.98 % (S1) and 2.43 % (S2) of the total N2O emissions through the denitrification of nitrite and nitrate (NOx) from the return sludge. The second reactor emitted 79.9 % (S1) and 69.0 % (S2) of the total N2O with the emission rates the same order of magnitude as the NOx production rates. Multiple pathways were considered to contribute to the high N2O emissions, and biotic NH2OH oxidation was one potential pathway at pH 6.2. Finally, the third reactor emitted 9.98 % (S1) and 16.8 % (S2) of the total N2O by nitrifier denitrification. Overall, this study showed that the large N2O emissions under nitrification-inhibiting conditions of the MAS process owed to the incomplete nitrification under acidic conditions and large abundances of denitrifiers. On the other hand, the lower N2O emissions at pH 6.2 evidenced the potential N2O mitigation under slightly more acidic conditions, underlining the necessity of further study on N2O mitigation when adapting to the trend of NH4-N recovery.