Evaluation of nitrous oxide emission during ammonia retention from simulated industrial wastewater by microaerobic...

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 (NO_x) 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 NO_x 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.