Characteristics And Mitigation Of N₂O Emission In Themultiple Anoxic And Aerobic Enhanced Biological Nitrogen Removal Process

Nitrogen is one of the important nutrients causing eutrophicationof surface water.Due to the aggravation of eutrophication,the discharge standard ofnitrogen for municipal wastewater treatment plant?WWTP?is getting stricter.Multiple AO processis one of the preferred process for update and optimization of WWTPs because of its high nitrogen removal efficiency.However,during the enhanced nitrogen removal process,there may be more emission of N₂O?a kind of serious greenhouse gas?.Therefore,it is important to study the characteristics and mitigation of N₂O emission in multiple AO process.Characteristics of biological nitrogen removal and N₂O emission in a lab-scale SBR with multiple AOprocess were examined,comparing with conventional AO process.Effects of aeration rate and anoxic to aerobic time ratio?AO time ratio?on biological nitrogen removal and N₂O emission were studied,to clarify key control parameters for optimization of multiple AO process.Then,the effect of step feed operation mode on nitrogen removal and N₂O mitigation of multiple AO process was also studied.According to results of above experiments,mechanisms of enhanced biological nitrogen removal and N₂O emissionof multiple AO processwere analyzed,and strategies for efficient N₂O mitigation were explored.In the multiple AO process,partial nitrification and denitrification?PND?,simultaneous nitrification anddenitrification?SND?and enhanced endogenous denitrification occurred;nitrogen removal efficiency was 38.7% higher and N₂O emission factor was 75.5% lower than that in the conventional AO process.N₂O was mainly released during aerobic phases,and N₂O emission was mainly affected by dissolvedoxygen?DO?concentration and heterotrophs activities.A lower N₂O emission factor of the multiple AO process mainly result from the lower N₂O emission of SND under high DO concentration,comparing with that in the conventional AO process.Under aeration rates of 600,1200 and 2400 mL/min,nitrogen removal efficiencies of multiple AO process were 92.0%,79.4% and 70.8%,respectively.Contrast to high aeration rate,obvious PND,SND and enhanced endogenous denitrification occurred under low aeration rate,so relatively higher nitrogen removal efficiency was obtained;while the N₂O emission factor under low aeration rate was 11.54%,much higher than that of 0.78% under high aeration rate.A higher activity of endogenous denitrification under aerobic condition was the main reason causing higher N₂O emission factor of low aeration rate reactor.Under AO time ratios of 50 min/30 min,40 min/30 min and 30 min/30 min,nitrogen removal efficiencies were 85.8%,81.6% and 73.0%,and N₂O emission factors were 1.93%,2.54% and 1.37%,respectively.Under high AO time ratio,the inhibitionof NOB activity was a little higher,so more nitrite was accumulated;under gas stripping condition,the activity and N₂O emission factor of endogenous denitrification were both relatively high.Therefore,N₂O emission of high AO time ratio reactor was higher than low AO time ratio reactor.An improvement of nitrogen removal efficiency and reduction of N₂O emission can be achieved in multiple AO process under both stable cultivation and short-term shock of step feed operation mode,comparing with one-step feed operation mode.With a step feed operation mode,external organic carbon could be effectively utilized,which could enhance nitrogen removal and reduce N₂O emission.