污水处理过程N_(2)O排放:过程机制与控制策略

N_(2)O Emission from the Processes of Wastewater Treatment:Mechanisms and Control Strategies

污水处理生物脱氮过程中氧化亚氮(N_(2)O)作为直接碳排放源,其大气升温效应较CO_(2)高出265倍.因此,国际上对N_(2)O排放机制与控制策略的研究层出不穷.N_(2)O产生源于硝化与反硝化过程,主要涉及亚硝化(AOB)及其同步反硝化、常规异养反硝化(HDN)、同步异养硝化-好氧反硝化(HN-AD)和全程氨氧化(COMAMMOX)等生物途径,以及硝化过程中间产物NH_(2)OH与NOH之非生物化学途径.常规硝化与反硝化(AOB+HDN)途径在正常运行工况下N_(2)O排放量并不是很大,约只占进水TN负荷的1.3%;即使是HN-AD与COMAMMOX代谢过程,两者N_(2)O产生量也不足TN负荷的0.5%.不可忽视的是AOB亚硝化及其同步反硝化,它们已被确认为是污水处理生物脱氮过程中N_(2)O排放的首要途径;AOB过程中间产物(NH_(2)OH与NOH)非生物化学过程以及AOB反硝化生物过程(主途径)共同导致的N_(2)O排放量可高达TN负荷的13.3%,主要是因为硝化过程溶解氧(DO)受限引起NO^(-)_(2)积累所诱发的AOB反硝化过程.需要特别注意的是,污水处理过程进水碳源不足而导致的HDN反硝化进行不完全情形,这会让NO^(-)_(3)反硝化止步于N_(2)O,致N_(2)O积累,其释放量可高达TN负荷的30%.此外,污泥絮凝体内部同步硝化/反硝化(SND)现象也是N_(2)O不容忽视的产生源,其释放量可高达TN负荷的7.7%,产生根源实际上是AOB反硝化.污水处理生物脱氮过程中为防止N_(2)O产生,应着力促进HDN反硝化进行完全和避免AOB反硝化过程.为此,运行过程中应控制曝气池中DO处于正常水平(~2 mg·L^(-1)),并尽可能延长污泥龄■,以避免AOB亚硝化积累NO^(-)_(2)并诱发AOB反硝化出现;同时,应及时补充进水碳源,以促进HDN反硝化进行完全至终点——N_(2).综述总结生物脱氮过程中涉及N_(2)O产生的所有机制,并根据过程机制讨论对其运行控制的策略.

As a direct carbon emission source,the amount of nitrous oxide(N_(2)O) produced in biological nitrogen removal of wastewater treatment is approximately 265 times higher than CO_(2) in atmospheric warming.Thus,there have been many global studies on the mechanisms of N_(2)O emission and its control strategies.N_(2)O emission is mainly associated with nitrification and denitrification,including such biogenic pathways as partial nitrification(AOB) and its simultaneous denitrification,ordinary heterotrophic denitrification(HDN),aerobic heterotrophic nitrification-denitrification(HN-AD),complete ammonia oxidation(COMAMMOX),etc.,as well as some abiogenic pathways via intermediate NH_(2)OH and NOH produced by AOB.Ordinary nitrification and denitrification(AOB+HDN) generally produce lower N_(2)O emissions,accounting for 1.3% TN under normal working conditions;both HN-AD and COMAMMOX together cause even less N_(2)O emissions,accounting for only 0.5% TN.Importantly,partial nitrification(AOB) and its simultaneous denitrification,which have been identified as a main approach to producing N_(2)O during biological N-removal,contribute to more N_(2)O emissions,up to 13.3% TN.This occurs through AOB abiogenic pathways(via intermediate NH_(2)OH and NOH) and its biogenic pathway of denitrification,which are caused by lower DO for nitrification and resultant NO^(-)_(2) accumulation for AOB denitrification(major pathway).Of note,incomplete HDN caused by insufficient carbon in influent would end at N_(2)O and thus produce higher N_(2)O emissions,up to 30% TN.Further,simultaneous nitrification and denitrification(SND) inside sludge flocs is also a non-negligible source of N_(2)O emission,up to 7.7%,which is actually caused by AOB denitrification.To control the N_(2)O emission during biological N-removal,complete HND and NO^(-)_(2) accumulation for AOB denitrification should be avoided to a large extent.For this purpose,DO in aerobic tanks should be controlled at a normal level(approximately 2 mg·L^(-1)),and solid retention time(SRT) should be extended,up to 20 d,which would avoid accumulating N_(2)O for AOB denitrification.Additionally,external carbon should be supplemented in time to promote HDN approaching the end,N_(2).This review summarizes the mechanisms of all the mentioned N_(2)O emission pathways and discusses the control strategies of N_(2)O emission according to the associated mechanisms.