Achieving Efficient Nitrogen Removal From Municipal Wastewater Via Partial Nitrification/Anammox Process

Partial nitrification/anammox（PN/A）is a novel and promising biological nitrogen removal technology.The application of PN/A to the nitrogen removal from municipal wastewater can save 100%of denitrifying carbon sources and 55%of aeration energy consumption of the wastewater treatment plants（WWTPs）and has considerably economical and environmental benefits,and thus has captured great attentions recently.Depending on whether nitritation and anammox take place in the same reactor,PN/A process can be distinguished into one-stage and two-stage processes.For the treatment of ammonium-rich wastewater,one-stage PN/A process is more widely applied because it is more simple and easier to operate than two-stage process.Till now,mainstream PN/A process is still in the stage of laboratory research,and most of which are also one-stage process.However,It has been found that under mainstream conditions（e.g.low temperature and low ammonium concentration）,one-stage PN/A process has poor operation stability and is easier to deteriorate,and the effluent is difficult to reach the discharge standard.Therefore,the process type of mainstream PN/A remains to be investigated.Targeting to this issue,one-stage and two-stage PN/A processes were both established in this study to treat municipal wastewater.The nitrogen removal performance,process stability and microbial mechanisms of these two systems were investigated.The results are useful for the determination of process type and system optimization when applying mainstream PN/A.The nitrogen removal performance and stability of one-stage mainstream PN/A process under low residual ammonium were investigated.One-stage PN/A was established in a sequencing batch reactor and fed with pre-treated municipal wastewater in which organic matter has been pre-removed.The residual ammonium was gradually decreased from 8-12 mg/L to 2-5 mg/L by prolonging the aeration time while keeping other operating parameters unchanged.The results showed that PN/A process was stable when the residual ammonium was higher than 5 mg/L.The anammox activity was basically maintained,and the activity of nitrite-oxidizing bacteria（NOB）was effectively inhibited.However,when the residual ammonium was further decreased,the anammox activity in situ disappeared,NOB activity increased significantly.As a result,nitrate production ratio increased to 87±6%,and the main nitrogen conversion pathway shifted from partial nitritation/anammox to complete nitrification.Therefore,residual ammonium is the key factor affecting the stability of one-stage PN/A process.In addition,the underlying mechanism of low residual ammonium on PN/A stability was discussed by investigating the responses of microbial activity and microbial community evolution in the reactor.The results showed that substrate competition（NO₂^-）from anammox bacteria was the key to controlling NOB activity.Under low residual ammonium,anammox bacteria is disadvantageous when competing with ammonia-oxidizing microorganisms（AOMs）for substrate ammonium.The reasons are:（1）Anammox bacteria mainly grew in the interior of granular sludge,while AOMs mainly resided in the outer layer of granular.AOMs is easier to obtain substrate than anammox bacteria;（2）Under low ammonium concentration,the dominant AOMs in the reactor changed from traditional ammonium oxidizing bacteria（AOB）to complete ammonium oxidizer（comammox）which has a high ammonium affinity.Therefore,the in situ activity of anammox in the reactor decreased significantly under low ammonium concentration,which led to the increase of NOB activity and the deterioration of nitrogen removal performance of the system.A novel strategy to start and maintain mainstream partial nitritation was proposed,and the feasibility of two-stage PN/A process for efficient nitrogen removal from municipal wastewater was investigated.The biological phosphorus removal sludge that has little nitrification capacity was used as seeding sludge.By extending the aeration time and controlling a extremely short sludge retention time（2 d）,partial nitritation can be rapidly started within 12 days and maintained stable in the following long-term operation.The accumulation rate of nitrite（NAR）was as high as 84.8±6.3%.This start-up method does not need to control a low dissolved oxygen concentration,and therefore has a high nitritation rate.Besides,phosphorus was removed simultaneously a with an average removal efficiency of 91%.The two-stage PN/A system showed good nitrogen removal performance.The partial nitritation reactor was stable,the NAR maintained at 82～98%.The average concentrations of effluent ammonium and nitrite were 15.2 mg/L and 13.2 mg/L,respectively,and the ratio was close to 1:1.The overall removal efficiency of ammonium and total nitrogen（TN）of the two-stage PN/A process was 92%and 86%,respectively.The average concentrations of effluent ammonium and TN were 2.3 mg/L and 5.7 mg/L,respectively,which met the First-A level of discharge standard for WWTPs in China.In conclusion,the stability of one-stage PN/A process is poor when treating ammonium-diluted municipal wastewater,and it is difficult to achieve efficient nitrogen removal.The operation strategy under low residual ammonium needs further study.On the contrary,the two-stage PN/A process presented good nitrogen removal performance and stability when treating municipal wastewater.These results can provide technical reference and guidance for the process type selection and optimization of mainstream PN/A in real application.