Characteristics And Influential Factors Of Nitrous Oxide Emissions For Endogenous Denitrification

Nitrous oxide(N2O),a powerful greenhouse gas is emitted from almost all biological nitrogen-removal processes(eg.A2/O,shortcut nitrification denitrification,Shortcut nitrification-ANAMMOX and so on).Meanwhile,it is recognized as a potential renewable energy source,it has become a hotspot for minimizing its emission or maximizing its production for recovery.Especially noteworthy is the obviously N2 O accumulation was occurs in endogenous denitrification.However,DPAOs(denitrifying phosphate-accumulating organisms)and DGAOs(denitrifying glycogen-accumulating organisms)are known to carry out endogenous denitrification.In most N2O-accumulating reactors,DPAOs and DGAOs coexist due to a high level of phosphate feeding,and it is still not clear which of these was the dominant contributor to significant N2 O emission,Especially,few studies have focused on N2 O emission from endogenous denitrification activated sludge excluding DPAO s.In this study,activated sludge with good endogenous denitrification.characteristics was domesticated.Batch tests were conducted to studied the emission of N2 O that was influenced by electron acceptor and external carbon source dosage.A better understanding of the mechanism of N2 O emission during Biological denitrification process is critical for the N2 O release control or maximizing its production for energy recovery.The main results obtained were as follows:(1)A laboratory-scale sequencing batch reactor(SBR)seeding with conventional activated sludge with decoupled acetate and nitrate additions,was conducted for endogenous denitrification characteristics running with anaerobic/anoxic cycles,DPAOs was gradually washed out of the system by gradually reducing the dosage of phosphorus in the initial phase of acclimation.From day 6~13,a good performance of denitrifying phosphorus removal was observed,and from day 15 on,the phosphorus dosage was gradually decreased and the TP removal was decreased to 6%.From day 50 on,stable performance of endogenous denitrification was obtained in the SBR,which anaerobic COD removal rate was 80%,and the removal rate of NO3--N in the anoxic stage was 85%,approximately 50% nitrate was converted to NO2--N and TN removal rate was only 42 %.(2)The removal characteristics of nitrogen conversion in endogenous denitrification during the whole cycle were studied,the results showed that: the concentration of NO3--N was decreased from 42.36 mg·L-1 to 4.23 mg·L-1,N2 O emission rate was the similar trend as NO2--N and both have a peak in the initial time.The maximum N2 O emission rate was 0.36?g·min-1·g SS-1 and was occurred almost the same time as the peak of nitrite concentration,and 2.04% of removed nitrogen was emitted as nitrous oxide(N2O).(3)During the running of the SBR,the activated sludge was gradually converted from flocs to granular sludge,the diameter of the granules was around 0.3~0.5mm.the dominant species were Candidatus Competibacter sp.(accounting for 45.07%),combined with the SBR operating conditions,it should be DGAOs(Denitrifying Glycogen-Accumulating Organisms),the proportion of other species were: Denitratisoma sp.(13.63%)?Terrimonas sp.(9.33%)?Nitrosomonadaceae sp.(7.93%)?Thauera sp.(7.72%)?Halomonas sp.(3.93%).(4)The effects of different electron acceptors(NO3--N and NO2--N)on endogenous denitrification and the N2 O emission was conducted by batch test.the results showed that: the N2 O emission and the N2 O conversion rate were almost no significant changed with the increased of NO3--N concentration(10~60 mg·L-1),the N2 O emission was between 0.072~0.101mg·g-1 SS and the maximum N2 O conversion rate was 1.04%;However,the N2 O emission and the N2 O conversion rate were both increased with the increased NO2--N concentration.In addition,the endogenous denitrification rate and the total nitrogen removal rate were both decreased with the increase of initial NO2--N concentration,the endogenous denitrification rate was decreased from 5.66 mg·h-1g-1SS to 3.43 mg·h-1g-1SS with the increased NO2--N concentration(10~60 mg·L-1).(5)Batch tests were conducted to compare the emission of N2 O that was influenced by external carbon source dosage(COD/NO2--N was 0,0.75 and 2.5)using nitrite as the electron acceptor before and after poly-?-hydroxybutyrate(PHB)was synthesized.The results showed that,the N2 O emission was slightly decreased with the increase of total carbon source,and the N2 O conversion ratio was between 0.24% and 1.61%.15.90% of N2 O conversion ratio was obtained in the absence of external organic carbon,and the N2 O emission was 14~26 times higher than that of the other batch test results,indicating that when the endogenous PHB was the sole electron donor,the endogenous denitrification could produce more N2O.