Study On N₂O Emission Characteristics During Nitrogen Removal Process In CASS

Cyclic activated sludge system is widely used by both domestic and oversea wastewater treatment plants for nitrogen and phosphorus removal.However,biological nitrogen removal is an important anthropic source of N2O.It is very important to study the mechanism and mitigation strategy of N2O generation during biological nitrogen removal process.To date,limited research has been conducted on N2O emission from CASS and microbial mechanism of N2O production.Therefore,this thesis used low carbon source synthetic domestic wastewater as the the influent water and studied pollutant removal performance and N2O emission characteristics in CASS under different process parameters and cycle periods.By the use of molecular bio-techniques,relationships between denitrification enzyme activity,functional gene,microbial community and N2O generation were explored,which provided fundamental data for control of N2O release from CASS.The results were as follows:(1)Appling anoxic section in feeding phase of CASS improved nitrogen removal and reduced by 68%of N2O conversion rate.Step feeding and continuous feeding could also mitigate N2O emission from CASS with enhanced nitrogen removal.The expression of denitrification genes was promoted under these three cycle periods with improvement of nitrite reductase and N2O reductase.(2)N2O conversion rate decreased with the increase of aeration rate.During aeration period,higher aeration rate increased DO concentration and then repressed the production of N2O by nitrifier denitrification pathy.CASS with continuous feeding showed better nitrogen removal performance than batch feeding and released less N2O.N2O conversion rates of batch feeding CASS were 1.6?2.2 times of continuous feeding CASS.Continuous feeding enhanced denitrification process,mitigated electron competition amoung denitrifying enzymes and increased the abundance of N2O-reducing bacteria(3)With the increase of influent C/N ratio,ammonium and TN removal efficiencies of CASS were enhanced enhance and N2O conversion rate reduced from 20.79%to 2.71%.Adequate carbon source supply was beneficial to the growth of N2O-reducing bacteria Zoogloea,increasing the abundances of denitrification gene nosZ,nirS and nirK,which promoted full denitrification and decreased N2O procudtion from denitrification.(4)The use of acetate as carbon source in CASS improved nitrogen removal and leaded to no N2O production from denitrification and more N2O generation from nitrification due to higher ammoniurm oxidizing rate.When using glucose as carbon source,N2O produced from denitrification increased with decrease of N2O producd from nitrification.Adding three types of carbon source in CASS resulted in the least N2O emission.Acetate was more likely to be utilized by mitroorganism which increased the abundance of N2O-reducing bacteria Dechloromonas,Zoogloea,and denitrification genes.(5)N2O conversion rates were similar in CASS with different influent pH.Influent pH played an important role on N2O production pathy.When influent pH of CASS decreased to less than 7.2,nitrification was weakened and nitrate concentration was decreased,leading less N2O generation from denitrification.When pH was more than 8.4,ammonium removal efficiency and N2O produced from nitrification increased and activity of N2O reductase was strengthened with decrease of N2O produced from denitrification.The increase of pH had little influence on abundance of denitrifying bacteria,but higher abundance of denitrification genes were observed in pH 8.4 and 9.0.(6)N2O conversion rate decreased with the increase of sludge retion time(SRT)in CASS while no significant enhancement of nitrogen removal was obserbed.CASS with higher SRT produced more N2O during settling and decanting,which may caused by more intracellular carbon source using as electron donor for denitrification.Long SRT lead to higher abundance of N2O-reducing bacteria Dechloromonas and Opitutus with little difference of total denitrifier abundance.(7)Adding fillers to CASS had little positive effect on nitrogen removal and N2O control.Sludge biomembrane hybrid reactor produced the largest amount of N2O,which mainly produced from denitrification.Biomembrane reactor emitted least N2O due to low diffusion rate of N2O and thick biomembrance allow enough time for denitrigying bacteria to reduced N2O.However,its poor ammonium removal performance affected effluent quality.(8)After CASS running for 90 days under 10 ?,nitrite oxidizing bacteria activities were inhibited by low temperature.No nitrate was produced and nitrite accumulated during aeration phase,which was considered to be patial nitrification and denitrification.Low temperature and nitrite accumulation promoted N2O generation.With magnetic field strengthening,NH4+-N and TN removal efficiencies increased by 21.2%and 8.4%,and leaded to 2.86%decrease of N2O conversion rate.After anoxic section was added in feeding phase and acetate was used as influent carbon source,magnetic field also enhanced nitrogen removal performance and decreased N2O comversion rate by 1.80%.The growth of nitrifying and denitrifying bacteria was promoted by magnetic field while it increased denitrification gene abundances and enzyme activities.