Pilot Scale Study On Two-stage A/O-MBBR Process Optimization And Nitrogen Fate For Domestic Wastewater Treatment

With the rapid growth of population,the acceleration of urbanization and the rapid development of industry and agriculture,the problem of water eutrophication caused by nitrogen and phosphorus pollution is becoming more and more strict.In order to solve the problem of water environmental pollution,while strengthening sewage treatment,the national and local governments have increasingly strict requirements on the sewage discharge standards of urban sewage treatment plants.However,the biological denitrification treatment technology mainly based on the traditional activated sludge process generally has disadvantages such as high energy consumption in operation and poor treatment stability.Previous studies have shown that a certain amount of nitrous oxide（N₂O）and nitric oxide（NO）intermediate products will be produced in the process of biological nitrogen removal.In particular,as one of the six major greenhouse gases,N₂O has a 100-year greenhouse effect 265 times that of carbon dioxide（CO₂）,which has a strong destructive ability to the atmospheric ozone layer.Therefore,while strengthening the urban sewage treatment to solve the water environment problems,the transfer of pollutants in the sewage treatment process and its impact on the atmospheric environment should also arouse enough attention.Therefore,this study aimed at the two-stage A/O-MBBR biological denitrification technology for urban sewage treatment with biofilm as the main body,carried out the study on the optimization of the two-stage A/O-MBBR biological denitrification process and the nitrogen fate pollutants in urban sewage,and provided technical support for the construction and optimization of the two-stage A/O-MBBR system for urban sewage with high efficiency and low carbon.In this study,field experiments were carried out in A municipal sewage treatment plant in Qingdao City.Taking the actual inlet water of the sewage treatment plant as the research object,a two-stage A/O-MBBR pilot test system with the treatment capacity of12m³/d was constructed.Based on the optimized operation of the two-stage A/O-MBBR system,the exhaust gas sampling and detection method was used.The product changes of nitrogen,nitrous oxide（N₂O）and nitric oxide（NO）in each reaction unit of two-stage A/O-MBBR pilot-scale system were analyzed under different operating conditions.The research results show that:（1）After starting the two-stage A/O-MBBR pilot system for 7 days,the treatment effect of pollutants can reach stability.Under the conditions of temperature 20.6～26.9℃,organic volumetric load of 284.11±115.27g SCOD/（m³·d）,ammonia volumetric load of 89.19±31.31 g N/（m³·d）,the average effluent concentrations of SCOD,ammonia nitrogen and TIN were 26.43 mg/L,0.43 mg/L and 2.85 mg/L,respectively,and the average removal rates were 80.78%,98.85%and 94.11%.It has good treatment effect on organic matter,ammonia nitrogen and TIN,and can realize stable and efficient operation.（2）During the stable operation of the two-stage A/O-MBBR pilot-scale system,under the conditions of temperature 20.6～26.9℃and organic volume load is 284.11±115.27g SCOD/（m³·d）and ammonia volumetric load of 89.19±31.31 g N/（m³·d）.SCOD and ammonia nitrogen can be basically removed in the A/O subsystem of the first stage.There is a good linear relationship（positive correlation）between the surface load of BFCOD membrane and the surface removal load of BFCOD membrane in each aerobic reactor.When the nitrification load of O3 reactor is 1.42～4.27 g NH₄⁺-N/（m²·d）,the nitrification load and nitrification removal load have a wireless relationship.When the nitrification load of O4 and O5 reactors is less than 1.2 g NH₄⁺-N/（m²·d）,the nitrification load is positively correlated with the nitrification removal load.When the nitrification load is greater than 1.2 g NH₄⁺-N/（m²·d）,the nitrification load and the nitrification removal load have an infinite relationship.When the nitrification load of O5 reactor is 0-0.57 g NH₄⁺-N/（m²·d）,the nitrification load is positively correlated with the nitrification removal load.There was a positive correlation between denitrification load and denitrification removal load in each anaerobic reactor.（3）The key factor affecting the nitrogen removal in the two-stage A/O-MBBR pilot-scale system is dissolved oxygen,followed by the influent pollutant concentration（ammonia nitrogen,SOCD,nitrate,nitrite）,carbon source dosage,C/N ratio,hydraulic retention time,SS,reflux ratio,aeration rate,while the inlet water has little effect on the nitrogen removal in the pilot-scale system.In the process of denitrification in the two-stage A/O-MBBR pilot system,there was A positive correlation between dissolved oxygen and ammonia nitrogen removal rate and effluent TIN concentration,and a negative correlation between dissolved oxygen and effluent ammonia nitrogen concentration,effluent SCOD concentration,SCOD removal rate,TIN removal rate and effluent nitrite concentration.The denitrification efficiency of the system can be effectively increased through human co-regulation.（4）In the two-stage A/O-MBBR system,the conversion of nitrogen from oxidized ion nitrogen（NO₃^--N,NO₂^--N）to gaseous nitrogen（N₂,N₂O and NO）mainly occurred in the pre-denitrification A1 reactor and the post-denitrification A6 reactor,and the denitrification gaseous products in each anaerobic reactor all existed N₂,N₂O and NO.The product of nitrogen removal in each aerobic reactor is only N₂O.From the perspective of nitrogen reaction balance,the N₂O released in each aerobic reactor mainly comes from the stripping off of the denitrification product dissolved in the reaction liquid in the anaerobic reactor.The dissolved amount of water-soluble N₂O in each reaction section showed a decreasing trend in anoxic and aerobic reactor.In the two-stage A/O-MBBR pilot system,the highest average emission coefficient of N₂O is the O8 aerobic reactor after denitrification,whose emission coefficient is 309.31 mg N/（m2·h）,and the average release flux is 331.19 mg N/m3 and 13.51 mg N/g N.（5）The key factor affecting the denitrification product emission in the two-stage A/O-MBBR pilot system is the C/N ratio,and the C/N ratio is negatively correlated with the N₂O emission coefficient,the dissolved N₂O concentration,and the NO emission coefficient.Artificial control factors（dissolved oxygen,carbon source dosage,hydraulic retention time,reflux ratio）can not be ignored for N₂O and NO production.By adjusting the ratio of carbon to nitrogen,the effluent nitrogen concentration can be reduced,and the emission of N₂O and NO can also be reduced.The emission of N₂O in the two-stage A/O-MBBR pilot-scale system accounts for 0.07%～4%of the total nitrogen load treated by the system,which is much lower than 25%of the biological nitrogen removal process with activated sludge.