Operation And Nitrogen Removal Mechanism Of A Microaerobic Activated Sludge System Treating Swine Wastewater

High ammonia nitrogen（NH₄⁺-N）for dry swine manure wastewater having characteristics of low carbon and nitrogen（C/N）ratio,and biological nitrogen removal difficult problem,our previous developed micro liter of oxygen flow activated sludge.The reactor（UMSR）successfully cultured the activated sludge with anammox oxidation（ANAMMOX）as the main biological nitrogen removal pathway,achieving efficient simultaneous removal of carbon and nitrogen.However,the UMSR startup process is slow,and the operating state is susceptible to the influent water quality,hydraulic retention time（HRT）and temperature.The high energy consumption required for the effluent reflux ratio of 25:1 or more further restricts its engineering application.Therefore,how to quickly start UMSR under low reflux ratio and explore the influence of control parameters such as HRT on system processing efficiency and operation characteristics is of great significance for engineering applications.After 50 d of shutdown period,the UMSR was started again under the conditions of HRT 8 h,25°C and reflux ratio of 10:1.The long-term operation characteristics of the system and the effect of the shortening of HRT on the processing efficiency of the system were investigated and clarified.The subsequent changes in microbial community and functional flora.The results show that under the condition of reflux ratio of only 10:1,UMSR can be successfully started and stable operation within 75 days,and the removal rates of COD,NH₄⁺-N and TN are stable at 63.8%,87.7%and 74.5%,respectively.The effluent concentrations averaged 89,30.4 and 63.3 mg/L,respectively,which fully met the"Pests and Poultry Pollutant Emission Standards"（GB18596-2001）.In the subsequent continuous operation of 136 days,the system’s removal rate of COD,NH₄⁺-N and TN fluctuated greatly due to fluctuations in water quality,but eventually stabilized at 64.1%,88.4%and 81.2%.The microbial community structure and C/N equilibrium analysis based on the principle of biological nitrogen removal showed that although the proportion of anammox bacteria in activated sludge was only about6.1%,ANAMMOX was still the main biological nitrogen removal pathway of UMSR.When HRT decreased from 8 h to 7 h,UMSR had no significant effect on the removal rates of COD,NH₄⁺-N and TN,but after further reduction to 6 h,the removal rates were significantly reduced to 55.5%,64.1%and 63.7%,respectively.The effluent concentrations averaged 109,96.7 and 98.3 mg/L,respectively,which could not meet the requirements of the emission standard GB18596-2001.Despite this,ANAMMOX remains the main biological nitrogen removal pathway for UMSR,but the proportion of anammox bacteria in activated sludge has decreased significantly from 6.1%at 8 h HRT to 4.5%.It can be seen that too short HRT can significantly affect the growth and enrichment of anammox bacteria in the system,and finally cause the system to reduce the TN removal rate.