Effect Of COD/N On Partial Denitrification-anammox When Treating Anaerobic Effluent Of Municipal Wastewater

The activated sludge process used in traditional urban sewage treatment can not achieve the goal of energy saving and low carbon operation due to high energy consumption and large carbon dioxide emissions.The anaerobic digestion process can realize carbon separation and energy recovery via methanogenic.The low concentration ratio of COD to total nitrogen in the effluent just meets the requirements of the nitrogen removal process based on anammox.The combined process of anaerobic digestion and anammox has the advantages of low carbon,low energy consumption and low excess sludge.Anaerobic digestion process is relatively mature,and the process based on anammox still has challenges in the supply of nitrite nitrogen.In urban sewage treatment,the process of partial denitrification coupled with anammox(PDA)is considered to be more feasible than the process of partial nitrosation coupled with anammox(PNA)because of its higher stability and reduction of nitrate nitrogen.In the one-stage PDA process,the COD supply(usually characterized by the ratio of influent COD to nitrate concentration,COD/N)is the key factor affecting the system operation,but the impact mechanism at the microbial level needs to be further explored.Therefore,in this study,the anaerobic effluent was treated by one-stage PDA reactor(hydraulic retention time 1.8 h),and the COD/N of the influent of PDA reactor was adjusted by changing the system branch flow.The removal and transformation rules of pollutants in PDA were studied,and the succession relationship between different metabolic characteristics and different forms of microorganisms,as well as the impact on sludge granulation and system nitrogen removal performance were discussed.The main work and conclusions are as follows:(1)When COD/N was low(2.77),nitrogen removal efficiency(NRE)was less than 60%.When COD/N was 3.44,PD reaction and anammox reaction coupled and the NRE was 86.12 ± 2.90%.When COD/N was about 4.09,the anammox reaction was inhibited due to too much nitrite nitrogen consumed by denitrification,and the NRE is lower than 75%.The effluent COD concentration of PDA reactor was maintained at about 20 mg/L.With the increase of COD/N,the degradation efficiency of protein decreased with the increase of COD/N.(2)The activity test results showed that during the period when COD/N was increased from 2.77 to 3.44 and 4.09,the values of nitrate reducing activity(SDA3)and nitrite reducing activity(SDA2)gradually increased,while the ratio SDA3/SDA2 gradually decreased and was more sensitive to the change of COD,which proved that with the increase of COD/N,the nitrite reduction ability of denitrifying bacteria in the reactor was gradually improved.When COD/N was 4.09,the rapid proliferation of Denitrifying bacteria with strong nitrite reduction ability and competition for nitrite nitrogen were the main reasons for the reduction of NRE in the system.(3)Macrogenomic analysis shows that Acinetobacter(a filamentous denitrifying bacterium)was always the species with the highest relative abundance in PDA,but it gradually degenerated with the increase of COD/N.After COD/N was increased from2.77 to 3.44,An AOB(Candidatus＿Brocadia)adapted to the environment with higher substrate concentration gradually eliminated An AOB(Candidatus＿Kuenenia)adapted to the environment with lower substrate concentration because denitrifying bacteria provided more sufficient nitrite nitrogen;When COD/N was 4.09,the competition of class B denitrifying bacteria for nitrite nitrogen inhibited both anaob and class A and C denitrifying bacteria,resulting in the decrease of NRE.In addition,the decrease of PN degradation rate when COD/N was 4.09 was also explained by the change of functional genes and the degradation of Bacteroidetes,Chloroflex and acidobacteria.(4)The results of microscopic observation and fluorescence in situ hybridization showed that when the COD/N was 2.77,the sludge structure was broken and a large number of filamentous bacteria appeared when the carbon source was insufficient.The appearance of filamentous bacteria did not cause sludge bulking,but is conducive to the aggregation of small particles,so that the particle size in the system is about 476 μm with complete structure and smooth surface.When COD/N was 4.09,the decline of sludge sedimentation performance may be related to the degradation of filamentous bacteria.