Insight Into The Performance And Mechanism Of The Combined Systems Of A~2O Process For Enhanced Nitrogen And Phosphorus Removal And Sludge Reduction

Anaerobic-anoxic-oxic(A~2O)process has became one of the most widely used wastewater treatment processes in China,but it faces the problems of low nitrogen and phosphorus removal efficiency and high sludge yield.With increasingly strict wastewater discharge and sludge disposal standards,it is a major demand in China's environmental protection field to develop new combined systems of enhanced nitrogen and phosphorus removal and sludge reduction based on the A~2O process.Aiming at achieving efficient sludge reduction and effluent water quality satisfying the first-class A discharge standard,the A~2O-membrane bioreactor(MBR)-biological predation combined process was developed.In the combined system,the advantages of MBR in high-quality effluent were synergized with sludge reduction by biological predation.In order to achieve the effluent water quality reaching the similar class IV standard of surface water,a combination of A~2O and electrochemically active biofilm was developed.In the combined system,new carriers were developed based on the principle of microbial fuel cell to convert the energy in the sewage into electric energy,improve the activity of biofilm and strengthen the degradation of pollutants.Based on the development of the above two combined systems,the wastewater treatment efficiency and sludge reduction performance of the two combined processes were further investigated,and then the mechanisms of enhanced nitrogen and phosphorus removal and sludge reduction were analyzed.A worm reactor(WR)based on biological predation and microbial enhancement was developed,then a combined system of A~2O-MBR-WR was constructed.Results showed that the effluent water quality satisfied the first-class A discharge stander of Chinese regulation.Compared with conventional A~2OMBR,the sludge yield of A~2O-MBR-WR combined process was reduced by 49.8%,the membrane filtration period was extended by 65.5%,the average particle size of sludge was decreased by 24.7%,while the uniformity of sludge increased by 36.1% and the normalized capillary suction time of sludge was decreased by 16.7%,the sludge dewatering performance was improved in the combined system.The interaction between bacteria and worms in WR was analyzed,60.2% and 55.5% of the total nitrogen(TN)and chemical oxygen demand(COD)in the reduced sludge was consumed by bacterial metabolism,respectively,which played an important role in reducing the concentration of pollutants in WR supernatant.Denitrification phosphorus removal and the enhanced sludge phosphorus release were revealed in WR.The total phosphorus(TP)concentration in the supernatant of WR reached 34.8±4.8 mg/L,which realized auxiliary chemical phosphorus removal of WR through bypass reactor.A novel carrier with electrochemically active biofilm(EAB)was developed,then a combined system of A~2O-EAB was constructed.Results showed that the effluent concentrations of COD,ammonium(NH4+-N)and TP of the combined system were 23.0±4.7,1.2±0.4 and 0.26±0.11 mg/L,respectively,which satisfying the class IV water quality standards of surface water,and the concentration of effluent TN was 9.8±1.8 mg/L.Compared with the conventional A~2O,the biomass increased by carriers of the combined system was about 19% of the suspended activated sludge,the sludge yield of the combined process was reduced by 18.6%,the average particle size of sludge was decreased by 36.6%,the surface hydrophobicity of the sludge was increased by 102.7%,and the settling performance of the sludge was increased by 8.4%.During the operation,the electrochemically active biofilm carries had stable output voltage,with an average of 13 m V.The contributions of the carriers to pollutants removal mainly came from the promotion of sludge activity by current stimulation.The mechanisms of enhanced nitrogen and phosphorus removal and sludge reduction of the combined systems were studied.Compared with the conventional A~2O-MBR,the specific oxygen consumption rate,nitrite bacteria activity and sludge phosphorus absorption rate of A~2O-MBR-WR was increased by 23.1%,301.6% and 27.8%,respectively.In the A~2O-MBR-WR combined process,the abundance of aerobic denitrifying bacteria,autotrophic denitrifying bacteria and denitrifying dephosphorization bacteria were all enriched,which effectively improved the nitrogen and phosphorus removal efficiencies of the system.The relative abundance of Dechloromonas,Reyranella and other bacteria with degradation ability to protein and carbohydrate membrane pollutants in the cake layer of the combined system was increased,which could effectively reduce the membrane fouling.Moreover,the analysis of sludge reduction mechanisms revealed that WR contributed 41.7% to sludge reduction,energy uncoupling(2.3%)of the return sludge to A~2O-MBR,lysis-cryptic growth and microbial community changes(5.8%)were identified,which contributed to further sludge reduction.Compared with traditional biofilm,the specific oxygen consumption rate,nitrite bacteria activity and denitrification activity of the electrochemically active biofilm in the A~2O-EAB combined system were increased by 55.0%,141.2% and 2.4 times,respectively.Nitrifying bacteria such as Nitrosomonas and Nitrospira,anaerobic ammonium oxidation bacteria Candidatus Brocadia in the biofilm of the anoxic tank of the combined system were enriched by 75.5%,107.5% and 275.0%,respectively.The relative abundance of the phosphorus removal functional bacteria in the suspended activated sludge of the combined system was 1.2 times that of conventional A~2O.The enrichment of nitrogen and phosphorus removal bacteria in A~2O-EAB combined system could effectively promote the degradation of pollutants and improve the effluent water quality.The endogenous respiration rate and endogenous respiration ratio of the suspended sludge in A~2OEAB were 127.6% and 13.9% higher than that of conventional A~2O,respectively,which promoted the reduction of sludge yield.The A~2O-MBR-WR and A~2O-EAB combined systems could effectively improve the wastewater treatment efficiency and decrease sludge yield,providing important technical approaches for A~2O upgrading and application.