Effect Of Carbon-nitrogen Ratio On Nitrogen And Phosphorus Removal From Aerobic Granular Sludge

Aerobic granular sludge（AGS）is an innovative technology in the field of sewage treatment.Compared with conventional activated sludge（CAS）,AGS has the advantages of compact structure,high biomass retention,and multi-functional bacterial colonization.However,the low organic substrate concentration and the low carbon to nitrogen ratio（C/N ratio）in urban domestic sewage in China will be the most important factor limiting the effect of simultaneous nitrogen and phosphorus removal in AGS systems.Therefore,it is of great scientific significance and application value to explore the effect of C/N ratio in actual domestic sewage on nitrogen and phosphorus removal in AGS system.In this study,the actual low C/N ratio domestic sewage was used as the influent to cultivate AGS,and the effects of different C/N ratios on the formation,long-term stable operation and pollutant removal of AGS were explored.The microbial community structure,diversity and the activity of main functional bacteria in the system were analyzed,and the optimization strategy of nitrogen and phosphorus removal performance of aerobic granular sludge system was further explored.In the meantime,the effect of changing influent C/N ratio on nitrogen and phosphorus removal in mature AGS system was explored,which has indicative significance for the AGS process to treat the actual domestic sewage with low C/N ratio.The main experimental results are as follows:Three sets of sequencing batch reactors（SBR）were operated synchronously,using the actual domestic sewage as the matrix,and adding sodium acetate to the raw sewage to simulate the influent with three different C/N ratios for AGS cultivation respectively.The average influent carbon-nitrogen ratios of the three reaction systems were 4.29,5.19 and 7.93,respectively.The reaction systems（R1,R2,and R3）treating influent with different C/N ratios successfully achieved granulation within 60 days.The granular sludge was spherical or ellipsoid,with good settling performance.The mixed liquid suspended solid concentrations were about 5000,5000 and7000 mg·L^-1,respectively,and the average particle sizes were 351.5,294.4and 454.5μm,respectively.The AGS systems with sufficient carbon source and high C/N ratios has the largest particle size and higher biomass concentration.In the stable operation stage,the main functional bacteria activities of the three AGS systems were studied through batch experiments,and the microbial diversity and community structure of the AGS system were analyzed at the molecular biology level.Compared with CAS,the activities of AOB,NOB and denitrifying bacteria in AGS were higher.The R2 and R3 systems with the addition of carbon sources successfully enriched the phosphorus accumulating bacteria（PAOs）,and the biological phosphorus removal efficiency of AGS was 2-2.5 times that of CAS.However,the biological phosphorus removal performance of the R1 without the addition of carbon source was basically the same as that of CAS.At the molecular biology level,the microbial diversity of the AGS system was higher than that of the CAS,and the microbial diversity and richness were both R1>R2>R3.At the phylum level,Proteobacteria,Chloroflexi,Bacteroidota and Actinobacteriota were the main dominant phyla,and Nitrospirota related to nitrification was systematically enriched during the cultivation process.At the class level,the relative abundance of Gammaproteobacteria related to phosphorus removal was the highest in R3,which was about 3 times that of R1.At the genus level,the relative abundance of Candidatus＿Competibacter in R3 was 5 times higher than that in R2,but it was almost absent in R1.Sufficient organic substrate in the influent sewage will also lead to the proliferation of GAOs while enriching PAOs.Therefore,the appropriate operation strategy directly affects the microbial community structure inside the granular sludge,which in turn affects the efficiency of nitrogen and phosphorus removal.The layered structure of AGS will enrich different types of microorganisms,and increasing the C/N ratio of the influent is conducive to the enrichment of specific functional microorganisms,resulting in a decrease in the community richness.The low C/N ratio of the R1influent results in the reduced abundance and poor activity of PAOs.The effluent COD concentration of the three AGS systems were lower than 50 mg·L^-1,and the average TN concentrations in the effluent were 23mg·L^-1,14 mg·L^-1and 10 mg·L^-1,respectively.The removal rates of TN were54%,74%and 80%,respectively.The TP removal rates of R2 and R3 systems were both 100%,while the average effluent TP concentration of R1 was 2.7mg·L^-1with the TP removal rate of 26.6%.After further increasing the influent C/N ratio of R1,the nitrogen removal efficiency was significantly improved,while the phosphorus removal function showed limited improvement.After optimizing the operating conditions,the utilization rate of the organic substrate was improved,and the intermittent aeration was conducive to the occurrence of simultaneous nitrification and denitrification.However,due to the failuere to enrich PAOs during the granulation stage of R1,it took a long time for the phosphorus removal recovery of the R1 system.For domestic sewage with low C/N ratios,adding an appropriate amount of carbon source at the start of the culture can ensure that the system is enriched with a certain abundance of PAOs and enhance the phosphorus removal performance of the system.