Exploration Of Aerobic Granular Sludge Cultivation And Nitrogen Removal Performance Under Low C/N Ratio In Real Wastewater

Aerobic granular sludge is a type of microbial aggregate that forms spontaneously without the need for an external carrier.It has a compact layered spherical structure and is a new generation technology for wastewater biological treatment.Compared to traditional activated sludge/biofilm,it has advantages such as larger particle size,faster settling,higher concentration,and no need for a carrier.Its layered spherical structure facilitates simultaneous nitrification-denitrification for nitrogen removal and can achieve simultaneous nitrogen and phosphorus removal in a single tank.Therefore,this technology can improve treatment efficiency,significantly save space,and reduce energy consumption,meeting the industry’s development demands.Municipal wastewater in China generally has characteristics such as low organic matter concentration and low carbon-to-nitrogen ratio.Cultivating a stable aerobic granular sludge system and achieving efficient simultaneous nitrogen and phosphorus removal has always been a key scientific and technological challenge for the practical application of this technology.Therefore,focusing on municipal wastewater with a low carbon-to-nitrogen ratio(C/N),studying the aerobic granulation process,and exploring its nitrogen and phosphorus removal performance have important practical significance and theoretical value.This paper focuses on municipal wastewater as the treatment object and investigates the formation process of aerobic granular sludge under actual water quality conditions with a low carbon-to-nitrogen ratio.It explores the synchronous removal performance and stability of aerobic granular sludge for COD,N,and P under different parameters and operating modes.The microbial community structure at each stage is analyzed to explore the correlation between macroscopic operational performance and microbial community evolution.The specific experimental results are as follows:(1)Using low C/N municipal wastewater as the substrate,two sets of reactors(R1,R2)were started under different hydraulic conditions and subjected to gradually reduced settling time and increased organic load cultivation modes.Both sets of reactors successfully cultured aerobic granular sludge after about 80 days.The average particle size of the sludge was 200 μm and 222 μm,respectively,and the sludge volume index SVI30 reached 54.83 m L/g and 59.65 m L/g,respectively,indicating the successful start-up of the aerobic granular sludge system.As the C/N was gradually increased,the particle size of the R2 system sludge further increased,with an average particle size of 323 μm.(2)The study investigated the effects of operating modes and parameters on the aerobic granular sludge system.Operating in a single aeration mode significantly improved its phosphorus removal performance,with the average total phosphorus removal rate increasing from 10.9% to 87.9%.The system’s phosphorus removal performance was closely related to the sludge age,with the average total phosphorus removal rates of 83.0% and 74.5% when the sludge ages were controlled at 12～15d and 15～20d,respectively.Operating the system in an intermittent aeration mode greatly improved its denitrification performance,with average total nitrogen removal rates of 76.7% and 80.1%,respectively.Dissolved oxygen(DO)was a key factor affecting the system’s denitrification performance,and controlling the DO at the end of aeration significantly improved the efficiency of simultaneous nitrification and denitrification and the denitrification efficiency of the post-denitrification stage.(3)The study investigated the effects of different C/N ratios on the aerobic granular sludge’s denitrification and phosphorus removal performance.When the C/N ratio of the influent was adjusted to 4～5,the effluent met the first-class A discharge standard.When the C/N ratio of the influent was adjusted to 5～6,the system’s effluent met the quasi-class IV standard after stable operation.In addition,the fuzzy filamentous growth on the surface of the particles caused by the increased organic load could be improved by increasing the aeration intensity and controlling the sludge age.(4)High-throughput sequencing analysis of microbial populations showed that the microbial community structures of the two sets of reactors were similar after stable operation.At the phylum level,Proteobacteria,Chloroflexi,Actinobacteriota,Bacteroidota,and Firmicutes were the dominant phyla.The microbial groups related to denitrification and phosphorus removal were greatly enriched during the system’s cultivation process,with a large number of relevant phosphorus removal functional bacteria such as Gammaproteobacteria and Alphaproteobacteria being enriched.In addition,key functional microbial groups related to denitrification,such as Candidatus＿Competibacter and norank＿f＿＿Caldilineaceae,were also identified in the aerobic granular sludge system,demonstrating the correlation between the system’s excellent denitrification and phosphorus removal performance and its microbial community structure.