Succession Law Of Microbial Community And The Role Of EDNA In The Process Of Sludge Granulation With Low-strength Wastewater

Compared with ordinary activated sludge,aerobic granular sludge has a dense structure,strong impact load resistance,high biomass,good sedimentation performance and ability to remove pollutants,so it has received widespread attention from researchers at home and abroad.However,most of the current research on aerobic granular sludge has been based on high or medium organic load sewage,and it has rarely been reported how to quickly achieve sludge granulation and maintain stable operation under low organic load conditions.Aiming at this situation,this paper reveals the influence mechanism of different carbon sources on the granulation process of sludge under low organic load by studying the formation and characteristics of good oxygen-free granular sludge,the removal effect of pollutants and the microbial community structure.In addition,the role of eDNA（Extracellular DNA）in the formation of aerobic granular sludge has not been determined,so it is necessary to explore the mechanism of action of eDNA in sludge granulation under low organic load.The results show that:（1）Under low organic loading conditions,the aerobic granular sludge formed in the reactor R1(0.6 kg COD m^-3·d^-1)with sodium acetate and sodium propionate as the mixed carbon source has a larger particle size and can reach about 1102μm at the maturity stage.And the formed aerobic granular sludge has a small particle size and can reach about 676μm at maturity in the reactor R2(0.6 kg COD m^-3·d^-1,the proportion of starch in the total carbon source gradually increased from 0%to 75%)with sodium acetate,sodium propionate and starch as mixed carbon sources.Compared with the two groups of SBR reactors,R2 began to appear granular sludge 3 days earlier than R1,but the disintegration of the particles appeared earlier,indicating that starch as an influent carbon source can promote the formation of granular sludge,but it is not conducive to the tightness and stability of granular sludge structure.（2）The removal efficiency of the two groups of reactors was compared and analyzed.After the system was stable,the removal rates of COD in R1 and R2 were stable at 90%and 85%,respectively.But the removal rates of total nitrogen and total phosphorus in R1 and R2 were all below 40%.The removal rate of ammonia nitrogen in R1 is basically stable above 95%,while the removal rate of ammonia nitrogen in R2fluctuates widely,which is basically between 40%and 100%.With the increasing starch content in the R2 influent carbon source（0%-25%-50%-75%）,the ammonia nitrogen removal rate in R2 gradually decreases.It shows that the removal rate of ammonia nitrogen is negatively correlated with the proportion of starch in the influent carbon source.（3）The spatial distribution of microbial populations of aerobic granular sludge was analyzed by fluorescence in situ hybridization（FISH）.The results showed that:In the inoculated sludge,the proportion of nitrifying bacteria is about 8%of the total microbial biomass,and the average percentage of nitrifying bacteria in the total number of bacteria[（AOB+NOB）/EUB]is（3.8±0.7）%and（4.1±1.0）%),respectively.When the experiment was carried out until the 85th day,the reactors in the two groups were stable.At this time,AOB and NOB in R1 granular sludge accounted for 3.2±0.9%and5.9±0.7%of the total bacteria,respectively.And AOB and NOB in R2 granular sludge accounted for 2.7±1.1%and 2.4±0.7%of the total bacteria,respectively.The proportion of nitrifying bacteria in R1 granular sludge is higher than that of R2,which is consistent with the result that the removal rate of ammonia nitrogen in R1 is higher than that of R2.（4）The high-throughput technology was used to compare and analyze the microbial community structure in the initial sludge and the mature granular sludge in the two reactors.The results show that:The dominant bacteria in the initial sludge S0and the aerobic granular sludges R1 and R2 in the two reactors are Proteobacteria and Bacteroidetes.The dominant strains in the initial sludge and R2 are Betaproteobacteria and Sphingobacteriia,and the dominant strains in R1 are Betaproteobacteria and Gammaproteobacteria.The dominant genus in the initial sludge was Saprospiraceae（10.29%）.After the granulation process was completed,the bacterial community structure changed significantly.The dominant genus of mature aerobic granular sludge R1 was transformed into Plasticicumulans（22.23%）and Zoogloea（15.96%）and Cytophagaceae（14.71%）;the dominant genus in R2 was transformed into Zoogloea（22.25%）and Runella（14.66%）.（5）The analysis results of the content change and distribution of eDNA in sludge granulation are as follows.In the early stage of operation of the two SBR reactors（0⁴2d）,the eDNA content of granular sludge in the two reactors increased from 4055.5 ng/g to over 10000 ng/g.However,after the system was stable,the content of eDNA was basically stable at around 10000 ng/g.Therefore,eDNA may have a certain promoting effect in the formation of aerobic granular sludge,but may not be related to the growth and maturation of the granules.According to the staining results of the sludge samples in the two reactors,the NOB and the regions with high concentrations of eDNA are more consistent,indicating that NOB may be one of the main sources of eDNA in sludge.