Study On The Cultivation Of Aerobic Granular Sludge And Its Life-cycle In Membrane Bio-reactor Under A Continuous-flowing Mode

Compared with the traditional activated sludge method,aerobic granular sludge technology has the advantages of high density,good settling performance and rich biodiversity.Membrane bioreactor can save floor space and has good solid-liquid separation characteristics.Combining of aerobic granular sludge and MBR process has attracted more and more attention.In this work,an integrated moving bed biofilm-membrane bioreactor?IMMB-MBR?was built by introducing a certain amount of bio-reactor into a membrane bioreactor?MBR?under a continuous-flowing mode to cultivate aerobic granular sludge?AGS?.The mechanism of formation of AGS in the IMMB-MBR was proposed.Meanwhile,the performance of bio-reactor reactor during the sludge granulation process was investigated,and the whole life-cycle of AGS?formation,growing,maturing and disintegration?was also summarized.Artificially prepared domestic sewage as the influent water,and its carbon,nitrogen,and phosphorus ratio is 100:5:1.A cylindrical bio-carrier made of polyethylene was introduced to bioreactor,and it can freely moving in the bio-reactor under the action of aeration.The inoculated sludge was taken from the secondary sedimentation tank of the local sewage plant and its concentration was diluted to 2000 mg/L mixed liquor suspended solids?MLSS?.The bioreactor was operated for 95 days under a continuous-flowing mode without no sludge discharge.The effluent index of the IMMB-MBR reactor were determined by the standard method every two days,including COD_cr,ammonia nitrogen,nitrite nitrogen and nitrate nitrogen.Meanwhile,the temperature and dissolved oxygen?DO?in the reactor were continuously monitored.To investigate the growth status of the bioreactor,periodically measure the concentration of suspended liquor?MLSS?,mixed liquor volatile suspended solids?MLVSS?,total attached biomass?TAB?,and total attached volatile biomass?TAVB?.Scanning electron microscope?SEM?and optical microscope were employed to observe AGS at different growth stages and biofilms attached to the bio-carrier.To reveal the microbial composition of the biofilm and AGS,high-throughput sequencing?HTS?was used to analyze the microbial community of AGS and biofilms.Laser confocal microscope?CLSM?explored the distribution of on extracellular polymeric substance?EPS?of AGS.The variation of trans-membrane pressure,sludge particle size and EPS of bioreactor during sludge granulation were also revealed.The results show that the bio-reactor achieved an excellent performance in removing COD_Cr?>90%?and TN?>85%?during the whole process.The MLSS is gradually rising.AS the biofilm on the bio-carrier is in a cyclic growth process,TAB curve shows a trend of rising first and then falling.On the 28th day,the formation of AGS can be observed with the naked eye,and the AGS gradually became smooth and compact along with time.The results of microscopy and SEM indicate that the formation of AGS is the self-aggregation of microorganisms in the reactor under the longitudinal circulation in the reactor.The results of HTS also proved that the attached biofilms at different stages and AGS were significantly different on the microbial community.The surface of AGS is covered with filamentous bacteria,and a large number of short rod-shaped bacteria are accumulated.During the granulation process,the particle size of the sludge in the reactor continues to increase.The sludge in the reactor is hydrophobic,which is conducive to the cultivation of granular sludge.The reactor showed more serious membrane fouling in the late stage of operation.In the later stages of bioreactor operation,it was also observed that the life cycle of AGS:formation,maturity,and disintegration at the same time.SEM observations revealed that the main reason for the increase in AGS particle size was filamentous bacteria entanglement.At the same time,observation on the disintegrated AGS revealed that the microbial community was completely different from the AGS during its formation and maturity.In summary,this topic provides another new idea for continuous flow mode cultivation of AGS in MBR,and also reveals the variation of the whole life cycle of AGS in MBR.