Study On The Performance And Mechanism Of Anaerobic Dynamic Membrane Bioreactor For Sludge Anaerobic Digestion

With the development of urbanization and industrial modernization,the total amount of domestic sewage and industrial wastewater increased year by year.wastewater treatment plants（WWTPs）is a vital infrastructure to treat domestic sewage.The development of wastewater treatment technology has made great contributions to the control of water pollution and the realization of emission reduction targets.However,as the main by-product of wastewater treatment,the treatment and disposal of waste activated sludge（WAS）has become a serious burden of WWTPs.Through conventional anaerobic digestion（CAD）process of sludge,part of organic matter can be converted into methane to realize energy recovery,and various pathogenic bacteria and parasites can be inactivated,achieving sludge stabilization,volume reduction and harmless disposal.Anaerobic membrane bioreactor is a coupling process of anaerobic digestion and membrane separation technology,improving the efficiency of anaerobic digestion by using the advantages of membrane separation technology.Membrane separation decouples the solid retention time（SRT）and hydraulic retention time（HRT）,realizing successful treatment efficiency at relatively short HRT.The interception effect of membrane on microorganism can realize the enrichment of anaerobic microorganism in the reactor,enhance the substrate utilization efficiency and anaerobic digestion effect.However,the membrane fouling remails as a great challegen due to the high solid concentration and complex compositions,limiting the application of anaerobic membrane bioreactor for slude digestion.In the present work,the key bottleneck of anaerobic membrane bioreactor was studied,and the use of coarsh mesh for constructing anaerobic dynamic membrane bioreactor（AnDMBR）was proposed,utilizing dynamic membrane to make HRT-SRT decoupling and to control membrane fouling.The AnDMBR technology was applied to treatment of excess sludge.First,using laboratory-scale AnDMBR,the supporting materials,operating conditions and dynamic membrane formation rules were preliminarily explored.Then,the pilot-scale AnDMBR was constructed to optimize the pipe layout of the membrane tank,the accuracy and effectiveness of of biogas cycle for the scouring area were improved.Furthermore,the digestion effect,biogas resource recovery,dynamic membrane operation characteristics,sludge properties and filtrate characteristics of the AnDMBR under different HRT and SRT conditions were further studied.The enhanced digestion mechanism was revealed by analyzing the microbial community structure.Finally,the technical and economic performance and carbon emission level of AnDMBR and CAD were compared.The operating characteristics of stainless steel mesh,nylon mesh and polyester mesh of 38μm,58μm and 75μm were compared in laboratory scale AnDMBR.The running time of polyester mesh with three kinds of pore diameter under all membrane fluxs was relatively higher than stainless steel mesh and nylon mesh.At the condition of high flux of 10 L/（m²·h）,the running time of polyester mesh with three kinds of pore diameter was more than 10 days,which was obviously better than stainless steel mesh and nylon mesh.The running time of 38μm polyester mesh was close to that of 58μm and 75μm polyester mesh,therefore the 38μm polyester mesh was selected for follow-up study.According to formation characteristics of dynamic membrane,with the accumulation and thickening of dynamic membrane,the suspended particles size of effluent decreased,which reflected the improvement of filtration effect acaused by dynamic membrane formation process.The dynamic membrane had a stable filtration effect under the trans-membrane pressure（TMP）of 5 k Pa.When TMP reached 30 k Pa,it completely depended on the dynamic membrane to achieve filtration,while the suspended solids concentration in the effluent significantly reduced.The membrane tank and biogas circulation system were optimized by setting up a pilot-scale AnDMBR device,and the start-up characteristics of pilot-scale AnDMBR were studied.The circulation system is divided into fixed and detachable type,and the detachable type can be divided into double mian pipe with multipoint air inlet and double mian pipe with single point air inlet.By comparing the running time under different biogas circulation intensities,detachable type and double mian pipe with single point air inlet were selected as the biogas circulation system in membrane tank of pilot-scale AnDMBR from the perspective of experimental research and engineering application.Based on the construction of the pilot-scale AnDMBR and the optimization of operation parameters,the characteristics of the start-up stage were studied.The results showed that the dynamic membrane filtration and digestion effect were stable.The pilot-scale AnDMBR had the potential for long-term operation.On the basis of stable operation of pilot-scale AnDMBR,the operation characteristics were further analyzed by adjusting HRT or SRT.When HRT is equal to SRT,the effluent was stopped,and rector was operated at CAD mode.After reaching stable,HRT was adjusted to 20 days and 10 days,TMP and effluent SS were basically maintained at 2.0±1.5 k Pa and 5.5±1.6 g/L.The sludge concentrations in anaerobic tank were 18.0 g/L,23.7 g/L and 36.5 g/L at HRT of 30,20 and 10 days,respectively.The biogas production and VS reduction rate improved with the decrease of HRT.Compared with HRT of 30 days,they increased by 15%and 18%at HRT of 10 days,respectively.When SRT was adjusted to 30,40 and 50 days,the sludge concentrations in anaerobic tank were 36.5 g/L,41.6 g/L and 51.0 g/L,respectively.The biogas production and VS reduction rate improved with the extension of SRT.Compared with SRT of 30 days,the VS reduction rate and biogas production at SRT of 50 days increased by 38.0%and 33.8%,respectively,and methane proportion increased from69.7 vol%to 77.5 vol%.The concentration of dissolved organic matter（DOM）decreased with the decrease of HRT or the extension of SRT,suggesting that the degradation efficiency increased significantly.The humic acid content increased with the decrease of HRT or the extension of SRT,indicating that the sludge stabilization was improved.In order to explore the changes of HRT and SRT on the microbial community during the sludge thickening and improvement of anaerobic digestion effect,the microbial community structure under two operation conditions was further studied.In fixed SRT condition,as the decrease of HRT,the abundance of Chloroflexi increased,which could effectively utilize refractory organic matter,thereby improving the hydrolysis efficiency and VS reduction rate.Firmicutes and Actinobacteriota are mianly acidogenic bacteria,their abundance decreased to 5.4%and 4.4%at HRT of 10 days,respectively.While the content of dissolved organic matter and acetic acid decreased with the decrease of HRT,the overall digestion effect of AnDMBR improved,indicating that increasing organic load did not cause acid accumulation risk,but the reduction of their abundance was not conducive to the acetoclastic methanogenesis.Spirochaetota is related to the production of intermediates such as acetic acid,ethanol,H₂ and CO₂.The abundance of Spirochaetota increased with the decrease of HRT,which was beneficial to the hydrogenotrophic methanogenesis.Methanosaeta is a typical acetoclastic methanobacteria,the decrease of its abundance indicated that the acetoclastic methanogenesis significantly reduced.Methanolinea,Methanobcterium and Methanobrevibacter belong to hydrogenotrophic methanogens,and their abundance increased with the decrease of HRT.The abundance of Candidatus＿Methanofastidiosum increased with the decrease of HRT,which were methylotrophic methanobacteria.The system gradually changed from the dominant acetoclastic methanogenesis to multi-path methanogens,which indicated that the decoupling of HRT and SRT in the AnDMBR system promoted the digestion efficacy.In fixed HRT condition,Chloroflexi,as hydrolytic bacteria,became the dominant bacterial phylum,which could effectively utilize refractory organic matter,and was an important reason for the improvement of hydrolysis efficiency.The abundance of Chloroflexi increased with the extension of SRT,which was conducive to the improvement of VS reduction rate,so that the bio-refractory macromolecular substrate could be hydrolyzed into short chain volatile fatty acids,H₂ and other intermediates,which increased the hydrolytic bacteria and hydrotrophic methanogens.The increase of SRT further strengthened the symbiotic relationship,and H₂ also promoted the abundance of methylotrophic methanobacteria.With the extension of SRT,the diversity of archaea increased,and the abundance of acetoclastic methanogens（Methanosaeta）decreased from 50.7%to 29.5%.The abundance of hydrogenotrophic methanogens（Methanobacterium and Methanobrevibacter）increased from 20.0%to 29.5%,The abundance of methylotrophic methanogens（Candidatus＿Methanofastidiosum）increased from 19.4%to 33.1%.The system changed from the dominant acetoclastic methanogenesis to the coexistence of multiple methanogenesis,which promoted the digestion efficacy.This study comprehensively analyzes the characteristics of AnDMBR system from the perspective of technical economy.The methane recovery energy of AnDMBR is2476.2 k Wh/t DS,which is 32.2%higher than that of CAD.The net energy output of AnDMBR treating per unit DS mass was 50.0%higher than that of CAD.The overall carbon emission of AnDMBR was-5.3 kg CO₂/t DS,the overall carbon emission of CAD after covering compensation energy of coal-fired power generation was 210.8 kg CO₂/t DS.Compared with CAD,AnDMBR has obvious advantages in reducing carbon emission.