Study On The Filtration Properties And Stabilization Mechanism Of Biofilm-based Membranes In Membrane Bioreactors

At present,China is vigorously implementing the strategy of rural revitalization and promoting the construction of beautiful countryside.And the problem of rural decentralized sewage treatment is urgently to be solved.Membrane bioreactor（MBR）,combining membrane separation process with biological treatment technology,has high removal efficiency,and is easy to realize equipment integration and modularization,satisfying the requirements of rural sewage treatment,but severe membrane fouling in MBR requires frequent membrane cleaning and membrane replacement,restricting the wide application of MBR.In MBR,the biofilm forming on the membrane surface exhibits a continuous and permeable structure,which can act as a"secondary membrane"to separate solids and improve rejection.Meanwhile,microbial activity of the biofilm can help to reduce membrane fouling.Consequently,microfiltration membrane may be unnecessary in MBR,and the biofilm can be used as a membrane to achieve solid-liquid separation in MBR.Meanwhile microbial activity can control membrane fouling.In this thesis,40,70,100μm nylon mesh were used to support the biofilm to form a continuous and dense structure,namely biofilm-based membrane（BM）.The growth of biofilm on nylon mesh was observed to explore the characteristics of BM formation.The irregular growth of the biofilm on the mesh surface was mainly responsible for the formation of the BM.Although smaller nylon mesh pore size led to faster BM formation,the final BM thickness of different pore diameter was nearly the same and about 60μm.Then the filtration performance and mechanical properties of BM were investigated,and finally it was applied to the MBR for100 d filtration to explore the characteristics and mechanisms of self-stabilization.The BM showed good water permeability.The water flux was 4000 LMH·k Pa^-1.Meanwhile,the BM can effectively remove particles larger than 10μm,and the rejection rate of the MBR bulk solution was as high as 95%.The smaller the pore size of the nylon mesh,the higher the rejection rate.The BM also had good mechanical properties,enabling them to undergo high-flux（2000 LMH）filtration and intensive aeration（0.15 m/s）to keep complete structure and good separation performance.In the MBR,the BM had good effluent quality,with the average effluent turbidity as low as 0.6 NTU and removal rates of COD and NH₄⁺-N reaching 90%and 99%,which was equivalent to the polymeric microfiltration membrane.More importantly,the BM could maintain filtration resistance stable at a low transmembrane pressure of 0.3 k Pa during 100 d filtration,which solved the membrane fouling problem.The BM ensured that the SMP and sludge particles were not completely rejected,thereby reducing the deposition of foulants.In addition,the microorganisms in the BM could also remove EPS by biodegradation.Meanwhile,the eukaryotes induced a heterogeneous structure with cavities and channels through predation（Roughness:17.9μm,Porosity:44%）,greatly reducing the hydraulic resistance.Consequently,the filtration resistance could be stabilized without outside interference.In conclusion,the BM not only has good water treatment efficiency similar to microfiltration membrane,but also makes use of intrinsic structure and microbial activity to avoid the continuous accumulation of foulants and induce mushroom-like structure,self-stabilizing filtration resistance and overcoming membrane fouling problem.Therefore,MBR with biofilm-based membrane can achieve low maintenance,which is of great significance for rural sewage treatment.