| Rural sewage treatment is related to the construction of ecological civilization and the building of livable environment in China.Decentralized sewage treatment technology provides a new way for rural sewage treatment.Gravity-driven membrane filtration(GDM)has a great prospect in the application of distributed wastewater treatment due to its highly efficient filtration performance for particulate matters,colloid and pathogens,as well as its low energy consumption and low maintenance.However,it has the limitations of low stable flux and poor pollutant removal efficiency when treating sewage.Pretreatment which improving influent quality is an effective method to improve filtration performance.Vermibiofilter(VBF)is an ecological decentralized wastewater treatment technology,having some characteristics of low construction and operation cost,low management and maintenance requirements,good pollutant removal effect and high resource utilization.Therefore,the technology of Vermibiofilter combined with Gravity-driven membrane filtration is constructed to deal with the decentralized wastewater.Taking the internal measures to regulate the efficiency of vermibiofilter(Reflux-Vermibiofilter,RVBF).and systematically analyzing the efficiency and membrane poulltion control mechanism of the process,which contributes the membrane filtration technology in the application of the decentralized wastewater treatment process.Firstly,the pollutant removal efficiency of Vermibiofilter with the internal reflux combined with Gravity-driven membrane filtration process(RVBF/GDM),Vermibiofilter combined with Gravity-driven membrane filtration process(VBF/GDM)and ordinary Biofilter combined with Gravity-driven membrane filtration process(BF/GDM)were discussed.The removal rate of organic matter(measured in DOC)of the three combined processes could reach more than 95%,and the hydrolysis of organic matter in the cake layer in the GDM could be effectively alleviated.However,the pretreatment could improve the TN removal rate of GDM process,and the introduction of earthworms could further enhance its removal efficiency.In particular,the nitrogen removal efficiency of RVBF/GDM process was 1.6 times than BF/GDM process.The TP removal rates of RVBF/GDM and VBF/GDM were 48.06±9.94% and 51.38±8.19%,respectively.In addition,earthworms were found to relieve the blockage of the filter and improve the effluent quality.Then,the permeability and the structure and composition of cake layer were analyzed,in order to summarize the mechanism that how the Vermibiofilter to regulate membrane pollution.The stability flux values of RVBF/GDM,VBF/GDM and BF/GDM were increased by 183.7%,101.9% and 28.4%,respectively,compared with the GDM control group.Therefore,the introduction of earthworms could effectively improve the stable flux and alleviate membrane pollution.VBF can reduce the organic content in the cake layer on the surface of the membrane in GDM process,including protein substances and polysaccharides in EPS,which could alleviate membrane pollution.Moreover,internal reflux measure could enhance the alleviation effect.At the same time,it could be seen that the reduction of membrane pollution was often accompanied by the surface roughness and internal looseness of the cake layer structure.It was also found that Vermibiofilter could improve the bioactivity,species abundance and diversity of microorganisms in the cake layer.And Vermibiofilter control the stability of GDM system is discussed: Vermibiofilter could improve the influent quality of GDM system,on the one hand,it could reduce the pollution on the membrane surface and deposition in membrane pore,on the other hand,it could change the microbial community structure and activity of the cake layer,so that strengthening the feeding effect,improving the structure of the cake layer,and alleviating the membrane fouling which could improve the stability flux of the GDM system.Finally,the feasibility of combined process and the resistance to shock of water quality were studied by changing the simulated wastewater into the actual sewage.It was found that the Vermibiofilter had a weak ability to resist shock of water quality,but the measure of internal reflux was helpful to enhance the system's ability to resist shock of water quality.The shock of water quality influenced the biological community,morphological structure and composition of the cake layer,resulting in the decrease of its effluent flux.However,internal reflux measures can reduce the impact to a certain extent and increase the effluent flux.In addition,by comparing the operation performance of the new membrane system(replacing with a new ultrafiltration membrane which had not yet formed bio-cake layer)and the old membrane system(continuing the experiment with the ultrafiltration membrane that had formed the bio-cake layer),it is found that the cake layer formed by low load water inflow was helpful to promote the stability of system flux.In this paper,the pollution removal efficiency and membrane pollution control mechanism of Vermibiofilter combined with Gravity-driven membrane filtration process were systematically studied.The research results can provide technical and theoretical support for the application of Gravity-driven membrane filtration combination in decentralized wastewater treatment with low energy consumption and maintenance. |
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