Analysis Of Microstructure And Micro Mass Transfer Of Anaerobic Granular Sludge

Mass transfer is the key factor restricting the biological activity of granular sludge.The microstructure and macro movement of granular sludge are the two major factors affecting its mass transfer behavior.In this study,the mass transfer behavior in granular sludge was analyzed from the perspective of granular sludge structure and motion state,a comprehensive mass transfer model was constructed,and the reasons for the decrease of biological activity of granular sludge caused by adhesive adhesion were revealed from the perspective of mass transfer.The main research contents and conclusions are as follows:(1)The influence mechanism of granular sludge microstructure and macro movement on its internal mass transfer was studied.Based on the different states of granular sludge in the reactor,a pressure oscillation bioactivity test system was designed by pressurizing with nitrogen.The movement state of granular sludge in the reactor is simulated.Using the pressure oscillation bioactivity test system,the mass transfer law of calcified/uncalcified sludge in the simulated fluidization state was compared and studied.Combined with the observation results of the microstructure of granular sludge by scanning electron microscope and microcomputer tomography,the mass transfer mode and its influencing factors in granular sludge were determined.The results show that there are three mass transfer modes in granular sludge,which are molecular diffusion,convective mass transfer and seepage according to their contributions.The mass transfer driving forces are concentration gradient,external hydraulic fluctuation caused by granule fluidization and periodic internal pressure fluctuation caused by biogas accumulation.Calcification will block the orifice structure of sludge,reduce mass transfer area and inhibit biological activity.Microbial autolysis caused by the shortage of substrate supply leads to the cavitation of calcified granular sludge,and the cavity structure is the premise of convective mass transfer.(2)A mass transfer model of granular sludge based on microstructure and macro movement was constructed.The orifice and cavity structure of granular sludge were observed by scanning electron microscope and microcomputer tomography.The size exclusion chromatography was used to classify the pores of granular sludge,and the distribution coefficient was used to solve the porosity of granular sludge.The pressure oscillation biological activity test system is used to quantitatively analyze the activity of granular sludge in suspension,fluidization and sedimentation states.According to the ideal gas state equation and Fick’s law,a mass transfer model which can describe the influence of sludge microstructure and its macro movement state on the internal mass transfer behavior of granular sludge is constructed for the first time,and the accurate prediction of granular sludge biological activity is realized.(3)The inhibitory effect of adhesive adsorption on the biological activity of granular sludge was analyzed by mass transfer model.The effect of adhesive adhesion on the biological activity of granular sludge was studied by automatic methane test system and pressure oscillation biological activity test system.The internal structure of granular sludge was characterized by transmission electron microscope,scanning electron microscope and steric hindrance chromatography.It is confirmed that the adhesive is adsorbed on the surface pores of granular sludge,which can inhibit the molecular diffusion of sludge from reducing the diffusion area and diffusion rate.Due to the weakening of molecular diffusion that the rate and concentration of matrix entering the pores drop sharply.Considering the process digestion of microorganisms in the pores,the concentration of matrix reaching the cavity structure is almost 0 mg/L,and finally the phenomenon of convective mass transfer disappears.