Preparation And Characterization Of Polyvinyl Alcohol Microsphere Pre-coated Dynamic Membranes And Its Application Research In Submerged Membrane Bioreactor

Membrane bioreactor(MBR) that combines conventional biological wastewater treatment using suspended biomass with membrane separation is one of the most promising processes used for wastewater treatment.The membrane f'dtration used in the bioreactor can retain higher activated sludge concentration,and then improve the effluent quality significantly and reduce footprint.However,the high cost of membranes and the severe fouling have been demonstrated as the major obstacle for the widespread application of MBR. Therefore,development of anti-fouling and low-cost filtration medium are of great significance for the wide application of MBR,especially in the developing countries.In recent years,pre-coated dynamic membranes prepared by low-cost filtration medium and pre-coated regent with anti-fouling characteristics have attracted more and more attention.In this thesis polyvinyl alcohol microsphere pre-coated dynamic membranes(PVA-MS/ PCDMs) were investigated detailedly.It was prepared when the PVA-MS of the pre-coated reagent deposited on the surface and entered the pores of porous substrate common filter cloth which was low-cost by means of circulatory filtration.The morphology and structure, approximate maximal separated aperture,water contact angle,pure water permeation resistance and relative Zeta potential of the membrance surface were determined,and antifouling characteristics were also studied.The results showed that:(1) Pure water permeation resistance of PVA-MS/PCDMs was between 0.64×10¹⁰ m^-1 and 3.84×10¹⁰ m^-1 and the approximate maximal separated aperture reached microfiltration level.(2) The hydrophilicity of PVA-MS/PCDMs increased with increasing PVA-MS precoated mass.The relative Zeta potential of the PVA-MS/PCDMs,which were measured using 0.01 mol·L^-1 KCl solution,pH 8±0.2 and at 20℃,reached a peak value of -36.4 mV with PVA-MS pre-coated mass of 15.7 g·m^-2 on the membrance surface.(3) PVA-MS/PCDMs presented dramatically high anti-fouling characteristics when the pre-coated PVA-MS mass was 23.9-61.9 g·m^-2.Three flat PVA-MS/PCDMs modules which the mass of PVA-MS per square meter membrane were 0,23.9 and 61.9 g respectively were submerged in the bioreactor and operated in parallel.Transmembrane pressure,turbidity rejection abilityes,flux recovery and pollutants removal efficiency of PVA-MS/PCDMs,sludge characteristics and membrane fouling process were studied detailedly.The results showed that:(1) PVA-MS/PCDMs presented dramatically high anti-fouling characteristics when the pre-coated PVA-MS mass was 61.9 g·m^-2.Its transmembrane pressure rose to 45 kPa after 55 days on the stable operation condition of 10.5 L·(m²Â·h)^-1 invariable flux,and its treating effect that the average of COD,NH₄⁺-N and turbidity were 22.0 mg·L^-1,0.38 mg·L^-1 and≤2.34 NTU in effluent was good on simulated domestic sewage.(2) Membrane fouling phenomenon of PVA-MS/PCDMs occurred with the increasing of sludge volume index(SVI),and the rate of membrane fouling was also influenced by concentration and viscosity of activated sludge intermixture and the extracellular polymeric substances(EPS) concentration in SMBR and so on.Especially,EPS would influence significantly on the rate of membrane fouling after the cake layer formed was compacted.(3) The flux of the fouled membrane could achieve 100%recovery nearly just after being cleaned by physical washing and chemical soaking,but a part of the PVA-MS pre-coated layer deposited on the surface and entered the pores of porous substrate common filter cloth was still destroyed.Moreover,the tracking results of membrane fouling process proved that PVA-MS pre-coated layer in the pores of porous substrate was as important as on the surface which impacted on the rate of membrane fouling at very early stage.