Fabrication, Characterization And Performance Of Anti-fouling Dynamic Membrane For Filter Cloth Membrane Bioreactors

Membrane bioreactors(MBRs) are the most promising processes used for wastewater treatment and water reclamation.The membrane filtration used in the bioreactor can retain the sludge flocs,and then improve the effluent quality significantly.However,the high cost of membranes and the severe fouling has been demonstrated as the major obstacle for the widespread application of MBRs.Therefore,development of anti-fouling and low-cost filtration medium is of great significance for the wide application of MBRs,especially in the developing countries.Recent years,precoating the low-coat filtration medium(i.e., non-woven,mesh and filter cloth) by using some special particles(i.e.,powder activated carbon) has been proved as an interesting and potential alternative for MBRs.The powder activated carbon(PAC) with particle size of 22.8μm was used as precoating reageat to develop a dynamic membrane on the commercially used filter cloth.The impacts of the developed dynamic membrane thickness on membrane resistance and rejection ability were studied by measuring the distilled water permeation and the rejection of standard turbidity solution.The results indicate that the optimum thickness of the precoated PAC daynamic membrane was determined to be 0.3±0.05 mm.But,it was found that the aeration used in the submerged MBR could damage the precoated PAC dynamic membrane.As MLSS was lower than 8000 mg·L^-1,the activated sludge suspension featured non-newtonian fluid.In addition,according to the theory of boundary layer,the steady aeration intensity was estimated and optimized in this study.The results indicate that the optimized aeration intensity can guarantee the steady and sustainable operation of the precoated dynamic membrane bioreactor.Compared with self-forming dynamic membrane and hollow fiber membrane,it was found that the precoated dynamic membrane(PDM) could keep high permeation,the TMP of PDM rose up to 42 kPa after 43 days of the stable operation of PDMBR,and the removal efficiency of effluent COD(97.09%) and NH₄⁺-N(76.13%) were as good as traditional hollow membrane bioreactor(THFMBR).Furthermore,the flux of the fouled membrane could recover totally just after being cleaned by brushing,and it didn't consume any chemical reagent.Moreover,it was also found that PDM could prevent the biomass contaminations diffusing from the surface to the internal.The application of filter cloth is much attractive since it commercial price is only 25 RMB per square meter.Lastly,the PAC precoated filter cloth will become a more and more reliable and cost-saving when some more hydrophilic substances(i.e.,YiO2,SiO2) can be introduced into PAC precoated filter cloth.Since the major membrane foulants of EPS/SMP featured higher hydrophobicity and negative charge,a novel coating reagent,polyvinyl alcohol microsphere(PVA-MS),was prepared and was coated onto the surface of filter cloth.The PVA-MS was prepared by PVA and glutaraldehyde after acetal reaction.The PVA-MS prepared featured an excellent dispersal with a mean particle size of 1.2±0.1μm.The analysis of FTIR and zeta potential shows that the PVA-MS had several solar groups and behaved negative charge.It also was found that the pH value had significant influence on zeta potential and at pH=11 there was a lowest zeta potential of-87.25 mV.Subsequently,Three dynamic membranes of symmetric Standard blockage filtration type without filter cake,asymmetric Standard blockage filtration type,symmetric full blockage filtration type were prepared successfully by using self-assembly coating and pre-coating methods.It shows that the water contact angle of the coated PVA-MS membrane was determined as 0,and their filtration resistances were calculated to be 1.02×10¹⁰ m^-1,2.76×10¹⁰ m^-1,1.07×10¹¹ m^-1.The zeta potential of the dynamic membrane prepared was also monitored by measuring streaming potential with 0.01 mol·L^-1 KCl solution,and the zeta potentials were calculated to be -26.2 mV,-35.3 mV and -65.1 mV after corrected with membrane resistance. The coated PVA-MS on the filter cloth was found to be effective in the rejection of small particles.The symmetric Standard blockage filtration type without filter cake,asymmetric Standard blockage filtration type,symmetric full blockage filtration type could reject 78%, 93.7%and 98.9%turbidity of the sludge supernatant with 21.34 NTU.Their rejection abilityes on TOC were 6.56%,37.16%and 48.63%,respectively,when the sludge supernatant with 18.3 mg·L^-1 TOC was filtered.The results suggest that chemical cleaning can eliminate the irreversible fouling of uncoated filter cloth,self-assembly coated filter cloth and pre-coated cloth filter.The efficiency of physical cleaning would increase with increasing coated MS on the filter cloth. By comparing,the self-assembly coated MS are not readily shatter even after frequent chemical cleaning;however,regarding the pre-coated MS,physical cleaning could eliminate the irreversible fouling completely,but it needs a repeated precoating to maintain the MS layer on the surface of the filter cloth.The cross flow shear stress induced by aeration is an effective approach to control membrane fouling.In this study,a novel flat sheet membrane module,ladder-type flat membrane structure,was developed.In this way,the aeration bubbles near the membrane surface can be held,at the same time the intensity and times of elastic collision between bubbles and membrane surface will be increased.According to the mathematics model of collision between bubble and vertical flat developed by Vries,the inclination angle of the ladder-type was calculated and optimized by using computer iterative calculation technology. In view of the real application of SMBR,the design of membrane and aeration was optimized as following:the interval distance of membrane modules is 8～15 mm,and aeration will be operated at 5～7 mm among membrane modules,besides the optimal design angle of trapeziform membrane is 1.7°～2.5°.To confirm the role of ladder-type flat membrane structure in fouling control,a vertical and a ladder-type flat PVA-MS module were submerged in the bioreactor and operated in parallel.By comparing,it confirms that the ladder-type flat module can control membrane fouling effectively,but after long-term operation the TMP increased suddenly due to the compress of the cake layer.The effluent quality of ladder-type PVA-MS module was determined as turbidity 1.34 NTU,and the removal efficiency of effluent COD and NH₄⁺-N were 90%and 91.5%respectively.