| Membrane Bioreactor(MBR)has been widely used in the field of membrane water treatment because of its high separation efficiency,good effluent quality,small footprint and high degree of automation.Polyvinylidene fluoride(PVDF)and polytetrafluoroethylene(PTFE),as emerging fluoropolymer membrane materials,have stronger resistance to oxidation,high temperature,strong acid and alkali than polysulfone and polyolefin membranes,but there are few mature operating cases in MBR,and the high cost of the materials also restricts their further popularization.However,while chemical cleaning agents remove membrane fouling,they also have a negative impact on the physicochemical properties of membrane materials,which will cause irreversible deterioration of membrane performance,called "membrane aging" phenomenon.Facing the raw water quality which may cause serious membrane fouling,the chemical cleaning ability of membrane material is particularly prominent.Membrane aging may be accompanied by deterioration of effluent water quality,increase of membrane fouling and shortening of membrane life,which are the key factors limiting the large-scale use of MBR,so it is necessary to conduct systematic research on the mechanism of membrane aging and chemical cleaning efficiency.Firstly,PVDF and PTFE hollow fiber microfiltration membrane products with thermotropic phase separation process and stretching process were used as research objects in this paper to simulate the chemical cleaning equivalents for removing membrane contamination caused by printing and dyeing wastewater in a 5-year cycle using Ct value method.The effects of sodium hypochlorite & sodium hydroxide and hydrochloric acid immersion cleaning on the physical and chemical properties of the two membrane materials at different Ct values were investigated by material characterization techniques,respectively.The results showed that during the aging process of alkaline washing,the PVDF membrane and PTFE membrane underwent saponification and defluorination reactions,respectively,which generated new chemical groups,and the destruction of the inherent structure of molecular chains led to a significant increase of the average pore size;during the aging process of acid washing,the decomposition of PVDF cast membrane additives and the dissolution and precipitation of hydrophilic additives of PTFE membrane caused the increase of the maximum pore size.The cleaning agent changes its physical and chemical properties by chemically reacting with the membrane matrix(membrane material + additives)and destroying the molecular structure of both membrane material and additives.During the simulated 5-year cleaning cycle,the chemical cleaning agents affected the hydrophilicity and surface potential of PVDF/PTFE membranes,and the mechanical properties of membrane filaments decreased.aging of PVDF and PTFE membranes occurred mainly in the long-term sodium hypochlorite & sodium hydroxide cleaning phase,and PTFE had better anti-aging performance.Secondly,PVDF/PTFE membranes with different acid-base cleaning Ct values were selected for this paper to investigate the retention characteristics of aging membranes for model organic pollutants(humic acid,sodium alginate,bovine serum albumin)and the evolution of membrane contamination trends in treating actual water bodies(surface water,municipal wastewater),and to analyze the effects of membrane physical and chemical property changes on their own permeability performance.The results showed that the retention rate of both PVDF and PTFE membranes decreased significantly with the increase of maximum pore size after cleaning with sodium hypochlorite & hydroxide and hydrochloric acid,and the pore size and hydrophilicity were the main factors causing the membrane flux change.During the simulated5-year cleaning cycle,membrane fluxes increased by 35.52% and 47.56% for PVDF and PTFE,respectively,after alkaline washing,and by 14.83% and-8.54%,respectively,after acid washing.The retention of bovine serum albumin was highest for both membranes,followed by sodium alginate and lowest by humic acid.The smaller the pore size,the higher the retention rate;the greater the increase in pore size,the greater the decrease in retention rate.the MBR operation results showed that the increase in membrane pore size had a suppressive effect on the growth of transmembrane pressure difference,and the membrane fouling cycle was prolonged;the increase in hydrophobicity made the membrane more susceptible to fouling,and the membrane fouling cycle was shortened.Finally,this paper uses the PVDF/PTFE membrane filament samples that have been severely contaminated in the pilot site,using sodium hypochlorite & sodium hydroxide cleaning alone,hydrochloric acid cleaning alone,and sodium hypochlorite & sodium hydroxide combined with hydrochloric acid cleaning in three modes,for a comprehensive elaboration and analysis of the cleaning efficiency of removing membrane fouling.The results show that the membrane fouling caused by printing and dyeing wastewater is mainly composed of organic matter,iron and aluminum oxides and inorganic salt scale.The combined cleaning mode achieved a better cleaning effect than the individual cleaning mode,and the strong dispersion of sodium hypochlorite & sodium hydroxide combined with the strong permeability of hydrochloric acid effectively removed the filter cake layer on the membrane surface,which basically restored the fouled membrane to the new membrane flux level.In addition,according to the test results,it was found that the PVDF membrane mainly showed aging phenomenon of pore size increase while the PTFE membrane mainly showed aging phenomenon of hydrophilicity decrease during the membrane module treatment of printing and dyeing wastewater and membrane cleaning. |
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