Formation And Control Of By-products During The Chemical Cleaning Of Ultrafiltration Fouled Membranes

As an efficient physical separation technology,the ultrafiltration process has broad application prospects in drinking water advanced treatment.However,membrane fouling seriously restricts the further development of ultrafiltration technology.As a cheap oxidant,sodium hypochlorite(NaClO)is commonly used in the chemical cleaning process of membrane fouling.At present,the common membrane fouling substances are also the important precursors of disinfection by-products in the disinfection process of water treatment.Therefore,halogenated toxic by-products will also be generated in the NaClO cleaning process,which may result in secondary pollution problems.In this paper,the formation rule of halogenated toxic by-products in NaClO cleaning of membrane fouled by typical foulants was studied,and the formation mechanism of by-products was analyzed through the characterization of pollutants and membrane materials.The composition of toxic substances in the residual cleaning solution after membrane cleaning was revealed,which provided theoretical support for its subsequent treatment.On this basis,heat-activated peroxydisulfate(heat/PDS),the combined process of NaClO and peroxide were applied to remove membrane fouling and reduce the formation of toxic by-products.The efficiency and mechanism of these cleaning methods were synthetically investigated.(1)The performance of NaClO off-line cleaning on membrane fouling control were investigated by choosing humic acid as fouling materials.The effects of water matrices and cleaning process parameters on the formation of by-products were studied.The results showed that NaClO cleaning could effectively restore the membrane flux and remove pollutants from the membrane surface.C=O,C=C,–COOH and O–H in humic acid were the main functional groups that adhered to the membrane surface and caused the formation of by-products.Trichloromethane(TCM)and trichloroacetic acid(TCAA)were the main by-products generated during the cleaning,whose formation were enhanced with the increase of influent pollutants'concentration,Na OCl dosage and reaction temperature,while the generation of some nitrogenous halogenated by-products were inhibited with the increase of NaClO dosage.The elevation of solution p H promoted the generation of TCM,which is contrary to the continuously decrease of TCAA.Most by-products were favored during the cleaning of membranes with medium MWCO(3 k Da and 5 k Da).The presence of Ca²⁺with high concentration in feed water might reduce the formation of some selected chlorinated by-products.(2)The performance of NaClO off-line cleaning on membrane fouling control were investigated by choosing algae as fouling materials.The effects of water matrices and cleaning process parameters on the formation of by-products were also studied.The results showed that ultrafiltration membrane had excellent retention performance for algal cells,which would accumulate on the membrane surface and result in serious membrane fouling.NaClO cleaning could effectively remove the algal foulants on the membrane surface.Organic substances such as protein,amino acid and polysaccharide in algal cells which were attached to the membrane surface were the main precursors of by-products.The formation of most by-products was elevated with the increase of NaClO dosage,reaction temperature and Ca²⁺concentration.The cleaning of algal cell-fouled membrane generated fewer carbonaceous halogenated by-products and trichloronitromethane(TCNM),but larger amounts of haloacetonitrile(HANs)than that of HA-fouled membrane.(3)In view of the fact that toxic halogenated by-products generated during NaClO cleaning process,heat/PDS was developed for membrane cleaning.The performance of heat/PDS on different membrane fouling control was investigated,which was compared with other common cleaning agents.The results showed that the flux recovery ratio and resistance removal of humic acid-fouled membrane were close to 100%after treating by heat/PDS for 2 hours at the temperature of 50 ^oC and PDS dosage of 1 m M.Heat/PDS cleaning could improve the hydrophilicity of humic acid,reduce the content of fluorescent substances,thereby enhancing the removal of foulants from the membrane surface.Heat/PDS cleaning with proper dosage of PDS had limited influence on the membrane fouling behavior and mechanism.The cleaning performance could be enhanced with the increase of cleaning time,reaction temperature and p H,which might result in the excessive recovery of membrane flux.Heat/PDS showed comparable cleaning efficiency to NaClO,which could avoid the formation of halogenated by-products.Furthermore,heat/PDS can effectively remove membrane fouling caused by sodium alginate,humic acid-bovine serum protein-sodium alginate mixed organic compounds and algal organic compounds,which can also remove micropollutants trapped by the membrane.(4)Both NaClO and heat/PDS can effectively remove membrane fouling,but NaClO cleaning could produce halogenated toxic by-products,while the cost of heat/PDS for membrane cleaning was relatively high.Based on the comprehensive investigation of the advantages and disadvantages of the above two cleaning methods,the combined cleaning process of NaClO and peroxides was further developed for membrane cleaning.The results showed that the post process of NaClO had more advantages in controlling the generation of halogenated by-products than the pre-process of NaClO.All the three combined process with NaClO at postposition can effectively restore membrane flux,and PDS-NaClO cleaning resulted in the lowest generation of by-products,showing that PDS-NaClO was more suitable for membrane cleaning.The increase of reaction time and temperature in PDS-NaClO cleaning process could lead to the increase of some by-products.Overall,a large number of halogenated toxic by-products can be generated during the NaClO cleaning of membrane fouled by typical foulants,and the formation pattern is closely related to the properties of foulants,water matrices and cleaning process parameters.Thus,the toxicity of cleaning solution should be concerned.Heat/PDS showed good performance on membrane cleaning,and the combination of PDS and NaClO can both effectively restore membrane flux and control the generation of by-products.