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Sodium hypochlorite(NaClO)solution has been widely used to restore the flux of fouled membrane.The test of membrane flux herein also suggests that humic acid-fouled membranes could be efficiently cleaned by blending in NaClO solution at off-line mode.However,toxic halogenated by-products were found to form during the chemical cleaning of membrane fouling by NaClO solution.All the halogenated by-products except TCE and MCAA rose significantly with the increase of HA concentration.With prolonged chemical-cleaning time,the concentrations of formed chloral hydrate(CH)and trichloroacetic acid(TCAA)rose monotonically,while chloroform(TCM),trichloroacetonitrile(TCAN),trichloroethylene(TCE),trichloronitromethane(TCNM)followed an obvious increasing and then decreasing pattern.The formed TCM,TCAA and 1,1-dichloro-2-propanone(1,1-DCP)show obvious positive dependences on NaClO concentration,which was contrary to the formation of dichloroacetonitrile(DCAN)and 1,1,1-trichloro-2-propanone(TCP).With increasing the blending pH,both TCM and CH increased apparently,while haloacetic acids(HAAs)and haloacetonitriles(HANs)decreased continuously.Overall,the relatively stable halogenated by-products were favored at higher temperature compared to the unstable ones in the temperature range of 15-35℃.1,1-DCP,TCNM,TCM and TCAA were found to be easier to form when using the membranes with medium molecular weight cutoff(3 kDa and 5 kDa).The presence of calcium ions(Ca2+)also affected the formation of halogenated by-products as a result that lots of them were significantly reduced at a Ca2+concentration of 10 mg/L.This study suggests that the remained NaClO solution after treatment of fouled membrane would contain some toxic by-products and should be carefully treated.