Cleaning Efficiency And Formation Of By-products During Hypochlorite Cleaning Of Bacteria Fouling Membrane

Ultrafiltration（UF）has an extensive application in water treatment.However,membrane fouling is a critical factor restricting its further development.Sodium hypochlorite（NaClO）is extensively used to remove membrane fouling and recover membrane flux.However,a large number of halogenated toxic by-products will generate during the chemical cleaning of organic and biofilm fouling using NaClO.At present,not only the composition of toxic substances in the residual cleaning solution after chemical cleaning with NaClO is still unclear,but the corresponding treatment technology is faultiness.The discharge of residual cleaning solution without proper disposal may seriously impair the environment and human health.Bacteria in natural water is the primary foulant that form the biofouling of UF membrane,but there are few reports on the generation of toxic by-products during chemical cleaning of bacteriogenic membrane fouling with NaClO.In order to solve these problems,this paper selected Escherichia coli（E.coli）as the model foulant.The cell properties were analyzed,and the variations of membrane structure and properties in NaClO cleaning process of bacteriogenic membrane fouling were also studied.Moreover,the factors affecting the cleaning efficiency and the formation of halogenated by-products during NaClO cleaning were investigated.The composition of toxic substances in the residual cleaning solution was revealed to provide theoretical support for subsequent treatment.（1）E.coli cells mainly contained protein substances and soluble metabolites,providing a large number of precursors for the generation of halogenated by-products.The functional groups and elements of membrane surface changed after the filtration of the E.coli cells containing water.NaClO cleaning could effectively remove E.coli cells from the membrane surface and recover the membrane flux.The hydrophilicity and element composition of membrane surface changed during the cleaning process.（2）Temperature,pH,reaction time,NaClO dosage,E.coli cells concentration and Ca²⁺concentration affected the efficiency of NaClO cleaning.NaClO concentration in the range of 50～400 mg/L could effectively restore the membrane flux,while the membrane flux after cleaning with high concentration NaClO（1000 mg/L）was higher than the initial value.The membrane flux after NaClO cleaning could effectively recover to over 90%of the initial membrane flux with the concentration of E.coli cells increased from 10 mg/L to 40 mg/L in feed water.Temperature could inhibit the cleaning efficiency of NaClO.The increase of solution pH promoted the improvement of cleaning efficiency,but the membrane flux after cleaning under alkaline condition was significantly higher than the initial value.In the reaction time range of 2～24 h,the membrane flux was effectively recovered.The change of Ca²⁺concentration had limited effect on the cleaning efficiency of membrane fouling,and the recovery rate of membrane flux reached more than 90%.（3）Haloacetic acids（HAAs）,trichloromethane（TCM）,chloral hydrate（CH）and dichloroacetonitrile（DCAN）were the main halogenated by-products generated during NaClO cleaning process.The formation of carbonaceous by-products and nitrogenous by-products were enhanced with the increase of reaction temperature,E.coli cells concentration,NaClO dosage,reaction time and Ca²⁺concentration.However,the concentrations of dichloroacetone（1,1-DCP）and dichloroacetonitrile（DCAN）were significantly decreased at high temperature（35℃）and high concentration of NaClO,respectively.（4）The effect of pH on the formation of different halogenated by-products was significant different.For carbonaceous by-products,the concentrations of trichloromethane（TCM）,trichloro ethylene（TCE）,monochloroacetic acid（MCAA）and dichloroacetic acid（DCAA）increased first and then decreased with the rise of pH.While the concentration of other carbonaceous by-products increased continuously with the rise of pH.For nitrogenous by-products,the concentration of trichloronitromethane（TCNM）increased with the rise of pH,while the concentration of dichloroacetonitrile（DCAN）and trichloroacetonitrile（TCAN）also increased first and then decreased with the increase of pH,and reached the maximum value when pH was 7 and 5,respectively.In conclusion,a large number of halogenated by-products are generated during NaClO cleaning bacteriogenic membrane fouling.The cleaning efficiency and the formation of halogenates are affected by the characteristics of foulants and the operating parameters of cleaning process.In order to avoid the secondary pollution caused by the halogen in the residual cleaning solution,the treatment of residual cleaning solution should be paid attention to.