| Membrane bioreactor is one of the most widely used membrane technologies and has been widely applied in wastewater treatment.However,ultrafiltration fouling caused by secondary effluent organic matters has severely restricted the development of MBR,especially the irreversible fouling caused by proteins.Among all the fouling control techniques,chemical cleaning is widely used worldwide.As a representative of the oxidant cleaning agent,NaClO has a good effect on organic fouling and biofouling control of ultrafiltration membrane in MBR.However,traditional chemical cleaning always employ excessive chemical agents,which would pose great challenge on membrane matrix and microorganisms in MBR.In recent years,novel protocols employing low dose oxidants have been proposed,such as chemically enhanced back flush.However,systematical evaluation of the cleaning efficiency with low dose oxidants has not been reported.In this study,bovine serum albumin(BSA)was chosen as a model foulant.Traditional flux measurements were carried out by two commercial PES and PVDF membranes with negative charges.The oxidation mechanism between NaClO and BSA and its influence on the effect of membrane fouling control were mainly explored.In addition,another type of oxidant H2O2 and another model contaminant lysozyme with opposite electrical properties to BSA were also employed in order to explore the typ e of oxidants and the nature of foulants on the effects of ultrafiltration membrane fouling control.In this study,the reaction mechanism between NaClO and BSA was analyzed by the variations of physicochemical properties,functional groups,fluorescent properties and molecular weight before and after oxidation.Membrane flux decline and permeability variations were also employed to evaluate the membrane cleaning efficiency.The result showed that low dose NaClO scarcely affects the functional groups,fluorescent properties and molecular weight of BSA.However,zeta potential of BSA decreased which would increase the electrostatic repulsive force between BSA and negative membranes and alleviate membrane fouling in turn.After the filteration,the flux loss of PES and PVDF membrane decreased about 23%and 7%respectively.When the concentration of NaClO increased,aromatic proteins were attacked by oxidants.Though molecular weight was not affected,the decomposition of benzene rings led to the increase of unsaturated bonds.From the three-dimensional fluorescence excitation-emission matrix spectra,BSA was transformed to more hydrophobic fulvic-like substances.Increased hydrophobic interactions between BSA and membrane surface led to more severe membrane fouling.When increase NaClO doses continually,zeta potential of BSA decreased sharply.Furthermore,amide groups of BSA were attacked,the destruction of secondary structure leading to the unfolding of BSA.On the one hand,increased electrostatic repulsive force prevented the accumulation of BSA on membrane surface.On the other hand,increased hydraulic size of BSA prevented some protein molecules from entering the membrane pores,therefore reducing the pore blocking.When exposed to 2000ppm NaClO,the flux loss of PES membrane decreased about 26%.In conclusion,the variation of membrane flux and permeability existed a ’S-shaped’ trend,so membrane fouling caused by BSA could be well controlled upon exposure to low dose NaClO.To explore the effects of different kinds of oxidant on ultrafiltration membrane fouling control,the reaction mechanisms between H2O2 and BSA were deeply researched as well as the effects on membrane flux and permeability.The results showed that the physicochemical properties and functional groups were hardly affected upon exposure to H2O2.The fluorescent intensities of BSA decreased continually along with the formation of a new substance with a molecular weight of about 45kDa when the concentration of H2O2 increased.In the previous study,we find that NaClO would attack benzene rings preferentially and transform them to long chains.However,H2O2 promoted the proteolysis of BSA,leading to the formation of some small molecular peptides.As a result,these new substances could pass through membrane pores easily in the initial period of fouling.However,they will accumulate gradually in the membrane pores with the filtration process,leading to a more severe fouling in the later period of fouling.Therefore,low dose H2O2 cannot control BSA fouling well compared with NaClO.To evaluate the effects of foulant characteristics,the interactions between NaClO and LYS and the effects on membrane fouling control have been systematically studied.The results showed that low dose NaClO promoted the breakage of aromatic substituents,leading to the formation of hydrophobic condensed aromatic substances.As a result,the flux loss of PES and PVDF membrane increased about 46%and 53%respectively.On the contrary,at high NaClO loads,protein structures were destroyed completely,which promoted the recovery of membrane filterability.Over all,membrane filterability exhibited an obvious‘U-shaped’trend upon exposure to different concentrations of NaClO.Membrane fouling caused by LYS could not be alleviated but even be aggravated when low dose NaClO was applied.The results in this study indicated that low dose oxidants may have good effects on membrane fouling control under some circumstances.However,the results were highly related to the type of oxidants and the nature of foulants.Overall,the results in this study may provide some guidance for the application prospect and conditions of low dose oxidants in industry. |
