Study On Membrane Fouling Mechanism Of Extracellular Polymeric Substances

In recent years,membrane technology has developed rapidly in the field of wastewater treatment,but the problem of membrane fouling still limits its further promotion.After membrane fouling,the energy consumption increases,and the filtration performance is reduced,which reduces the economic and practicality.In membrane bioreactors,Extracellular Polymeric Substances(EPS)are important substances that cause membrane fouling.They can adhere to the surface of the membrane or block the pores of the membrane,reducing the permeate flux,resulting in frequent cleaning or even replacement of the membrane.In order to further explore the membrane fouling phenomenon caused by EPS and its mechanism,this study intends to use EPS as the research object.Based on the comparison of the extraction methods of EPS,the differences between EPS and model solution(bovine serum albumin and sodium alginate,BSA+SA),and the influencing factors of EPS membrane pollution,the main research results are as follows:(1)EPS is extracted by four methods:heating method,ultrasonic method,cation exchange resin method(CER)and alkaline extraction method(NaOH).The EPS content follows the NaOH method>heating method>CER method>ultrasonic method,however the NaOH method has the most severe cell lysis.The EEM plot shows that the intensity of the fluorescence peak is directly proportional to the content of EPS.FTIR results show that the CER method can extract more substances with different chemical properties.(2)The order of the magnitude of stable flux in membrane filtration behavior is NaOH method<heating method<CER method<ultrasonic method.By the data fitting of membrane fouling mechanism,NaOH method and the heating method obey cake formation and intermediate blocking model,and the CER method and the ultrasonic method obey cake formation model.The order of hydrophobicity of EPS obtained from the surface free energy parameter is NaOH method>heating method>CER method>ultrasonic method.Meanwhile,the energy barrier and minimum secondary energy in total potential energy curve between EPS and the membrane indicate that the ranking of membrane pollution severity also follows the NaOH method>heating method>CER method>ultrasonic method.Considering the large extraction amount,small cell lysis and low membrane pollution potential,the cation exchange resin method(CER)was determined as the best extraction method in this study.(3)Calcium ion(Ca2+)will combine with polysaccharides in actual EPS and model solution to form hydrocolloids,leading to an increase in average particle size and a wide particle size distribution range.The average particle size of EPS-Ca2+(498±21nm)is larger than the average particle size of BSA+SA-Ca2+(384±11 nm).Owing to the interaction of different functional groups with Ca2+,a shift of the characteristic peaks in FTIR spectrum was observed.The functional groups with EPS-Ca2+are C=O and N-H which shift is small,and the BSA+SA-Ca2+offset functional groups are C=O and O-H which shift is large.(4)Rheological behavior confirms that BSA+SA-Ca2+hydrogel system is more stable than EPS-Ca2+.The XDLVO curve shows that the membrane pollution potential of BSA+SA-Ca2+is higher than that of EPS-Ca2+,which results in the BSA+SA-Ca2+hydrocolloid structure not being easily destroyed,and the membrane fouling is more serious.Further fitting of the membrane fouling mechanism shows that BSA+SA-Ca2+obey cake formation and intermediate blocking model,while EPS-Ca2+obey cake formation,intermediate blocking and standard blocking model.There are significant differences between actual EPS and model solution,and further research on EPS alternatives is needed in the future.(5)One-way analysis of variance(ANOVA)results show that pressure,temperature,stirring rate,and concentration are all factors that affect membrane flux.Pressure and temperature are positive contributing factors,and stirring rate and concentration are negative contributing factors.In addition,the response surface methodolgy was used to analyze the interaction of various influencing factors,and it was found that the order of influence on the membrane flux was concentration>temperature>pressure>stirring rate.The model fitting found that the coefficient of determination R2 was 0.9612,which was highly significant,while R2adj was 0.9225,which fit the real data well.The optimal operating conditions are determined to be a stirring rate of 396.438 rpm,a concentration of 1.216 g·L-1,a pressure of 0.017 MPa,and a temperature of 26.69?.Under this condition,the model predicted the highest membrane flux to be 0.187 L·(m2·s)-1.The study aims to provide a scientific basis and technical support for mitigating membrane fouling.Meanwhile,it has important theoretical and practical significance for the control of membrane fouling and the popularization and application of membrane technology in the field of wastewater biological treatment.