Biofouling Control And Cleaning In Sugar-making Process By Membrane Technology

Sugar is an essential food additive in our daily life,most of which is derived from sugarcane.Conventional sugar making process needs to add a large amount of chemicals,and this traditional refining method suffers from inferior and unstable product quality,serious environmental problems caused by solid waste,and potential risks in food safety.In recent years,a membrane-based sugar-making process has been developed which applies membranes to remove impurities from sugarcane juice via physical separation.However,during sugarcane cutting,L.mesenteroides subsp.dextranicum may grow on the fractured sugarcane surface.This kind of bacteria would enter into the membrane system,produce dextran and inorganic acid using sucrose as substrate.The bacteria,cell debris and metabolites result in serious biofouling in the membrane modules,which greatly hindered the long-term and stable operation of membrane sugar making process.Therefore,in order to ensure the long-term stable operation of this process and improve the yield and quality of sugar products,we herein investigated the growth of L.mesenteroides subsp.dextranicum,the fouling behaviours and biofouling mechanisms during decolorization of cane juice by nanofiltration membrane,and offered an effective cleaning strategy for biofouling control.First,the effect of feed properties on bacterial growth was observed in the laboratory.It was found that the bacterial growth in culture medium is the fastest under pH 5?6,temperature 30?,and 10%of the initial sucrose concentration.And more dextran is produced at faster bacterial growth.At the same time,lactic acid and acetic acid were released from bacteria fermentation,which led to the decrease of pH value in the culture solution.Then,the specific situation of membrane biofouling was investigated in the pilot-scale tests,and it was found that the juice acidification was serious after the membrane system was operated for 5 h,indicating that the biofouling would be more serious with filtration time.In this regard,we adopted some fouling control strategies,such as increasing temperature,changing process sequence and adding bactericide,and preliminary inhibitory effect on the bacteria have been emerged,but the biofouling problem cannot be solved completely.Then,enzymatic cleaning(dextranase,trypsin,lysozyme,etc.)was applied to target specific foulant for clarifying the fouling composition and structure in the process of dead-end nanofiltration.It was found that the main foulant causing permeate flux decline was dextran.At this time,alkaline and dextranase cleaning displayed a super high permeability recovery,95.7%and 97.6%respectively.Various characterization methods showed that when the dextran was mixed with other foulants,much denser fouling layer was formed.They would adsorb on the membrane,leading to a recalcitrant fouling.Finally,in order to provide more reliable reference for the real applications,the pristine,fouled and cleaned membranes after filtering culture solution and real cane juice(with bacteria addition)respectively were evaluated using a cross-flow filtration device.In the cross-flow filtration,the bacterial cells would be broken by the shearing force on the membrane or in centrifugal pump,releasing proteins to construct fouling layer with pigments in cane juice.At the same time,the compact dextran layer would cover on the adsorption fouling layer and/or enwrap other foulants such as protein,pigments,cell debris and bacteria,adhering them on the membrane and protecting them from chemical cleaning.At this time,either dextranase or alkaline cleaning could not well recover the membrane permeability.Therefore,an enzymatic(i.e.dextranase)cleaning is required to degrade the dextran layer before an effective sodium dodecyl sulfate(SDS)alkaline cleaning for removing the other organic fouling.Only in this way can the biofouling be effectively cleaned,and the membrane permeability recovery can reach 98.9%.The outcome of this work not only elucidated the fouling mechanisms during decoloration of cane juice by nanofiltration membrane,but also offered an effective cleaning strategy for biofouling control in the practical application.