Study On Biofouling Characteristics Of RO Membrane And Control Mechanism By Chlorine Pretreatment At Low Temperature

Water crisis has become a major barrier obstacle to the high-quality development of society.Wastewater reclamation is a significant approach to relieve the shortage of water resources.RO technology has been widely used all over the world,but it faces serious problems of membrane fouling.Previous studies were more focused on low latitude and high temperature areas,but few studies on RO membrane fouling under low water temperature conditions.The commonly used chlorine disinfection pretreatment were found that it couldn’t control the development of RO membrane biofouling at room temperature,and the effect of chlorine disinfection on RO membrane biofouling at low temperature has not been clearly studied.In this study,RO membrane fouling development at low feed water temperature was investigated by comparing with that at room temperature.The variation of flux,foulant characteristics and surface morphology of fouled RO membrane were investigated.It was found that the permeate flux decreased more slowly at low temperature,and the fouling layer was thinner.The fouling composition was more organic.The biomass of fouling layer on the membrane surface at low temperature was similar to that at normal temperature,and mature biofilm fouling was formed on the membrane surface at 3-7 days.Chlorine disinfection couldn’t alleviate the RO membrane biofouling at low feed water temperature.The abundance of chlorine-resistant bacteria with high fouling potential increased.The specific results of the experimental study are as follows:（1）The initial permeate flux at low temperature（10℃）was about 1/2 of that at25℃,but the flux decreased more slowly than that at 25℃.After 3 days of operation,thepermeate flux at 10℃and 25℃reached the same level.By the fitting of the intermediate blockage model,it was found that the influent fouling potential k value were 0.004 m^-1at 10℃and 0.019 m^-1 at 25℃.（2）After the same operating time,the foulants per unit area at 10℃were much less than that at 25℃.The fouling layer was thinner at 10℃.After 14 days of operation,the fouling layer thickness at 10℃was about 7μm,while that at 25℃was about 16μm.With the operating of the system,the proportion of fulic acid and humic acid on the RO membrane increased.The surface roughness at 10℃was smaller,and the proportion of calcium element was much lower than that at 25℃.（3）With the increase of operation time,the diversity of foulant microbial community at 25℃gradually increased,while the diversity of foulant microbial community at 10℃decreased.Proteobacteria,Patescibacteria and Bacteroidota were the main phyla in the foulant on the membrane.At the class level,the relative abundance of Parcubacteria and Bacteroides at 10℃was significantly higher than that at 25℃.While the relative abundance of Alphaproteobacteria was lower at 10℃.There were significant differences in the dominant genera,for example,Flavobacterium and Pseudomonas were more abundant at 10℃and 25℃,respectively.（4）Experiment on the effect of chlorine disinfection on membrane foulingat low feed water temperature was carried out.Within 28 days of operation,the membrane salt rejection of the two groups remained above 97%.Chlorine disinfection did not reduce the fouling potential of influent at low temperature.With the development of membrane fouling,the permeate flux decreased continuously.After 28 days of operation,the normalized flux of 5 mg-Cl₂/L disinfection group decreased to 0.29,while that of 0 mg-Cl₂/L group remained at 0.33.A greater reduction in normalized flux was observed after 5 mg-Cl₂/L disinfection.There were more foulants on the RO membrane and DOC concentration in 5 mg-Cl₂/L group were much higher than those in 0 mg-Cl₂/L group.（5）Analysis of microbial community of feed water and membrane foulant after chlorine disinfection showed that chlorine disinfection significantly reduced feed water community richness and diversity.5 mg-Cl₂/L disinfection could effectively remove the bacteria in Proteobacteria.Most of the remaining bacteria belong to Actinobacteria and Firmicutes which were more resistant to chlorine disinfection.The abundance of Mycobacterium,which is a common chlorobacteria,was significantly increased.Proteobacteria,Myxococcota and Patescibacteria were the main phyla in 0mg-Cl₂/L foulant,while Proteobacteria and Bacteroidetes were the main groups in 5mg-Cl₂/L foulant.In 5 mg-Cl₂/L foulant,the relative abundance of Pseudomonas and Sphingomonas was higher than that of 0 mg-Cl₂/L foulant.