Study On Water Purification Efficiency And Membrane Fouling Of Biological Pretreatment Enhanced Coagulation Ultrafiltration Combined Process

With the rapid economic growth,the surface water environment has been severely damaged,and the shortage of water resources has become increasingly severe.The development of green and sustainable water treatment technologies is of great significance to improving the water environment and alleviating water shortages.Coagulation ultrafiltration（C-UF）combined process is often used in surface water treatment,but it has poor treatment capacity for water bodies with high total organic carbon（TOC）,and high TOC will aggravate membrane pollution.In addition,TOC is positively correlated with the generation potential of disinfection by-products,too high will increase the safety risk of water effluent from the disinfection process.According to reports,biological pretreatment（BP）can reduce the concentration of organic matter in the influent,but there are few related studies as a pretreatment process for coagulation and ultrafiltration.In view of the above problems,this study adopts raw water（AT）with higher TOC,selects commonly used coagulants AlCl₃（C1）and PAC（C2）,and initially constructs a new type of combined process（BP-C-UF）,and investigates the combined process water purification efficiency,analysis of the corresponding membrane fouling mechanism.The research aims to build a green and sustainable combined process to provide new ideas for purifying surface water and alleviating membrane pollution.The research conclusions are as follows:（1）The TOC concentration of at raw water is high（11.8 mg / L）,and the TOC removal rate of conventional coagulation ultrafiltration process is limited.After adding biological pretreatment,the TOC removal rate of BP-C1-UF system is increased from 17.0% to 80.0%,and the TOC removal rate of BP-C2-UF system is increased from 19.6% to 82.4%.The effluent quality is effectively improved,and the organics removed are mainly hydrophobic substances and transitional hydrophilic substances.（2）In the experiment on the influence of the generation potential of the disinfection by-products,the sodium hypochlorite disinfectant was selected and the influence of the BP-C-UF system on the generation of trihalomethanes（THMs）and haloacetic acids（HAAs）was investigated.The results showed that the AT-C1-UF system and AT-C2-UF system reduced the total THMs production by 39.1% and 39.7%,respectively,it is difficult to effectively inhibit the formation of THMs and HAAs in the direct coagulation ultrafiltration system.Biological pretreatment can effectively remove the precursors of disinfection by-products（such as polysaccharides,proteins and humic acid）,and the combination of coagulation ultrafiltration process can greatly reduce the generation of THMs and HAAs.Among the BP-C-UF systems,the BP-C2-UF system has the best effect,with a decrease of 96.1% in total THMs generation and 78.1% in total HAAs generation.（3）The final flux of the BP-C1-UF system and the BP-C2-UF system are 3.86 times and 8.41 times that of the AT-UF system,respectively,which indicated that BP-C-UF system could effectively alleviate membrane fouling,which was attributed to the joint action of microbial degradation and coagulation system of BP system.Among them,the BP system relies on biodegradation to remove hydrophobic substances（such as polysaccharides,proteins and other biopolymers）in the AT raw water,and then coagulation further removes the hydrophobic substances in the water to enhance the effect of membrane pollution control.When the supernatant is finally filtered,a large number of hydrophilic groups are attached to the surface of the ultrafiltration membrane to increase the membrane flux.In addition,PAC,as a high-valent cationic coagulant,uses electrical neutralization to destabilize and easily remove negatively charged small and medium molecular weight organics.AlCl₃ water purification mainly relies on mesh cloth sweeping,which has relatively limited removal capacity for such organics.The flux of BP-C2-UF system is higher than that of BP-C1-UF,and the effect of alleviating membrane pollution is better.