Operation Optimization And Pilot Study Of Nanofiltration Process In Pipeline Drinking Water Treatmen

With the development of the economy and society,people’s requirements for drinking water quality are becoming increasingly higher,and high-quality direct drinking water has become a standard configuration for many newly built communities.Traditional treatment processes can meet drinking water quality standards,but they are not effective in removing hardness,inorganic salts,and other pollutants,resulting in a poor taste.Reverse osmosis processes remove most ions from the water,which is not suitable for long-term consumption by residents.Nanofiltration-based deep treatment technology is a mature method for preparing direct drinking water,but low water production rates and high operation and maintenance costs due to membrane fouling are urgent problems to be solved in engineering applications.This project focuses on the efficient preparation technology of direct drinking water,constructs a nanofiltration process,and optimizes operating parameters to enhance the pollutant removal performance.It proposes microbubble membrane fouling control and membrane cleaning technologies,constructs pilot and semi-pilot scale process systems,and explores and verifies the effectiveness and engineering application potential of microbubble membrane fouling control technology,laying the foundation for practical production applications.（1）The nanofiltration membrane used in the direct drinking water treatment process was optimized.The response surface method was used to optimize the process parameters of the selected nanofiltration membrane and verify the predicted optimal operating parameters.Four nanofiltration membranes were compared for their removal efficiency of inorganic pollutants,and the 1#membrane,which has better removal effects on divalent ions and total hardness,was selected for the study.The response surface method was used to optimize operating parameters,and the effects of feed water pH,pressure,and recovery rate on the performance of the nanofiltration process were analyzed.The regression model of membrane flux and desalination rate was established.The optimal operating parameters were determined to be pH=7.344,feed water pressure of 5 bar,and a recovery rate of 13.50%.The optimized operating parameters were used to study the pollutant removal efficiency of the nanofiltration process,and the parameter changes during the process operation were recorded and analyzed.SEM-EDS was used for membrane fouling analysis.The results showed that under optimal operating conditions,good removal effects were achieved for turbidity,conductivity,total hardness,total alkalinity,total dissolved solids,inorganic salt ions,TOC,and UV₂₅₄.The changes in membrane flux,transmembrane pressure difference,and SEM-EDS analysis during operation indicated that the main cause of nanofiltration membrane fouling was inorganic scaling,and the degree of membrane fouling was relatively mild.（2）Membrane fouling control and membrane fouling cleaning were carried out using microbubbles and compared with traditional membrane fouling control and cleaning methods.The effects of different methods on the membrane were analyzed through characterization.The membrane fouling control and cleaning effects were verified using microbubbles with particle sizes ranging from 0.1μm to 200μm,and the results were compared with those obtained by continuous input,intermittent input,and the addition of antiscalants.The results showed that when microbubbles were continuously and intermittently input,the final specific membrane flux was increased by 6.3%and 2.9%compared with no control measures,and by1.5%,3.6%,1.9%,and 0.2%compared with the addition of antiscalants.This indicates that the input of microbubbles can effectively control membrane fouling and partially recover membrane flux,but the membrane cleaning effect is limited.Fourier infrared results show that,compared with antiscalants,microbubbles do not change the membrane surface functional groups,have little impact on the membrane polymer layer,and do not cause secondary pollution.Cleaning technologies for membrane fouling using 1%EDTA-4Na,2%citric acid,and microbubbles were simulated,respectively,for alkali washing,acid washing,and microbubble washing over four cycles.The results showed that the membrane flux recovery rate gradually decreased with the increase in the number of cleaning times,and the decrease with microbubble cleaning was more pronounced,indicating that the cleaning effect of microbubbles is limited.However,both acid washing and alkali washing led to an increase in the average pore size of the membrane and affected the hydrophobicity of the membrane,causing changes in the contact angle.The physicochemical properties of the membrane hardly changed after microbubble cleaning,which is more friendly to the membrane material.（3）A pilot-scale pipeline direct drinking water device centered on nanofiltration technology was further constructed.Under optimal operating conditions,the pollutant removal performance of the process was examined,and the effect of microbubbles on delaying membrane fouling under pilot-scale conditions was verified through parameter changes and SEM-EDS analysis.During continuous operation for 100 days,the removal rates of conductivity and turbidity were 80%and 50%,respectively,the removal rate of divalent ions was over 94%,and the removal efficiencies of TOC and UV₂₅₄ were between 67%-71%and 75%-81%respectively.As the device continued to operate,the membrane flux gradually decreased and the transmembrane pressure difference gradually increased,but the magnitude was small.This indicates that the introduction of microbubbles slowed down the decrease in membrane flux and had a good antiscaling effect.SEM-EDS analysis showed that there were larger particles deposited on the membrane surface,but the structure of the deposits was loose,indicating that the inorganic pollution on the membrane surface was light,and no obvious organic membrane pollution and biofouling were found.