Optimization And Pilot-scale Study Of Nanofiltration Process For Treatment Of Composite Hardness Water

With the development of society,various pollution problems of drinking water sources are becoming more and more serious.The quality of raw water tends to be complicated and complex.The pollution of composite hardness of raw water is becoming more and more serious.At the same time,there are inorganic salts such as sulphate and chloride exceeding the standard in raw water with high hardness.Most of the traditional water treatment processes in water plants are facing the problems of upgrading and transformation.In recent years,nanofiltration technology has been widely used in various fields of water treatment due to its low operating pressure,high rejection rate of divalent ions and retention of beneficial trace elements in water.However,membrane fouling has always been a bottleneck problem affecting the application and development of nanofiltration membranes.Aiming at the problem of increasing hardness in raw water,this paper optimizes the operation parameters of nanofiltration process for treating water source polluted by composite hardness,studies the removal efficiency of pollutants and membrane pollution of nanofiltration membrane for treating water polluted by composite hardness,and constructs a pilot-scale deep treatment combinedprocessofnanofiltrationwithbiologicalcontactoxidation precipitation-ultrafiltration as pretreatment process,which lays a foundation for practical production and application.The main research contents and results are as follows:Aiming at the selection of membranes and three parameters of raw water pH,influent pressure and water yield in the experiment of nanofiltration membrane removal of polluted water with complex hardness,the effects of interaction among the factors on the desalination rate and TMP change of nanofiltration process were studied by response surface methodology using Design-Expert 10.0 software.A regression model was established with the desalination rate and TMP change as the response values,and the optimal combination conditions were obtained;The accuracy of the optimal conditions determined by the model is verified by experiments.By comparison,the average removal rate of inorganic pollutants by 1#membrane with a desalination rate of 90%⁹5%reached 77.1%,while the average removal rate of inorganic pollutants by 2#membrane with a desalination rate of 85%⁹0 was only27.9%.The effect of 1#membrane in treating composite hardness water was better than that of2#membrane;when the influent pH of NF equipment is 8,the influent pressure is 1 MPa and the water yield is 27.976%,the TMP changes to 0.18KPa,the desalination rate is 98.252%,and the membrane performance is the best.According to the optimization results,the purification efficiency of nanofiltration process for composite hardness water was studied,and the removal effect of the process on various pollutant indexes of underground and surface composite hardness water was investigated.;the changes of parameters such as transmembrane pressure difference,membrane flux and so on were monitored during the continuous and stable operation of the nanofiltration equipment under the optimum operating conditions.The surface of the nanofiltration membrane was analyzed by SEM-EDS to observe the membrane fouling.The removal rates of turbidity,total hardness,total alkalinity,total soluble solids,K⁺,Na⁺,Ca²⁺,Mg²⁺,SO₄^2-,Cl^-,HCO₃^-,NO₃^-in underground composite hardness water by nanofiltration membranes reached 98%,99.4%,95.1%,97.8%,68%,94.1%,99.6%,99.6%,98.3%,95.2%and 90.6%respectively;the components of removal rates of surface hardness water were 99.4%,89.3%,84.9%,75.8%,95.4%,72.5%,92.0%,99.7%,89.5%,83.1%and 62.8%,respectively.The removal rates of UV₂₅₄ and TOC were 95%and 78%respectively,and the removal effects were good;the nanofiltration pilot plant operated continuously for more than 30 days under the optimum operating conditions.Through parameter changes and SEM-EDS analysis,it can be seen that the membrane fouling situation is not obvious and the operation performance is good.A pilot-scale nanofiltration combined process with biological contact oxidation precipitation and ultrafiltration as pretreatment process was constructed.The water from the Yellow River Diversion Reservoir was used as water source.The pollution removal efficiency of the combined process under continuous and stable operation conditions was investigated,and the pollution situation of the membrane process and the recovery of membrane fouling by membrane cleaning were monitored.The results show that the combined process has good removal effect on conventional pollutants and characteristic pollutants under the optimal operating conditions,and the membrane fouling is not obvious.The removal rates of turbidity,TOC,UV₂₅₄ and COD_Mn were 95%,90%,90%and 85%respectively;the removal rates of total hardness,total alkalinity and total soluble solids reached 97%,86%and 91%respectively,and the total desalination rate was above 95%;the reduction rates of fluorescent organic substances such as tryptophan,tyrosine,SMPs,fulvic acid,humic acid and precursors of disinfection by-products reached 88%and over 50%.The pollutant removal efficiency of the combined process is mainly concentrated in the nanofiltration unit.Ultrafiltration unit can remove turbidity of raw water and macromolecule organic matter to a certain extent,which can reduce the operation pressure for the follow-up process.Regular cleaning of ultrafiltration and nanofiltration membranes can effectively restore the process parameters and prolong the service life of the membrane process.