Research On Performance Evaluation Of Nanofiltration/reverse Osmosis Membrane Elements For Water Treatment

With the rapid development of China's urbanization construction,urban residents have put forward higher and higher requirements for the stable supply of high-quality drinking water.Nanofiltration(NF)and reverse osmosis(RO)membranes have become an important drinking water advanced treatment technology for its advantages,such as low-input even without adding chemical agents,small floor area,high automation.At present,the quality of membrane products is irregularity and the basic data is scarce.Moreover,there is no scientific evaluation method system to evaluate the key performance indicators of membrane elements.Therefore,the popularization and application of membrane elements are severely restricted.This thesis based on the 13th Five-Year Plan of water project,a comprehensive evaluation of NF/RO membrane elements was carried out to understand its current technical and economic development level.The performance evaluation index systems of NF/RO membrane elements are established to evaluate the product characteristics,which provide support for the subsequent technical development and popularization.Firstly,the production and application status of NF and RO membrane elements were evaluated by fuzzy comprehensive evaluation method.According to the principle of maximum readiness,the technology readiness level of NF membrane elements'overall level is TRL7.NF membrane elements are lack of unified standards and manuals to guide the production and application;they have a certain gap in applying the standardized.The technology readiness level of RO membrane elements'overall level is TRL8.RO membrane elements have realized the localization of components and formed a certain scale of market application;at present,the technical standards are relatively perfect,and the promotion and application should be strengthened.Secondly,based on the existing laboratory testing platform,according to the relevant national,industrial and local standards,combined with the corresponding product manual and other technical requirements,key performance indexes of membrane elements were tested and compared in laboratory.The results show that the four brands of NF membranes,except NF2,are selective separation nanofiltration membranes.The application performance of domestic NF membranes NF4 and NF5 is not significantly different from that of imported NF1 and NF3.The results of laboratory performance and small-scale application tests of RO membrane show that under the same test conditions,the performance of membrane elements is significantly different.Domestic ultra-low RO membrane elements XLP4 has a great discrepancy compared with others,for its lower desalination rate and water yield.The performance of two domestic brackish RO membranes,BW4and BW5,is not different from that of imported membranes.Thirdly,pilot and production tests were carried out.Five kinds of NF membrane elements were applied to Changsha direct drinking water system.The application efficiency showed that,the average removal rates of COD_Mn,TOC,UV₂₅₄,turbidity,total dissolved solids,total alkalinity and total hardness by four NF membrane elements were 66.6%,70.2%,71.2%,64.0%,65.0%,53.8%,65.6%,respectively.Six kinds of membrane elements from a water plant in Taian,Zhejiang,Inner Mongolia and a direct drinking water system in Changsha using RO technology were selected for production efficiency test.The system running effect shows that RO membrane for most of the pollutants removal rate can reach more than 90%.By scan electron microscopy and atomic force microscope for membrane surface morphology observation,show that the groundwater in the reverse osmosis membrane used in processing is easy produce inorganic pollution.However,when applied to surface water treatment,it is easy to produce organic pollution and the pollution degree will be more serious.Finally,according to the preliminary investigation and technical performance evaluation results,chosen the indicators of product quality,application effect and economic performance reflected in the small-scale bench test,pilot test and production test,the specific index system of membrane products was summarized and formed.Correlation-principal component analysis was used to screen the index system to be verified.The final evaluation index system of NF membrane elements includes 11 indexes:membrane flux,desalination rate,operating pressure,influent salt concentration,recovery rate,COD_Mn removal rate,chloride removal rate,total hardness removal rate,operation cost,membrane life-time and health permit.The final evaluation index system of NF membrane elements includes 10 indexes:operating pressure,desalination rate,temperature,recovery rate,integrity,operating cost,membrane flux,Zeta potential,membrane life-time and health permit.The selected indexes cover product quality,application effect,economy efficiency,so the selected indexes have good rationality.