Pilot Study On Seawater Nanofiltration Softening Technology Based On The Integrated Membrane System

The softening technology of seawater and brackish water is one of effective waysto solve the problem of shortage of fresh water resources. As one of the main seawatersoftening technology, Nanofiltration can effectively remove divalent ions in seawater.Based on this characteristic, it can be set as a pretreatment portion of the reverseosmosis process, which can reduce membrane fouling and improve the systemrecovery; in addition it can be directly used as industrialized seawater softeningprocess, such as offshore oilfield water injection. However, relatively few commercialapplications were established and reported till now.In this study, a novel UF–NF IMS system was established and the NF processwas composed of ten8-inch commercial membrane elements. Pilot trial of two kindsof NF processes were conducted with coastal seawater to investigate the performanceand stability of the IMS system, and also to assess the feasibility of the UF–NF IMSsystem for surface seawater softening and provide operating parameters for theapplication of the two NF processes.The results indicate that a novel pilot-scale ultrafiltration UF–nanofiltrationNF integrated membrane system IMS with capacity of100m3d–1NFpermeate water was established for seawater treatment. During the pilot trial, a kindof UF process with combination of dead-end and crossflow cycling filtration wasinvestigated, and the results show that the UF process could provide water with lowturbidity and SDI value, with all of the turbidity less than0.1NTU, and all of the SDIvalue less than2.6, which totally meet the requirement of the feed water of NF. Twokinds of NF processes were also investigated during the trial; one was mainlycharacterized by high permeate flux and recovery, and the other mainly focused onachieving good quality of permeate water. The performance of the two processesunder different operating conditions such as operating pressure, recovery and influentflow rate was extensively investigated. The results show that under relatively low pressure, the high-flux process had high permeate recovery which could reach70%, and high rejection of divalent anion; the quality of the production water of the high-flux process was much higher than that of the traditional pretreatment, so it could be used as the pretreatment of seawater reverse osmosis (SWRO) process to alleviate the SWRO membrane fouling. The high-quality process had good permeate quality, especially the rejection rate of Ca2+and Mg2+which could keep higher than90%, thus the production water could meet the demand of certain industrial water standards, such as offshore oilfield water injection and polymer solution preparation for polymer flooding. During the long-running the high-quality NF process was relatively stable and had an excellent anti-fouling performance.