Study On Application Of Ultrafiltration-reverse Osmosis Membrane Technology In Sea Water Desalination

To satisfy the demand for local potable water and industrial water, Qingdao government is planning to establish a 100,000m3/d sea water desalination project.As one of biggest desalination projects in China, studies are required to evaluate the features of sea water in Bohai Gulf and the effects of desalination process by pilot tests. After treated by disk filter, sea water from Bohai Gulf was fed to ultrafiltration(UF) system in this study. Both the contaminants removal efficiency and operating stability of UF system were investigated. Then the same targets were also assessed for reverse osmosis(RO) system fed by UF effluent. Meanwhile, the feasibility of UF-RO system was verified, and the results of pilot tests would provide a reference for the engineering design of future full-scale projects. The main conclusions are as follows:The average removel rates of chemical oxygen demand(CODMn), total organic carbon(TOC) and turbidity in sea water were 20.3%, 5.64%, and 98.1%, repectively.The silting density index(SDI) value of UF permeate was in the range from 1 to 3with the average value of 2.2. The operation stability, trans-membrane pressure(TMP)increasing rate,and permeability recovery after chemical enhanced backwash(CEB).were investigated under three filtration cycles, three membrane fluxes and three CEB intervals, repectively.The results showed that the UF system was operatedstably and the lowest permeability decreasing rate was obtained under the condition of 45 min filtration cycle.When the system was running at the membrane flux of 85 L/(m2.h)and the backwash interval of 23, 31, and 46 times, the system was operated stably and TMP was almost recovered to the original value. FeCl3 made positive effects to control TMP as a type of coagulant with online dosage of 0~1.0 mg/L, whereas coagulants could also cause membrane fouling if overdosed. Moreover, Scanning Electron Microscope-Energy Dispersive X-Ray Spectroscopy(SEM-EDX) was applied to analyze the contaminants on the surface of membrane. The results indicated that amounts of Fe, Si, Ca, and Mg were presented on the membrane surface fouling compared with virgin membrane, which was related to overdosedcoagulants and the scaling formed on membrane surface due to Ca and Mg.The RO process had favorable pollutant removal effects during the pilot tests.Average permeate conductivity was 130 μs/cm, permeate boron concentration was around 0.33mg/L, and desalination rate was over 95%. RO system was operated stably under the membrane flux of 15 L/(m2?h) and the anti-scaling chemical dosing of 1~1.3 mg/L. And average recovery of permeate approached to 43.2%.Furthermore, the operation cost of UF-RO process in this study was approximately3.75 RMB/m3, which was lower than thermal desalination methods.The product water can be used as potable water for Qingdao local residents and as process water for Qingdao alkaline plant.The RO concentrate water can also be used for alkaline producing. Hence, the integration of the sea water desalination and industrial alkaline producing could be achieved.