에너지 절감형 해수담수화를 위한 역삼투-압력지연삼투막(RO-PRO) 공정의 적용

Abstract

Seawater desalination is the technique of obtaining fresh water from seawater by removing salinity. Among the desalination technologies available, reverse osmosis (RO) is currently considered as the most economical process. RO membranes are susceptible to a wide variety of organic and inorganic foulants. Fouling is an unavoidable phenomenon and major obstacle caused by dissolved or dispersed materials on membranes. So pretreatment process is the most critical step of RO membrane in order to prevent membrane fouling. Ineffective or unreliable pretreatment can lead to problems with the RO system including high rates of membrane fouling, high frequency of membrane cleanings, lower recovery rates, high operating pressure, poor product quality, and reduced membrane life; all having a direct impact on plant productivity and operational costs. UF membrane systems have been shown to be very efficient in removing turbidity and non-soluble and colloidal organics contained in the source water, and seem to be, by far, the most common choice in research studies and pilot testing for RO pretreatment. The performance of RO membranes downstream UF exceeded the usual operating conditions encountered in seawater desalination. The combined effect of higher recovery and higher flux rate promises to significantly reduce the RO plant costs. Coagulation has been successfully used inline with MF, UF, and NF membranes to prevent fouling during RO feed water pretreatment. Applying coagulation process before membrane filtration showed not only reducing membrane fouling, but also forming a cake structure that is more porous than the supporting membrane, thus avoiding permeate flux decline. The combination of UF with a pre-coagulation helped in controling the UF membrane fouling and providing filtered water in steady state conditions. Ferric and aluminum salts are commonly applied in MF/UF system for coagulation. Ferric chloride (FeCl3) has been shown to be the most successful coagulant in RO pretreatment application. According to the study, ferric chloride would cause accelerated fouling in the RO membrane when the oxidized form of iron is more than 0.05 mg/L. Most iron is removed by sand filtration, although some iron could pass through the sand filtration and negatively affect the RO membrane. Aluminum salt (such as alum) are avoided, because it is difficult to maintain aluminum concentrations at low levels (0.05mg/L) in dissolved form since aluminum solubility is very pH dependent. Aluminum residuals most notably from alum coagulant, were observed to cause colloidal fouling of membranes through interactions with ambient silica to form aluminum silicates. Meanwhile, a major problem in the RO is the high energy requirements for seawater desalination. Many researches have been conducted to use renewable energy sources for reduction power consumption to operating RO plant. One method to generate osmotic power is through pressure retarded osmosis (PRO) process. Experimental studies have demonstrated the technique potential for power generation using RO membranes of high water permeability and salt rejection rates. In the PRO process, water permeates through a semipermeable membrane from a low concentration feed solution to a high concentration draw solution due to osmotic pressure. Recently, flat-sheet and hollow fiber PRO membranes have been used for the PRO process, but most of those have limitations of the membranes, such as a limited hydraulic pressure applied and membrane deformation during operation. Also, despite many studies were carried out to investigate the PRO system, there have been only few studies which have evaluated the performance of the spiral wound membrane module using RO concentrate for a draw solution and RO permeate for a feed solution. The main objectives of this study is to assess the power production from a PRO pilot-plant using 8 in spiral wound membrane modules. In this study, a major objective is to investigate the effects of coagulants on UF-SWRO membrane process performance and application of PRO process for reduction of SWRO operating cost. The Coagulation-UF experiment was conducted by using different coagulant doses and coagulation type and UF flux was measured for evaluating the efficacy of coagulation process. The result of the coagulation-UF experiment by FeCl3 was better than alum on the reducing flux decline. And Al concentrate on the UF permeate did not exceed 0.05 mg/L. The RO experiment was also conducted by different coagulant doses and coagulation type, Fe concentration, Al concentration and RO flux, RO salt rejection were evaluated. The results showed that most filtered water contained the low Al concentration (0.05 mg/L) at the different dosing condition. But low silica seawater (such as UF filtered) with low Al concentrate show the similar flux decline and salt rejection at UF permeate. For the SWRO-PRO pilot plant, using low turbid seawater with UF pretreatment could enable the SWRO pilot plant to be run at stable TMP and flux. Both flux and TMP were maintained stable during the experimental period, which indicated that the RO membane was not fouled or fouled slightly. The two-stage PRO was better than the single-stage PRO for recovery with power density. Application of PRO-2PX process can reduce the SWRO energy consumption about 20 %.