Seawater reverse osmosis plants desalination

Pohland HW, Seawater desalination and reverse osmosis plant design. Desalination 1980, 32,157-167. 

Reverse osmosis processes for desalination were first appHed to brackish water, which has a lower I DS concentration than seawater. Brackish water has less than 10,000 mg/L IDS seawater contains greater than 30,000 mg/L IDS. This difference in IDS translates into a substantial difference in osmotic pressure and thus the RO operating pressure required to achieve separation. The need to process feed streams containing larger amounts of dissolved soHds led to the development of RO membranes capable of operating at pressures approaching 10.3 MFa (1500 psi). Desalination plants around the world process both brackish water and seawater (15). 

Reverse osmosis is now extensively used to reduce salt concentrations in brackish waters and to treat industrial waste water, for example, from pulp mills. Reverse osmosis has also proved economical (the cost can be as low as about 1 per 1000 liters) for large-scale desalination of seawater, a proposition of major interest in the Middle East, where almost all potable water is now obtained by various means from seawater or from brackish wells. Thus, at Ras Abu Janjur, Bahrain, a reverse osmosis plant converts brackish feedwater containing 19,000 ppm dissolved solids to potable water with 260 ppm dissolved solids at a rate of over 55,000 m3 per day, with an electricity consumption of 4.8 kilowatt hours per cubic meter of product. On a 1000-fold smaller scale, the resort community on Heron Island, Great Barrier Reef, Australia, obtains most of its fresh water from seawater (36,000 ppm dissolved salts) directly by reverse osmosis, at a cost of about 10 per 1000 liters. 

Dalvi, A. G. I., Al-Rasheed, R., and Javeed, M. A. (2000) Studies on organic foulants in the seawater feed of reverse osmosis plants of SWCC. Desalination 132,217-232. 

Figure 26-11 shows how seawater can be desalinated under pressure by reverse osmosis. The largest desalination plant in the world is located in Jubail, Saudi Arabia, where it produces 50% of the country s drinking water by the reverse osmosis of seawater from the Persian Gulf. Smaller desalination plants are in operation in Israel, California, and Rorida. 

A single pass reverse osmosis plant is used for the desalination of seawater (3.5wt% NaCl). 

Reverse osmosis is used for desalination of seawater, treatment of recycle water in chemical plants and separation of industrial wastes. More recently the technique has been applied to concentration and dehydrogenation of food products such as milk and fruit juices. See ultrafiltralion. 

The first reverse osmosis modules made from cellulose diacetate had a salt rejection of approximately 97—98%. This was enough to produce potable water (ie, water containing less than 500 ppm salt) from brackish water sources, but was not enough to desalinate seawater efficiently. In the 1970s, interfacial composite membranes with salt rejections greater than 99.5% were developed, making seawater desalination possible (29,30) a number of large plants are in operation worldwide. 

Applications RO is primarily used for water purification seawater desalination (35,000 to 50,000 mg/L salt, 5.6 to 10.5 MPa operation), brackish water treatment (5000 to 10,000 mg/L, 1.4 to 4.2 MPa operation), and low-pressure RO (LPRO) (500 mg/L, 0.3 to 1.4 MPa operation). A list of U.S. plants can be found at www2.hawaii.edu, and a 26 Ggal/yr desalination plant is under construction in Ashkelon, Israel. Purified water product is recovered as permeate while the concentrated retentate is discarded as waste. Drinking water specifications of total dissolved solids (TDS) < 500 mg/L are published by the U.S. EPA and of < 1500 mg/L by the WHO [Williams et ak, chap. 24 in Membrane Handbook, Ho and Sirkar (eds.). Van Nostrand, New York, 1992]. Application of RO to drinking water is summarized in Eisenberg and Middlebrooks (Reverse Osmosis Treatment of Drinking Water, Butterworth, Boston, 1986). 

Co-location of a power plant and a seawater reverse osmosis desalination plant allows for the cooling water from a neighbouring power plant to be blended with the waste from a desalination plant before discharge (Voutchkov 2004). In such a process, seawater is used as the cooling water for the condensers in a power plant. This water is then used as both the feed for the desahnation process, and for blending to dilute the concentrate from the desalination plant. 

The advent of the Loeb-Sourirajan asyimnetric membrane some twenty years ago gave birth to an industry now exceeding 200 million dollars in annual sales. Reverse osmosis (RO) and ultrafiltration (UP) were previously only laboratory curiosities. Today, there are many large membrane plants (up to 16 million gallons per day) in service for applications as diverse as desalinating seawater concentrating serum proteins, or the recovery of paint and other by-products from waste streams. 

Some of the largest plants for seawater desalination, wastewater treatment and gas separation are already based on membrane engineering. For example, the Ashkelon Desalination Plant for seawater reverse osmosis (SWRO), in Israel, has been fully operational since December 2005 and produces more than 100 million m3 of desalinated water per year. One of the largest submerged membrane bioreactor unit in the world was recently built in Porto Marghera (Italy) to treat tertiary water. The growth in membrane installations for water treatment in the past decade has resulted in a decreased cost of desalination facilities, with the consequence that the cost of the reclaimed water for membrane plants has also been reduced. 

FIGURE 29 A large-scale reverse osmosis seawater desalination facility, (a) Plant layout schematic and (b) simplified process diagram. (Du Pont Company.)... 

Bou-Hamad, S., et al. (1997). Performance evaluation of three different pretreatment systems for seawater reverse osmosis technique. Desalination Int. Symp. Pretreatment of Feedwater for Reverse Osmosis Desalination Plants, March 31-April 2, 110, 1-2, 85-92. Elsevier Science B.V., Amsterdam, Netherlands. 

FIGURE 43.2 The three alignments suggested for the realization of a seawater reverse osmosis (SWRO) desalination plant between Israel and the Hashemite Kingdom of Jordan. (From http //www.mfa.gov.il/mfa.)... 

Currently another process for the production of potable water from seawater is becoming established reverse osmosis (RO). The RO-process is particularly suitable for small plants. Therefore almost 70% of all plants operate according this principle, but they account for only 35% of the desalination capacity. In osmosis, water permeates... 

As pressure on existing freshwater supplies tightens, seawater desalination plants, using multieffect vacuum distillation or reverse osmosis, are required in increasing numbers for provision of freshwater. The residual evaporated brines from these plants contain much higher salt concentrations than ordinary seawater and this is also obtained near potential salt markets. 

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