Reverse osmosis brackish water desalination

Water treatment systems have now come to include plastics, such as Du Pont s Permasep permeators for reverse osmosis (RO) water desalination, introduced in 1969 (see Figs. 12-5 and 12-6). Since that time such systems have been used in thousands of installations around the world for desalination of brackish water and seawater and to treat waste effluents. These permeators come in four product types, according to the type of water to be treated (see Table 12-1). 

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). 

H. Dach, J. Leparc, H. Suty, C. Diawara, A. Jadas-Hecart, A. Lhassani, M. Pontie, Innovative approach for characterization of nanofiltration (NF) and low pressure reverse osmosis (LPRO) membranes for brackish water desalination, Desalination, 2006 (submitted). 

Reverse Osmosis in Seawater and Brackish Water Desalination.1132... 

REVERSE OSMOSIS IN SEAWATER AND BRACKISH WATER DESALINATION... 

Reverse osmosis membrane is widely used in seawater and brackish water desalination processes. Compared to traditional distillation, there is no energy-intensive phase change involved in membrane processes. Therefore, desalination with RO membrane is more energy efficient. In addition to the traditional desalination processes, RO membranes have also found wide application in industrial and municipal wastewater treatment, in pure water production for the electronic and pharmaceutical industries, and in the food industries. 

Hon-celluloslc Membranes. Despite an Intensive search for more favorable membrane polymers, cellulose acetate remained the best material for reverse osmosis until 1969 when the first B-9 permeator for brackish water desalination was Introduced by Du Font. Richter and Hoehn ( ) Invented aromatic polyamide asymmetric hollow-fiber... 

As of the end of 1984, the desalination of brackish water accounted for 82% of capacity. This is due to the fact that early reverse osmosis membranes were incapable of single stage seawater desalination and, thus, they were limited to brackish water desalination. Within the last 10 years, significant advances have been made in both the flux and rejection capability of membranes and reverse osmosis is technically able to desalt seawater in a single stage. In the recent past, it has been an effective competitor to the distillation process in seawater desalination. In fact, reverse osmosis is now beginning to replace existing distillation capacity in the Middle East.4... 

Figure 4.13 shows a flow diagram for a reverse osmosis unit with 75% recovery on a brackish feed. The pretreated feed is routed to the high pressure pump where the feed pressure is raised to between 250 and 400 psig as required for brackish water desalination. The pressurized feed is then pumped to the first pass pressure vessels where about 50% of the feed is recovered as product and 50% is reject. The reject from the first pass pressure vessels is then routed to the second pass pressure vessels where, again, about 50% of the first pass reject is recovered as product and 50% is reject which is sent to waste. Thus, the overall recovery of the unit is 75% as product. As can be seen, a normal array for a 75% recovery unit is two first pass pressure vessels feeding one second pass pressure vessel or a 2-1 array. If the system recovery were from 40 to 60%, all of the pressure vessels would be in parallel. However, if the system recovery were raised to between 85 and 90%, the pressure vessels would be arranged in a 4-2-1 array. 

Post-treatment of the desalted permeate from the reverse osmosis unit (Section 7.2.1.2) consists of the addition of CI2 and Ume for disinfection and corrosion protection. If H2S is present, it is eliminated by air stripping. For brackish-water feeds containing hydrocarbons, an activated carbon adsorber is used to remove dissolved hydrocarbons prior to microfiltration, which is followed by steps needed during the pretreatment and post-treatment processes dechlorination is required as a pretreatment if the RO membrane for desalination cannot tolerate residual chlorine dissolved otcygen is often removed to avoid damaging the RO membrane via vacuum based deaeration or addition of sodium bisuMte. An introduction to the pretreatment and post-treatment processes for membrane based sea-water and brackish-water desalination has been provided by Williams et al. (2001). The scale of such desalination plants is quite large, as much as 87 million gallons per day at Ashkelon, Israel, for example. 

To overcome the problems of cellulose acetate membranes, many synthetic polymeric materials for reverse osmosis were proposed, but except for one material, none of them proved successful. The only one material, which could remain on the market, was the linear aromatic polyamide with pendant sulfonic acid groups, as shown in Figure 1.2. This material was proposed by DuPont, which fabricated very fine hollow fiber membranes the modules of this membrane were designated B-9 and B-10. They have a high rejection performance, which can be used for single-stage seawater desalination. They were widely used for mainly seawater or brackish water desalination and recovery of valuable materials such as electric deposition paints, until DuPont withdrew them from the market in 2001. 

Eusaoka, Y. (1999). Super ultra low pressure composite reverse osmosis membrane elements for brackish water desalination and ultrapuie water production. Membrane 24(6), 319-323. 

Dance, E. L., Davis, T. E., Mahon, H. I., McLain, E. A., Skiens, W. E., and Spano, J. O. (1971). Development of cellulose triacetate hollow fiber reverse osmosis modules for brackish water desalination. Report No. 763. U.S. Office of Sahne Water Res. Develop. Progr., New York. 

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. 

Although the principal appHcation of reverse osmosis membranes is still desalination of brackish water or seawater to provide drinking water, a significant market is production of ultrapure water. Such water is used in steam boilers or in the electronics industry, where huge amounts of extremely pure water with a total salt concentration significantly below 1 ppm are required to wash siUcon wafers. 

The pressure to be used for reverse osmosis depends on the salinity of the feedwater, the type of membrane, and the desired product purity. It ranges from about 1.5 MPa for low feed concentrations or high flux membranes, through 2.5—4 MPa for brackish waters, and to 6—8.4 MPa for seawater desalination. In desalination of brackish or sea water, typical product water fluxes through spiral-wound membranes are about 600—800 kg/m /d at a recovery ratio RR of 15% and an average salt rejection of 99.5%, where... 

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). 

Reverse osmosis (RO) pressure difference desalination of sea/brackish water concentration of whey and fruit juice waste water treatment... 

The successful development of asymmetric cellulose acetate membranes by Loeb and Sourirajan in the early sixties, at the University of California, Los Angeles, has been primarily responsible for the rapid development of Reverse Osmosis (RO) technology for brack sh/sea water desalination. Reverse Osmosis approaches a reversible process when the pressure barely exceeds the osmotic pressure and hence the energy costs are quite low. Theenergy requirement to purify one litre of water by RO is only O.OO3 KW as against 0,7 KV required just to supply the vaporisation energy to change the phase of one litre of water from liquid to vapour by evaporation. Thus RO has an inherent capability to convert brackish water to potable water at economic cost and thus contribute effectively to the health and prosperity of all humanity. 

In summary, the NS-300 membrane system actually comprises a family of membranes, with reverse osmosis properties determined by the isophthallc trimesic ratio. Exceptionally high fluxes are possible at high retentivity levels for dissolved salts containing polyvalent anions. This membrane type may find applications in the desalination of brackish sulfate ground waters or industrial... 

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. 

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