Production using Reverse Osmosis

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 

Production of potable water from brackish water or seawater by reverse osmosis  

Membranes mostly made of acetylcellulose or more preferably polyamide. Large pressure differences mean complicated desalination plant construction (in some cases multistage). Pretreatment of water necessary as for distillation plants 

The membranes are manufactured from acetylcellulose or, more preferably, polyamide. The technical construction is complicated and made expensive by the large pressure differences and the need for thin membranes. Bundles of coiled thin hollow capillaries (external diameter 0.1 mm, internal diameter 0.04 mm) are, for example, placed in a pressure cylinder (Fig. 1.1-3). The.se capillaries protrude from the ends of the cylinder through plastic sealing layers. Of the (high salt content)-water fed into the cylinder from the other side, 30% passes through the capillary walls into the capillaries and the rest is run off as concentrate and disposed of. An intensive and expensive pretreatment of the feed water is also necessary in addition to the removal of all colloidal and biological impurities, treatment of the feed water is also necessary e.g. by acid addition. The use of feed water from wells in the neighborhood of beaches is particularly favored. 

In water production, reverse osmosis requires less than 50% of the energy required by multistage flash distillation (8 to 10.6 kWh for freshwater for a capacity of 19- 10- m-Vd). 

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G.A. Pittner, High Purity Water Production Using Reverse Osmosis Technology, in Reverse Osmosis, Z. Amjad (ed.), Van Nostrand Reinhold, New York (1993). 

Kitis, M., Kilduff, J. E., and Karanfil,T. (2001). Isolation of dissolved organic matter (DOM) from surface waters using reverse osmosis and its impact on the reactivity of DOM to formation and speciation of disinfection by-products. Water Res. 35, 2225-2234. 

Polymeric membranes also have vast applications in several processes, such as desalination using reverse osmosis membranes. Filtration, in a wide sense, with polymeric membranes can be applied in gas separation processes, biochemical processing, wastewater treatment, food and beverage production, and pharmaceutical applications [59-61],... 

Achieving low particulate levels in the final drug or parenteral solution usually requires filtration of the constituent water. The United StatesPharmacopoea defines specifications and methods for production of water for injection. Ultra-pure water systems in the pharmaceutical industry use reverse osmosis, ion exchange, and MF just as in the electronics industry (see below). Both industries seek to produce sterile/particle free water. However, in the pharmaceutical in-... 

In most plants that use reverse osmosis in the preparation of process water, the reject stream is routed directly to waste discharge without any additional posttreatment. In industrial plants that use reverse osmosis to treat industrial wastes, the reject stream may contain valuable materials and this stream would be sent back to the process. In other applications of industrial waste treatment by reverse osmosis, the reject stream may require additional treatment prior to ultimate discharge. In this case, the reverse osmosis unit will have provided a large volume of water that is disposable or can be reused (the product) and a smaller volume of reject which can be treated more economically. 

In the semiconductor and pharmaceutical industries, reverse osmosis is w ell-established as a critical step in providing ultra-high-purity w ater for processing. These industries also use reverse osmosis W ith in w astew ater systems for removing contaminants. Mcllvaine predicts that sales of reverse osmosis systems into these tw o industries wdll continue to expand wdth ongoing increases in production,... 

Typical sea water desalination plants using reverse osmosis produce 30 to 50 gfd of water [gal (U.S.) per square ft per day] (1 gfd = 4.72 X 10 m /m s). Suppose it is decided to double the operating pressure from 50 to 100 atm. Will the production rate increase by less or by more than a factor of 2 Assume 100% salt rejection. 

A polymer products operation in Arizona uses reverse osmosis to eliminate the discharge of cyanide-containing rinse water from one of the company s four plating units. The process, which concentrates the cyanide and separates it from the rinse water, reduces the environmental impact of the discharge and conserves valuable plating materials and water treatment chemicals. 

This chapter, in addition to surveying membrane types and production, overviews applications of gas and liquid membrane separation and polymer films as banier layers. Water purification for reuse and in desalination using reverse osmosis and nano-, ultra-, and microfiltration are discussed. Electrodialysis, dialysis, and hemodialysis are also covered. Membranes in emerging technologies are described including fuel cell membranes, membranes in lithium batteries, conducting polymer membranes, and thin film membranes used in LED and photovoltaic applications. 

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. 

In reverse osmosis membranes, the pores are so smaH, in the range 0.5— 2 nm in diameter, that they ate within the range of the thermal motion of the polymer chains. The most widely accepted theory of reverse osmosis transport considers the membrane to have no permanent pores at aH. Reverse osmosis membranes are used to separate dissolved microsolutes, such as salt, from water. The principal appHcation of reverse osmosis is the production of drinking water from brackish groundwater or seawater. Figure 25 shows the range of appHcabHity of reverse osmosis, ultrafiltration, microfiltration, and conventional filtration. 

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. 

Membrane Sep r tion. The separation of components ofhquid milk products can be accompHshed with semipermeable membranes by either ultrafiltration (qv) or hyperfiltration, also called reverse osmosis (qv) (30). With ultrafiltration (UF) the membrane selectively prevents the passage of large molecules such as protein. In reverse osmosis (RO) different small, low molecular weight molecules are separated. Both procedures require that pressure be maintained and that the energy needed is a cost item. The materials from which the membranes are made are similar for both processes and include cellulose acetate, poly(vinyl chloride), poly(vinyHdene diduoride), nylon, and polyamide (see AFembrane technology). Membranes are commonly used for the concentration of whey and milk for cheesemaking (31). For example, membranes with 100 and 200 p.m are used to obtain a 4 1 reduction of skimmed milk. 

Following ultrafiltration of whey, the permeate passes over a reverse osmosis (qv) membrane to separate the lactose from other components of the permeate. Reverse osmosis can be used to remove water and concentrate soHds in a dairy plant, giving a product with 18% soHds and thus decreasing the difficulty of waste disposal. Concentration of rinse water gives a product with 4—5% total soHds. Proper maintenance of the membrane allows for use up to two years. Membranes are available for use up to 100°C with pH ranges from 1 to 14 the usual temperature range is 0—50°C. 

Memhra.nes. Liquid separation via membranes, ie, reverse osmosis (qv), is used in production of pure water from seawater. The chief limit to broader use of reverse osmosis is the high pressure required as the concentration of reject rises. 

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... 

Completion of Esterification. Because the esterification of an alcohol and an organic acid involves a reversible equiUbrium, these reactions usually do not go to completion. Conversions approaching 100% can often be achieved by removing one of the products formed, either the ester or the water, provided the esterification reaction is equiUbrium limited and not rate limited. A variety of distillation methods can be appHed to afford ester and water product removal from the esterification reaction (see Distillation). Other methods such as reactive extraction and reverse osmosis can be used to remove the esterification products to maximize the reaction conversion (38). In general, esterifications are divided into three broad classes, depending on the volatility of the esters ... 

Membrane Pervaporation Since 1987, membrane pei vapora-tion has become widely accepted in the CPI as an effective means of separation and recovery of liquid-phase process streams. It is most commonly used to dehydrate hquid hydrocarbons to yield a high-purity ethanol, isopropanol, and ethylene glycol product. The method basically consists of a selec tively-permeable membrane layer separating a liquid feed stream and a gas phase permeate stream as shown in Fig. 25-19. The permeation rate and selectivity is governed bv the physicochemical composition of the membrane. Pei vaporation differs From reverse osmosis systems in that the permeate rate is not a function of osmotic pressure, since the permeate is maintained at saturation pressure (Ref. 24). 

In conclusion, the editors thank most sincerely the contributors to this book, both for complying with our strictures as to the length of their contribution and for providing their material on time, and our publishers for their friendly courtesy and efficiency during the production of this book. We also wish to thank Dr H. J. Smith for his advice on various chemical aspects, Dr M. I. Barnett for useful comments on reverse osmosis, and Mr A. Keall who helped with the table on sterilization methods. 

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). 

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