Desalinated seawater

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. 

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. 

Desalinated seawater and desalinated brackish water are important new sources of fresh water. Although this fresh water is more costly than fresh water from natural sources, one could argue that the higher cost reflects fresh water s true value. In the United States, natural sources of fresh water are relatively plentiful, allowing companies to sell fresh water at rates of a fraction of a penny per liter. Nonetheless, consumers are still willing to buy bottled water at up to 2 per liter Each year Americans spend about 400 million dollars on bottled water, and the market continues to grow rapidly. Unless we conserve fresh water, it is easy to project a growing reliance on distillation and reverse osmosis. 

The lowest point on our planet is the Dead Sea (elevation —413 meters), located in Israel about 80 kilometers from the Mediterranean Sea and about 175 kilometers from the Red Sea. Plans are under consideration to build a canal connecting the Dead Sea to either the Mediterranean or the Red Sea. The elevation difference along the canal would provide enough pressure to desalinate seawater by reverse osmosis, yielding as much as 800 million cubic meters of fresh water per year. Identify some of the pros and cons of such a plan. 

In Example 18.6, a reverse osmosis unit was designed to desalinate seawater based on provided data. It is required to rate an existing reverse osmosis unit, to determine its product rates and compositions. Data such as permeances, pressures, solution properties given in Example... 

General quality improvement of present supplies Upgrade total municipal supply Potable water from degraded supplies Brackish water desalination Seawater desalination... 

Provide usable water where none available Brackish water desalination Seawater desalination Pure water production Industrial rinse waters Food industry Electroplating Power plant boiler feed Beverage production Medical... 

The most important application of RO is the desalinization of seawater. Since 1990, RO has become the dominant process for seawater desalinization. Seawater contains about 3.5 wt% dissolved salts and has an osmotic pressure of 24.1 bar. The preferred RO membrane for desalinization is a spiral-wound module of polyamide membrane operating at a feed pressure of 55 to 70 bar. With a transmembrane water flux of 365 kg/m2-day, this module can recover 45% of the water at a purity of 99.95 wt%. Atypical cylindrical module is 20 cm in diameter by 1.0 m long, containing 34 m2 of surface area. Such modules resist fouling by colloidal and particulate matter, but the seawater must be treated with sodium bisulfate to remove oxygen and chlorine. 

Seawater is an unlimited source of water. Currently, seawater RO is the best available technology for desalinating seawater [48]. There is room for improvement to make the process more affordable and less energy intensive, although the progress will be incremental at best [21]. 

Attempts to alleviate the water shortage which exists in many parts of the world have included extensive efforts to desalinate seawater. This continuing effort will be discussed later. 

In this chapter we will introduce mathematical modeling in the context of desalinating seawater. 

In order to desalinate seawater in a small-sized plarrt, it is envisaged to use a hollow-fiber modttle (Figure 14.10), the sketch of which has been formd in the Techniques de Tlngenietrr. Salt water flows inside drcular cylindrical hollow fibers of inner diameter 40 rm and outer diameter 80 jrm through which the permeate (salt free) is filtered by reverse osmosis as it passes from the inside to the outside of the fibers. In this cross-flow filtration device, a substarrtial fraction of the salt feed flow rate leaves without being filtered. 

Today, seawater desatinalion is mostly used to produce fresh potable water for human consumption and crop irrigation. Industrial applications of desalinated seawater are... 

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