Because of RO's unique abilities, there is growing interest in implementing membrane processes for treating salty groundwater (brackish water), impaired rivers, and post-consumer reclaimed waters in addition to producing fresh water from the world's most plentiful water resource - our oceans. On average, seawater contains approximately 35,000 mg/L of salt, nearly two orders of magnitude
higher than that of potable water (United States has a secondary standard of 500 mg/L). Brackish water has lower salinity than seawater and ranges between 1,000-25,000 mg/L.

Use of desalination technology dates back to the ancient times, when Greek sailors purified water by seawater evaporation. During
the middle of the twentieth century, desalination technology became the primary source of fresh water supply in the arid regions of the
world where water is in short supply. Conventionally, thermal technologies (evaporative processes), which are energy intensive and
costly approaches, have been employed for desalination by oil-rich nations. The largest thermal based desalination facility in the
world; Shoaiba (located in Saudi Arabia) began operating in 2003 and has a production capacity of 93 MGD.

Recent advancements in membrane technologies along with energy recovery systems have made RO, and possibly nanofiltration, a more cost effective approach to desalination. These membrane technologies use high pressure to force water through semi-permeable
membranes, which in turn, retain dissolved and suspended solids. The largest RO facility in the world is located in Ashkelon, Israel and began operating in 2006 with a production capacity of 73 MGD. The first large-scale seawater RO desalination plant in the U.S. began operations in Tampa, Florida, early in 2004. Worldwide, there are over 21,000 desalination plants (in more than 120 countries) producing more than 10.5 billion gallons of potable water a day. Following table summarizes a list of key desalination facilities
around the world:

It is estimated that 50% of the future population growth in the United States will occur in the coastal states of California, Florida, and Texas. Interestingly, these three states are also the nation's leaders in seawater desalination. In California, many water agencies are interested in reducing their dependence on imported water and view seawater desalination as a viable local source of water. Currently, there are about a dozen small and large size desalination plants in the coastal regions of California. Proposals for more than two dozen additional seawater desalination plant are under review; some of these proposed plants could be the largest in the US and the western hemisphere.

In the past, prohibitive costs restricted widespread applications of desalination technology and conventional treatment of traditional water sources (groundwater, surface water and out-of-state transfers) was more cost effective. Increasing population, industrialization, frequent and prolonged droughts, dwindling traditional sources, and new stringent regulations are continuously pushing the costs of the conventional water supplies up, bringing seawater (and brackish water) desalination back to the world stage.

Driven by advances in material science, modern desalination membranes (such as thin-film composite membranes) use much lower pressures to produce a gallon of water than their predecessors. Also, dramatic improvements in the design of feed pumps and pressure exchangers have significantly reduced energy costs, which is a significant economic advantage; as energy costs can account for as much as one-half the total cost of the process. Additionally, these modern membranes have better mechanical and chemical stability, which results in longer life span than membranes produced just a decade ago. Other significant forces, such as increasing desalination plant capacity, co-location with power generation facilities, and the concept of “built-operate-own-transfer”, (which increases competition among the desalination contractors), and new stringent regulations are bridging the gap between cost of conventional water supplies and desalinated water.

Today, water desalination is a viable option for providing high-quality drinking water that can meet, typically exceed, the most stringent Federal drinking water quality standards. Although significant institutional hurdles exist, desalination holds the key to a drought proof, environmental friendly, and sustainable future potable water supply.

Terminology
Brackish water: Brackish water is characterized by the presence of higher total dissolved solid (TDS) levels than potable water, but lower TDS levels than seawater (in the range of 1,000 mg/l TDS to 25,000 mg/l TDS). Brackish water can be found in the coastal areas where fresh water mixes with the ocean (such as bays and estuaries), in aquifers (where it is usually referred to as saline water), and in surface waters (salt marshes, for instance, contain brackish water).
Conventional water treatment technologies: Typical conventional water treatment consists of screening to remove debris; coagulation to combine solids so that they settle; sedimentation to settle suspended solids; and filtration. Conventional water treatment processes have been around for more than 100 years.
Ground water: Water normally found underground and obtained from wells. It should not be confused with surface water such as rivers, ponds, lakes, or waters above the water table.

References

Water Science and Technology Board (2004) Review of the Desalination and Water Purification Technology Roadmap, The National Academy Press.

California Coastal Commission (2004) Seawater Desalination and the California Coastal act.

Texas Water Development Board (2004) The Future of Desalination in Texas Volume 1: Biennial Report on Seawater Desalination.

Young, M., Proctor, W., Qureshi E., and Wittwer, G. (2006), Without Water: The economics of supplying water to 5 million more Australians, Policy and Economic Research Unit, CSIRO Land and Water; and Centre for Policy Studies, Monash University, Australia.