How Is Seawater Converted to Freshwater?

The process of converting seawater to freshwater is known as desalination. Desalination is one of the earliest forms of water treatment known to mankind and is a widely used form of water treatment today. Modern desalination is done on ships and in regions of the world where water is scarce. Desalination plants are the primary means of converting seawater to drinking water and also removing contaminants found in water to make water potable.

Seawater not Fit for Drinking

• Seawater, or saline water, is not fit for drinking because of the high content of saline. The concentration of saline in water is measured by parts per million (ppm). Moderate saline water contains saline in the 3,000 ppm to 10,000 ppm range, while highly saline water ranges from 10,000 ppm to 35,000 ppm. Seawater is 35,000 ppm. Seawater causes dehydration because of the high concentration of saline. It also disrupts your body on a cellular level, causing your cells to hydrate in a process called a hypertonic condition.

How Nature Purifies Water

• Nature performs its own form of desalination through a process called the hydrologic cycle. The sun evaporates water from surfaces like lakes, streams, rivers and the ocean and creates water vapor. The water vapor mixes with cooler air, re-condensing to create rain or dew.

Desalination as an Industry

• Desalination plants are found all over the world. In the United States they are found in coastal states like California and Florida. The Middle East has many desalination plants because of the arid conditions and lack of fresh drinking water.

  Desalination converts seawater to freshwater in one of two ways: Reverse osmosis forces seawater through a membrane which captures salt and other minerals, while allowing water molecules to pass through. Distillation uses heat to create steam, which is then condensed to produce water with a low salt concentration and less of the impurities found in the water before distillation.

  The final product of either technology can lower the saline concentration in sea water from 1 to 500 ppm, which meets drinking water standards in the United States.

The Cost of Desalination

• Desalination is a very expensive means of purifying and providing drinking water; as of 2008, the average cost to produce one acre-foot of desalinated water from seawater ranged from $800 to $1,400, compared with $200 per acre-foot for water from other supply sources that don't need desalination. California has been on the leading edge of the desalination movement with plants in several coastal towns, taking the issue of water purification head-on.

Benefits and Effects of Desalination

• The benefits of desalination become more apparent when you consider population growth, water crises in several coastal states like Florida and California, and droughts. Disposal of waste created by desalination is another concern. The sediment and brine left behind by desalination must be disposed of without harming other ecosystems. A technology that requires immense start-up capital, has environmental drawbacks and has a high cost/benefit ratio is sure to be a topic of heated debate for years to come.