Is It Better To Recycle Waste Water or Desalinate Seawater?

Barbara10Broek By Barbara10Broek, 25th Aug 2011 | Follow this author | RSS Feed
Posted in Wikinut>Guides>Science>Environmental Science

Discusses ways to recycle waste and sea water for human use.

Which is better?

It may seem surprising, but according to the City of San Diego Water Department, it currently costs about twice as much to desalinate seawater as it costs to take the same quantity of water “from toilet to tap.” The reason is that ocean water is about 25 times saltier than the starting point for recycled water.

Removing dissolved salts is the most energy-intensive step in producing drinking water. The more salt in the water, the more energy required to remove it. Salt removal is accomplished either via distillation or, more commonly, reverse osmosis. In reverse osmosis, water is pushed through a membrane that allows water molecules through, but not the dissolved salts.

According to a 2006 report by the Pacific Institute for Studies in Development, Environment and Security, the cost of desalinating water in California and delivering it to users may be as high as 1 cent per gallon and is unlikely to fall below one-third cent per gallon. Although considerably cheaper than bottled water, even the lower estimate is more than the price paid by most urban water users and is about 10 times the price paid by farmers in the western United States.
Desalination Technology: Health and Environmental ImpactsSan Diego imports about 90 percent of its drinking water from Northern California and the Colorado River. As the costs of alternative sources of water rise due to drought and increased demand, desalination will become a more viable option. It already has in the Middle East, which is home to more than half of the world’s desalination plants.

In the United States, direct recycling of wastewater to drinking water is not accepted practice, probably for the psychological reasons mentioned. On the other hand, indirect recycling of wastewater into drinking water is common. For example, cities upstream discharge treated wastewater into rivers that serve as the drinking water supply for cities downstream. Also, in some places, including Los Angeles and Orange County, recycled water is used to top off underground aquifers that supply drinking water.

Recycled water does not contain levels of bacteria, heavy metals, or organic compounds that exceed drinking water standards. However, levels of dissolved salts are higher than those in the drinking water supply.

In San Diego, recycled water is used mainly for irrigation. Some new high-rises are being built with a dual plumbing system via which the city supplies recycled water for flushing toilets. The dual system adds around 10 percent to the cost of installing the plumbing.

The future of seawater?

As population growth increases the demand on existing sources of freshwater, desalination of seawater is becoming increasingly economically viable. The worldwide market for water desalination is increasing about 15 percent annually.

Reverse osmosis, which involves forcing seawater through a membrane that allows only water molecules through, is the most popular desalination technology in the United States. Improvements in the membrane material that make it longer-lasting and less likely to clog are increasing its cost-effectiveness. The concentrated salt solution that remains behind typically is dumped back into the ocean.

Several new desalination technologies are being explored. Freeze separation involves freezing seawater to get ice crystals of pure water. In vacuum distillation, saltwater is vaporized at low pressure, which requires less heat than distillation at atmospheric pressure. In electrodeionization, seawater passes between two parallel membranes on the inside of oppositely charged plates. Because ions in the seawater are attracted to the plates, the sodium, chloride, and other ions are pulled out through the membranes, leaving behind pure water.


Desalination, Human Consumption, Recycle, Reverse Osmosis, Seawater, Waste Water

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author avatar Mark Gordon Brown
25th Aug 2011 (#)

With water as important as it is, with populations exploding, we cannot afford to waste it. Good points here.

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author avatar Steve Kinsman
26th Aug 2011 (#)

Very informative and interesting Barbara. Does not dumping the salt back into the ocean increase the ppm's of salt in the ocean, ultimately making the process more costly?

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