desalination | US Geological survey (2023)

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Thirsty? How about a refreshing glass of seawater?

No, don't take us literally! Humans cannot drink salt water. But salt water can be transformed intofresh water, which is what this portable, inflatable solar still is all about (it even converts to a small package). The process is called desalination, and it is used increasingly around the world to provide people with drinking water. Most of the United States has, or can have access to, ample supplies of drinking water. But, fresh water can be in short supply in many parts of the country and the world. And as the population continues to grow, freshwater shortages will occur more frequently, if only in certain places. In some areas,saltwater(ofOcean, for example) is being transformed into drinking water.

The "simple" hurdle that must be overcome to convert seawater to freshwater is to remove the dissolved salt in the seawater. This may seem as easy as boiling some seawater in a pot, capturing the steam, and condensing it back into water (distillation). Other methods are available, but these current technological processes must be done on a large scale to be useful for large populations, and current processes are expensive, energy-intensive, and involve large-scale facilities.

What makes water saline?

What do we mean by "salt water"? Saline water contains significant amounts (referred to as "concentrations") of dissolved salts. In this case, the concentration is the amount (by weight) of salt in the water, expressed in "parts per million" (ppm). If the water has a concentration of 10,000 ppm dissolved salts, then one percent of the water's weight is dissolved salts.

These are our parameters for salt water:

• Fresh water: less than 1000 ppm
• Slightly saline water - From 1,000 ppm to 3,000 ppm
• Moderately saline water - From 3,000 ppm to 10,000 ppm
• Very saline water - From 10,000 ppm to 35,000 ppm

By the way, ocean water contains about 35,000 ppm of salt.

The world's need for fresh water

The scarcity of freshwater resources and the need for additional water supplies are already critical in many arid regions of the world and will become increasingly important in the future. Many arid areas simply lack freshwater resources in the form of surface water, such as rivers and lakes. They may have only limitedunderground water resources, some of which are becoming more brackish as the extraction of water from aquifers continues. Evaporation from solar desalination is used by nature to producerain, which is the main source of fresh water on Earth.

According to the International Desalination Association, globally, more than 300 million people receive water from desalination plants. The largest desalination plant in North America, the Claude "Bud" Lewis Carlsbad Desalination Plant, is located near San Diego, California. It generates about 10 percent of the fresh water used by about 3.1 million people in the region and costs about twice as much as other water sources. There are currently 11 desalination plants in California and 10 more plants have been proposed. The largest desalination plant in the world, the Sorek Desalination Plant is located south of Tel Aviv, Israel and produces more than 137 million gallons of fresh water per day. In contrast, the Carlsbad plant produces approximately 50 million gallons of water per day.

As of 2015, in the US, in some states, public water sources include desalinated seawater or brackish groundwater that has been treated to reduce dissolved solids. A combined 7.21 million gallons per day of saline surface water withdrawal for utility use was reported in the US Virgin Islands, Texas, Florida, and Massachusetts. A combined 263 million gallons per day of saline groundwater abstraction for public use was identified in Florida, California, Texas, Virginia, Kansas, and Utah. (Deiter et al., 2018).

Other Method: Reverse Osmosis

Another way that salt water is desalinated is through the "reverse osmosis" process. In the simplest terms, water, containing dissolved salt molecules, is forced through a semi-permeable membrane (essentially a filter), in which larger salt molecules do not pass through the holes in the membrane, but the smallest water molecules do.

Reverse osmosis is an effective way to desalinate salt water, but it is more expensive than other methods. As prices drop in the future, the use of reverse osmosis plants to desalinate large amounts of salt water should become more common.

Desalination is not modern science

Desalination by distillation is one of humanity's earliest forms of water treatment and remains a popular treatment solution around the world today. In ancient times, many civilizations used this process on their ships to turn seawater into drinking water. Today, desalination plants are used to convert seawater into drinking water on ships and in many arid regions of the world, and to treat water in other areas polluted by natural and unnatural contaminants. Distillation is perhaps the only water treatment technology that most completely reduces the widest range of contaminants in drinking water.

In nature, this basic process is responsible forwater cycle (hydrological). The sun provides energy that causes water to evaporate from surface sources such as lakes, oceans, and streams. Hewater steamit eventually comes into contact with cooler air, where it condenses again to form dew or rain. This process can be artificially mimicked and faster than in nature by using alternative sources of heating and cooling.

Why don't we see more desalination plants if fresh water is so scarce?

The main reason we don't see more desalination plants is because of the cost of running these plants. Salt dissolves easily in water and requires a lot of energy to break chemical bonds. Energy consumption adds to the cost of running a plant, and the cost of the technology to desalinate water also takes its toll.

Sometimes, environmental protection is a factor that can prevent the construction of desalination plants. In 2022, following a 20-year plan to build a desalination plant in Huntington Beach, California, the California Coastal Commission rejected the bid to build the plant due to concerns about higher water rates, loss of marine life, and impacts on poor families. who already lived nearby. Industrial Areas. Area residents did not like the fact that the plant took in 100 million gallons of seawater per day, desalinated half of it, and discharged the other half as saltier brine.

Your own personal desalination plant

Do you remember seeing the photo at the top of this page of a floating solar still? The same process that drives this device can also be applied if you are in the desert and need a glass of water.

The low-tech approach to accomplishing this is to build a "solar still" that uses the sun's heat to run a distillation process to cause dew to form on something like a plastic sheet. The diagram to the right illustrates this. The use of seawater or plant material in the body of the still creates moist air which, due to the enclosure created by the plastic sheet, is heated by the sun. Moist air condenses water droplets on the underside of the plastic sheet and, due to thesurface tension, the drops of water adhere to the sheet and fall into a drinker, from where they can be consumed.

Some facts about desalination

• It is estimated that about 30% of the world's irrigated areas suffer from salinity problems and remediation is considered too expensive.
• According to the International Desalination Association, as of June 2015, 18,426 desalination plants were in operation worldwide, producing 86.8 million cubic meters per day and providing water to 300 million people. This number increased from 78.4 million cubic meters in 2013, an increase of 10.71% in 2 years.
• The largest users of desalinated water are in the Middle East (mainly Saudi Arabia, Kuwait, the United Arab Emirates, Qatar and Bahrain), which uses about 70% of the world's capacity; and in North Africa (mainly Libya and Algeria), which uses about 6% of the world's capacity.
• Among industrialized countries, the United States is one of the largest users of desalinated water, especially in California and parts of Florida. The cost of desalination has prevented desalination from being used more frequently.

To learn more about desalination/desalination plants, visit the links below.

• Desalination: Limitations and Challenges (Hinkenbien, 2004)
• Safe drinking water from desalination (World Health Organization)
• Desalination Data (Texas Water Development Board)
• Desalination Overview (International Desalination Association)
• Estimated water use in the United States in 2015 (USGS)

*Some of this information comes from the Educational Water Foundation and the Corpus Christi Public Administration TAMU-CC.