Desalination is the process of converting seawater into potable water by removing salt and other minerals. Although rudimentary forms of desalination have been used since antiquity, only in the mid-20th century did industrial-scale desalination methods become widely available for water-insecure coastal communities around the world. Today, about 300 million people in more than 150 countries get water every day from some 16,000 desalination plants.

Only 2.5% of surface water on the planet is freshwater, and only a fraction of that is available and suitable for human consumption. As climate change intensifies, desalination provides alternative drinking water and irrigation source. However, it also has significant environmental impacts. Emerging technologies can help mitigate some of these effects, but desalination is a tradeoff between meeting the increasing human demands on freshwater sources and the environmental problems the process exacerbates.

Process and Technologies

Throughout history, people have used various methods of distillation and filtration to supplement freshwater supplies. But not until the mid-20th century did desalination become a large-scale, industrial process capable of supplying water to major population centers. Today, there are three basic categories of desalination in wide use: membrane technologies, thermal technologies (distillation), and chemical processes. Currently, membrane and thermal techniques are the most commonly utilized desalination methods.

Thermal Distillation

Thermal desalination involves boiling water until it evaporates, leaving behind the salt. The water vapor, now salt-free, is then recollected through condensation. The heat energy necessary to accomplish this on a large scale comes from steam generators, waste heat boilers, or by extracting steam from power station turbines.

One of the most prevalent thermal techniques is multistage flash distillation (MFS), a type of facility that is relatively simple to construct and operate, but extremely energy-intensive. Today, MSF desalination is most common in the Middle East, where abundant fossil fuel resources make this feasible, according to the International Water Association.

Membrane Separation

The basic technology with membrane desalination involves application of intense pressure to force saltwater through several tiny, semi-permeable membranes.

These membranes allow the water to pass through, but not the dissolved salts. That sounds simple, but it’s another very energy-intensive undertaking. The most common membrane process is reverse osmosis, first developed in the 1950s and commercialized in the 1970s. This is now the most widely used type of desalination outside the Middle East and North Africa.

Environmental Benefits and Consequences

Desalination is an important technology for supporting water security and resilience in arid, drought-prone communities close to sources of salt water or brackish water. By reducing demand for freshwater sources like groundwater, rivers, and lakes, desalination can help preserve habitats reliant on those same water resources.

Although expensive, desalination is generally a dependable local source of clean water, not only for human consumption but for agriculture. Small-scale desalination facilities in rural, water-scarce areas can help ensure water security for some of the most vulnerable communities. Larger facilities can play an important role in ensuring urban residents have access to safe, reliable drinking water. The use of desalination will likely expand in the coming years as climate change intensifies drought and contributes to the diminishing quantity and quality of freshwater resources.

But desalination is not without drawbacks. The biggest concerns are its energy footprint, the amount of wastewater produced and released back to the ocean, and the detrimental effects on marine life at both ends of the process. With more facilities coming online all the time as communities seek more climate-resilient water supplies, desalination isn’t going away. New technologies may reduce some of its environmental impacts.

Energy Use

The vast majority of desalination plants are still powered by fossil fuels. That means desalination contributes to greenhouse gas emissions and worsening climate change. Renewable-powered desalination facilities do exist, however, but are so far mostly limited to small-scale operations. Efforts are underway to make them more common and more cost-effective. Recent evidence suggests that renewable-powered desalination can work almost anywhere that has access to ocean water or brackish waters.

Solar, wind, and geothermal already provide viable options for powering new desalination facilities, with solar the most common source of energy for renewable-powered desalination plants. A hybrid approach that alternates renewable sources such as wind and solar may provide greater reliability during times of fluctuating energy production. Harnessing ocean power for desalination is another emerging area of research.

In addition, a number of technologies in development aim to achieve greater energy efficiency in desalination. Forward osmosis is one nascent technology showing promise. Another involves the use of low-temperature thermal desalination, which evaporates water at lower temperatures to reduce energy consumption and then reconstitutes it in liquid form. Less energy-intensive technologies like this may pair well with renewables, as detailed in this study by the National Renewable Energy Lab that explores powering low-temperature thermal desalination with geothermal energy.

Impacts on Marine Life

Half of the seawater used in desalination ends up as briny wastewater mixed with toxic chemicals that get added during purification. High-pressure jets flush this wastewater back to the ocean, where it threatens sea life.

A recent study found that the amount of brine in that wastewater is 50% greater than previously estimated. Standards for releasing wastewater back into the ocean vary considerably. In some regions, particularly the Arabian Gulf, Red Sea, Mediterranean Sea, and the Gulf of Oman, desalination plants are frequently clustered together, continuously pouring warm discharge into shallow coastal waters. This can raise seawater temperature and salinity and lower overall water quality, adversely affecting coastal marine ecosystems.

The initial intake of seawater also poses risks to marine life. Drawing water from the sea results in the death of fish, larvae, and plankton as they are inadvertently pulled into the desalination plant. Every year, millions of fish and invertebrates get sucked into desalination facilities and trapped on intake screens. Those small enough to pass through the screens enter the system and die during chemical salt water processing.

Design changes may reduce the number of marine organisms killed in this process, including the use of larger pipes to slow the intake of water, which allows fish to swim out and escape before they get trapped.

New technologies can reduce the amount of wastewater that flows to sea and more effectively disperse that waste to mitigate impacts on marine life. But these interventions can only work if they are adopted and properly enforced.

Toward More Data, Better Standards

Powering desalination systems with renewable energy and building facilities that mitigate potential harm to marine life require investing in research to better understand environmental impacts and using that data to develop better regulations for designing and operating plants. A useful example comes from California, which enacted the Desalination Amendment to its ocean water quality control plan. This mandates a consistent statewide process for seawater desalination facility permitting, requiring certain site, design, and operational standards be met to minimize harm to marine life.

Do the Benefits Outweigh Environmental Impacts?

According to the United Nations, some 2.3 billion people live in water-stressed countries. And 4 billion people—nearly two-thirds of the world’s population—experience severe water scarcity at least one month of the year. These numbers are likely to increase with intensifying drought and freshwater depletion.

Water managers and policymakers know that desalination cannot be the sole solution to water security. It’s too expensive, and it doesn’t guarantee an endless supply of freshwater free from environmental consequences for our ever-growing global population. Instead, it must be combined with smart water conservation technologies to prevent waste in agricultural, residential, extractive, and industrial sectors. Investing in water conservation represents an alternative strategy with far less environmental cost.

Water-scarce cities around the world are showing how conservation can be accomplished through a combination of use restrictions and innovative strategies, like greywater recycling and wastewater reuse. Las Vegas, Nevada, for example, recently imposed a permanent ban on decorative grass—one of several restrictions the city has placed on water use as its main water source, Lake Mead, reaches dangerously low levels. At the same time, the region’s water district uses a high-tech wastewater treatment process to purify greywater and sewage for reuse by local golf courses, parks, and businesses, and returns a portion of the clean water to Lake Mead for future use.

Humanity will need to use every trick in the book—and a few tricks we haven’t dreamed up yet—to ensure a safe, steady supply of water for a growing population. New desalination technologies will certainly be among them, but desalination must be coupled with strong, consistent standards and enforcement to ensure that the costs don’t outweigh the benefits.