Renewable water

Water is naturally replenished through a process known as the water cycle or hydrologic cycle, in which water evaporates from liquid into vapor, collects in the atmosphere in clouds, then returns to the ground as precipitation. This ongoing process of replenishment is why scientists classify water as a renewable resource.

Despite its inherent renewability, the supply of water available for human use and consumption is limited. The relative availability of potable and usable water in a given area depends on many factors including geography, climate, population size, and local demand. According to the World Health Organization (WHO), more than two billion people live in water-stressed countries, and 27 percent of the global population lacked access to safely managed potable water services in 2022. Experts believe that the number of people living in water-stressed regions is likely to rise by a considerable margin because climate change is projected to worsen existing rates of water scarcity.

Given the critical importance of water resources and the challenges associated with improving and maintaining them, scientists have developed systems and technologies to make water more accessible and available. These techniques and systems are sometimes described as renewable water technologies.

Background

Scientists and human development agencies broadly define renewable water as surface or subsurface freshwater resources that can be naturally replenished by precipitation. Precipitation is one of the key steps in the hydrologic cycle. Multiple models of varying complexity are used to describe the stages of the water cycle, with the simplest examples characterizing it as a four-step process that includes evaporation, convection, precipitation, and collection.

Evaporation occurs when heat from the sun causes water on Earth’s surface to change its form from liquid to vapor. Some models include a process known as transpiration as an additional step in the evaporation stage. Transpiration happens when a plant obtains water by drawing it out of the soil through its roots before releasing the water through its leaves as vapor.

Convection covers the process of water vapor rising into the atmosphere with warm air, where it loses heat and changes back into a cooled liquid form through a process known as condensation. When condensation occurs, the liquid water suspended in the atmosphere joins with other water molecules to form clouds.

Clouds become heavier as they draw more and more water molecules into their structures. They eventually become so heavy that the air in the atmosphere can no longer support the cloud’s weight. When this happens, clouds release their water as precipitation. This precipitation can take multiple forms, including rain, snow, sleet (freezing rain), and hail. Precipitation returns previously evaporated water to Earth’s surface, where it can be used and consumed again.

At the collection stage, also known as the storage stage, some surface-level water gets stored in structures and systems that functionally remove it from the water cycle. These structures and systems include oceans, glaciers, snow caps, and underground reservoirs. Lakes and rivers also function as water storage systems.

The hydrologic cycle has been recycling the global water supply on a continuous basis since water first appeared on Earth. While scientists are not sure of water’s precise origins, two leading theories have emerged. The first theory suggests that water has extraterrestrial origins and first reached Earth via meteors and other celestial objects that crashed into the young planet billions of years ago. This theory holds that Earth got most of its water during a hypothesized event known as the Late Heavy Bombardment. During the Late Heavy Bombardment, meteors and other objects originating in deep space are believed to have crashed into Earth with regular frequency, bringing water with them and releasing the water directly onto the Earth’s surface upon impact. Earth’s gravity and magnetic field then prevented the water from being removed by the Sun, leading to the formation of the oceans and providing a critical catalyst for the development of life.

The second and more recent hypothesis theorizes that water formed on Earth during the planet’s early history and gradually rose to its surface over time through natural processes. Some experts think that a combination of both explanations is more likely, with the planet’s native water supply combining with water introduced by celestial objects to generate all the water on Earth.

Overview

The concept of renewable water as it applies to engineered systems involves these approaches: atmospheric water generation (AWG) technology and wastewater treatment strategies and systems that recycle wastewater.

Atmospheric Water Generation

AWG systems extract moisture out of humid ambient air, turning it from vapor into a liquid form suitable for human use or consumption. These systems draw on various techniques to achieve results. Most of the functional and commercially viable units available as of 2024 use condensation as the main mode of action.

Some condensation-based AWG systems use networks of condensers and cooling coils to draw water molecules out of the air using a mode of action comparable to that of a dehumidifier. Once they are drawn into the system as vapor, water molecules are cooled until they reach a threshold known as the dew point, at which time the water returns to a liquid state. The liquefied water then passes through a filtration system that removes impurities before depositing the water into a collector unit, from which it can be retrieved and used.

An alternative process known as hygroscopic water harvesting generates similar results by using fan networks powered by solar energy. Hygroscopic systems consist of solar-powered hydropanels, which capture and cool ambient air to draw moisture out of it. Built-in ozone generators are then used to purify the water once the fan system cools it beyond its dew point and back into liquid form.

Other water generation technologies include a system known as Wood-to-Energy Deployable Water (WEDEW), which was developed by a California-based social impact organization known as Skysource. The WEDEW system is capable of creating more than 528.3 gallons (2,000 liters) of water per day using an input composed of organic waste material from plants or animals. The input is exposed to humidity, which accelerates the natural process of decomposition. As decomposition continues, water vapor is produced as a byproduct and captured. In 2018, the system won the Water Abundance XPRIZE, valued at $1.75 million, for its practicality in addressing global water challenges.

Atmospheric water generators in 2024 used salt to pull water out of the air. The wet salt was heated to the boiling point, and the steam was condensed and transported to the filters. Some of these systems also mineralized water on its way out of the generator to improve its nutritional value and taste. Atmospheric water generator technology was used in the Hard Rock Community of the Navajo Nation in Arizona. It produced 200 gallons of clean drinking water for the community each day.

The drawbacks to atmospheric water generators included their cost; a medium-sized generator cost between $30,000 and $50,000. They also only work well in places with temperatures slightly above freezing and humidity above 40 percent.

Wastewater Recycling

Potable water reuse systems first became operational in the United States in the 1970s. As of 2024, such systems contributed to the drinking water supplies in areas such as Los Angeles and San Francisco, California; Phoenix, Arizona; Miami, Florida; and Austin, Texas. In 2022, municipal officials in Los Angeles unveiled a plan to recycle 100 percent of the city’s wastewater by the mid-2030s.

Potable reuse treatment plants use a variety of techniques, including biofiltration, ozonization, and reverse osmosis, to remove pollutants and impurities from wastewater before cycling it back into municipal drinking water systems. Biofiltration uses bacteria to break down the contaminants found in industrial wastewater, while ozonization treats water with ozone gas. When introduced into supplies of liquid water, ozone gas oxidizes organic contaminants including bacteria, parasites, and viruses as well as inorganic pollutants such as metals and other residues. Reverse osmosis forces water through precision filters at very high pressures, with the filter removing virtually all the pollutants and contaminants the water contains. According to a 2022 study published by Stanford University researchers, reverse osmosis cleans wastewater more effectively than the natural filtration systems that produce pure supplies of groundwater.

Wastewater treatments are also used on water that is not intended to be cycled back into drinking water systems. Instead, these treatments use screens, storage tanks, biofiltration techniques, and other advanced processes to remove harmful pollutants from wastewater before it is released back into natural bodies of water. Treated wastewater reintroduced to natural environments using these techniques can then re-enter the water cycle.

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