In most years, winter and spring rains refill Lake Lanier, the reservoir that supplies the city of Atlanta, Georgia with water. But from 2007 to 2009, water levels in the 38,000-acre reservoir reached a historic low, signaling that the region was facing one of the worst droughts of the century. Cities across the Southeast declared a state of water emergency, as conflicts ensued among Alabama, Florida, Georgia, North Carolina and South Carolina over access to river waters.

Though the drought has passed, the possibility of water scarcity hovers over the Southeast. Power generating facilities in the region today use 65 percent of total area water withdrawals to cool equipment. Meanwhile, just downstream, water systems rely on large amounts of energy produced by these facilities to treat and distribute water to homes and businesses. But neither power plants nor water providers need to use as much water or energy as they do. In fact, this nexus -- the water used to produce energy and the energy required to treat and deliver water -- presents a strong opportunity for smarter, more efficient planning in how we use both resources. And, as cities and regions across the world work to mitigate and adapt to a changing climate, the energy-water nexus may provide some answers.

Water for Energy Generation

Water is used through the entire energy production process -- from mining to transport, generation, and disposal. In the United States, mining accounts for nearly one percent of all freshwater withdrawals, with coal, oil, natural gas, and uranium mining as key industries within this sector. Coal mining alone uses 50 to 59 gallons of water for every ton of coal mined, while oil shale requires 4 gallons of water for every gallon of fuel extracted. Renewable energy resources such as dedicated energy crops and some geothermal installations can require significant quantities of water, while other sources, like photovoltaic cells and wind turbines, have minimal water requirements.

Transporting fossil fuels also requires water. Transportation networks like coal-slurry pipelines mix coal with water to move the material from source to power plants. Nevada’s Mojave Power Plant, for example, used one billion gallons of water annually before shutting down in 2005. Shipping crude oil by tanker or barge across waterways can result in oil spills, limiting the availability of clean water for drinking, fishing, and swimming, and incurring high cleanup costs.

By far, however, the most water in the energy process is used by thermoelectric power plants, which account for 80 percent of electric generating capacity in the United States. Thermoelectric plants use heat from raw materials, including coal, oil, natural gas, nuclear, and biomass, to convert purified water into steam. This steam then spins a turbine connected to a generator to produce electricity. Plants withdraw large quantities of cooling water to convert steam back to purified water to repeat the process. Thermoelectric facilities in the United States are the second largest water user, accounting for 39 percent of all freshwater withdrawals , just after irrigation, which accounts for 41 percent.

Not surprisingly, limited water supplies and heat waves have actually stalled power production in many regions. In 2006, the Idaho state assembly unanimously passed a moratorium on the construction of two coal-fired power plants due to water and other environmental constraints. Similarly, power plants in the Southeast were shut down due to a drought between 2006 and 2007. And, during the 2003 European heat wave, which claimed 35,000 deaths across the continent, nuclear facilities in France had to restrict power production because of low water levels and concern over the impacts heated water discharged from plants would have on river systems and fisheries. Because thermoelectric power plants face increasing competition with essential water needs like agriculture and household use, decisions to produce reliable energy supplies must take water into account.

Energy for Water Treatment and Use

Just as water is used for energy generation, large quantities of energy are used for water collection, treatment, distribution, and end use in our buildings and industries. Most of the water we consume is collected from rivers, lakes, streams, and underground aquifers and delivered to centers to be treated for human and commercial use, using large quantities of energy. In California, the State Water Project is the largest single energy consumer in the state, moving water from its source in Northern California up 2000 feet and over the Tehachapi Mountains to customers in Southern California. The electricity used in this process is equivalent to one-third of the electricity used to power all homes in Southern California.

After collection, water is delivered to treatment centers where it passes through physical and chemical processes to meet water quality standards. Pumping water through treatment typically accounts for 15 percent of all energy use in the water treatment and distribution process. Irrigated water for agriculture generally does not undergo treatment, but still requires significant quantities of energy for use; an astonishing 85 percent of electricity on farms is used to pump groundwater for crop irrigation. And while some communities have turned to desalination plants to overcome freshwater scarcity, the energy requirements for treating water from saline aquifers and seawater off our coasts are still quite costly. Forty percent of all desalination costs go towards energy use, which is usually produced from fossil fuels.

Once water is treated, it is distributed for household and commercial use, accounting for the other 85 percent of energy used in the treatment and distribution process. Water providers spend the vast majority of their electricity bills on pressurizing and pumping water from treatment facilities to distribution blocks. When water arrives in homes and businesses, it requires additional energy for circulating, filtering, and pumping water for domestic and commercial use. The energy required to run a faucet for five minutes is equivalent to the energy used to power a 60-watt light bulb for 14 hours. And heating and cooling the water we use requires additional energy. Nationally, more energy is required to heat water for domestic use than to light our homes.

Finally, when water leaves homes and businesses it undergoes wastewater treatment, which, like initial water treatment, relies on biological and physical processes. Combined, the initial water treatment and post-use wastewater treatment account for an astounding 4 percent of total electricity use nationwide. Research suggests that the demand for electricity from wastewater plants alone will increase 20 percent in the next 15 years as a result of population growth. Hence, ensuring energy-efficient water treatment and use is imperative.

The Way Forward: Integrated Planning

Energy and water security are achievable, and with careful planning and efficiency measures, we can greatly reduce the amount of water used to produce energy, and the amount of energy used to provide and use water. But this requires an integrated understanding of the relationship between these two vital resources, research and development of efficient technologies, and effective policy measures to ensure that appropriate solutions can be adopted at the state and local level.

Water security depends on our ability to adopt water-efficient technologies on a large scale. Existing plants can be retrofitted with dry-cooling or hybrid wet-dry cooling technologies, which rely entirely or partially on air to cool equipment. In addition, renewable energies such as photovoltaic systems, wind turbines, and organic waste material can significantly reduce the need for water to generate energy.

Nevertheless, these solutions require strong support at the state and federal level. State regulators should ensure that all new power plants assess water availability and evaluate options to minimize water requirements. States should also implement policies to encourage for existing thermoelectric power plants to reduce water withdrawals and make accessible information on best practices. Finally, federal officials should consider water impacts in all federally funded and reviewed energy projects to ensure that energy production does not compete adversely with our ability to use water for essential purposes.

Reducing energy demands in water treatment and use is equally crucial. Cities often devote significant fractions of their budgets to water and wastewater treatment, and 30 percent of operational costs at treatment plants are for energy. To achieve real cost savings, plants can install energy efficient technology such as high-efficiency pumps, motors, and variable frequency drives to manage energy use. After installing an energy management system, the Encina Wastewater Authority in California saves approximately $611,600 per year . Similarly, the East Bay Municipal Utility District upgraded to energy-efficient equipment and processes and is now saving $2.8 million annually . Some wastewater treatment facilities are investing in advanced anaerobic digestion where methane from municipal wastewater is used as an energy source. This process reduces solid wastes, improves water quality, and lowers energy costs. In addition, households and businesses can save on energy bills by installing high-efficiency toilets, weather- and sensor-based irrigation and other energy saving technologies.

Still, strong policy is key. States should ensure that water providers take into account the distance and energy to deliver water when siting water treatment facilities, provide assistance to measure savings associated with energy efficiency, and supply information and financial incentives to water providers and users on energy efficient technologies. Furthermore, federal support for research, development, and deployment of water technologies that use less energy is of critical importance.

Such an integrated approach, including water planning into energy policy and energy planning into water policy, can reduce costs for energy generators and water providers and users, encourage the innovation and adoption of new technologies, and offer a reliable way to achieve water and energy security in the near future.