The United States is a thirsty nation: We use about 150 trillion gallons of water a year, according to the U.S. Geological Survey—to grow food, generate electricity and run our homes. The collection, transport, storage, treatment and eventual return of most of this deluge back to nature requires significant amounts of energy. The Environmental Protection Agency calculates that the energy spent countrywide to supply drinking water and process wastewater releases 116 billion pounds of carbon dioxide annually, on par with the emissions of 10 million cars.
Unlike national greenhouse-gas emissions, which have increased 17 percent since 1990, water demand in the United States has actually dipped since historical peaks from 1975 to 1980, according to USGS data, even as the population has increased 30 percent. It's one of the biggest environmental success stories of the past 30 years. By taking water conservation further, and by thinking differently about how we treat and move water, analysts believe the U.S. can achieve dramatic reductions in greenhouse-gas emissions fairly quickly--—while also helping to address a growing water crisis that has scientists and water managers worried in nearly every region of the country.
Fossil-fuel-burning power plants are America's biggest water consumers, partly because there are so many of them: Last year, carbon-intensive coal, natural gas and other fossil-fuel-fired plants provided two-thirds of America's electricity. And a significant amount of that energy went into pumping and treating water. Essentially, energy is needed to provide the nation's water supply, and water is needed to provide the nation's energy supply, says Michael Hightower, a researcher at Sandia National Laboratories and a member of a Department of Energy team that has studied the relationship. Researchers call this the "water-energy nexus."
As demand for energy increases—the Federal Energy Information Agency by 2030—so will the demand for water. But the country faces the potential for steep water shortages independent of energy consumption as well, and that will only exacerbate both problems. In a nasty feedback loop modeled by Heather Cooley, a senior research associate at the Pacific Institute, the growing scarcity will drive up energy usage, as water becomes harder to procure, deliver and treat. Intensified regional droughts will force some utilities to pump more water from deeper groundwater sources or to import the H2O from elsewhere, lengthening supply lines and driving up energy needs and carbon emissions.
So what can be done to keep the water flowing—and the lights on—while actively reducing carbon dioxide?
When it comes to water-related energy use, the lowest hanging fruit is obvious: "Water conservation and efficiency are going to be the key way to meet greenhouse-gas reductions," the Pacific Institute's Cooley says. She points to huge opportunities in agriculture, including improved irrigation efficiency through the widespread adoption of simple technologies such as rain sensors. While residential and commercial water use may be small compared to water consumption from agriculture and power production, the energy cost does add up: Water must be treated and heated or cooled before it reaches the faucet, then it must be treated again after it goes down the drain and is piped away. For that reason, appliances such as high-efficiency washing machines and low-flow toilets can make a cumulatively deep dent in water-related carbon emissions. The EPA's WaterSense program, which promotes water-efficient fixtures, says that retrofitting just one out of every 100 American homes with such hardware would shave 80,000 tons of greenhouse-gas emissions a year.
In cities, capturing storm water for local use, instead of draining it for subsequent treatment and re-delivery, would cut carbon emissions related to transporting water; several states and municipalities in the water-strapped West have recently eased permission to catch water or even mandated rainwater-catchment measures. Graywater recycling can also divert nonpotable water from showers and washing machines to irrigate landscapes and gardens. In a case study of urban areas in Southern California, the Natural Resources Defense Council says that carbon emissions would shrink up to 292,000 metric tons per year if all new residential and commercial developments and redevelopments implemented such practices by 2030.
Carbon dioxide isn't the only greenhouse gas related to water use. Wastewater treatment facilities emit about 5 percent of the nation's anthropogenic methane, which can trap heat in the atmosphere with 21 times the efficiency of carbon dioxide. Mark Shannon, director of the Center of Advanced Materials for Purification of Water with Systems (WaterCAMPWS) and a professor at the University of Illinois, calls for greater utilization of anaerobic bioreactors: Microbes feed on wastewater to produce methane that, rather than being released directly into the atmosphere, can be captured and burned to generate electricity.
Another step that can be taken in the near term to identify emissions-reduction opportunities is to evaluate the country's overall water resources with a new national water census. The last such assessment occurred in 1978, although the USGS is gearing up for another one soon. Sandia's Hightower says that a good analysis of brackish water supplies is lacking, and that accounting for non-freshwater resources will be integral in planning to utilize them. "There's not a lot of new freshwater out there," he says, "so there's a movement in the country to use nontraditional water resources much more extensively." This will help reduce carbon emissions associated with conveying freshwater from distant aquifers and surface-water sources.
Over the next decade, upgrading the nation's decaying water infrastructure could also greatly reduce the energy used to treat water that leaks from the system, rather than reaching consumers. "Many of these pipes have been in the ground for a hundred years," Cooley says, and it often takes a catastrophe like a water-main break or a sinkhole before anything is done to plug a leak. Illinois' Shannon ballparks the annual number of U.S. water-main breaks at 250,000. Mark Modzelewski, executive director of the Water Innovations Alliance, says that Chicago loses perhaps 60 percent of its water in transit from treatment to tap because of crumbling infrastructure.
To tackle this decaying infrastructure, engineers have proposed a national "smart water" grid, in the same spirit of the much-touted smart electricity grid. Informational technologies that keep real-time tabs on water circulation and quality could spot leaks early, and automated sensors, connected metering systems and better analytics could lead to more efficient distribution. Modzelewski says such a grid could slash U.S. water use by 30 to 50 percent, leading to a commensurate cut in carbon emissions.
As the nation looks for alternatives to gas and diesel, the kind of transportation fuel it settles on could also skew the water-energy nexus for the better—or the worse. Fuel from oil shale and tar sands, for example, requires several times the amount of water per mile driven as conventional gasoline, says Carey King, a research associate at the Center for International Energy and Environmental Policy at the University of Texas at Austin. And by 2030, nearly 10 percent of the national consumption of freshwater will go into making biofuels for vehicles, according to a study King co-authored. (The figure stood at 2.4 percent in 2005.) Shifting the emphasis to less water-intensive fuels, such as cellulosic, rather than corn-based, ethanol and algae, which can thrive in brackish water, would help head off water scarcity.
Renewable energy sources for electricity production, including wind, biomass, geothermal, hydroelectric and solar (although solar thermal production may require considerable amounts of water), could also change the carbon-from-water equation for the better. The Energy Information Administration's most recent projections to 2030 have electricity from clean sources increasing output by 3.3 percent a year, which more than makes up for annual energy demand increases of half a percent. So as the U.S. weans itself off burning fossil fuels for energy, the carbon emissions caused by supplying water to these power plants may go down as well.