Wastewater recycling that seeks to supplement fresh water supplies may create more greenhouse gases than a standard water treatment plant.
A new study, performed by Amy Townsend-Small of University of Cincinnati and a team of researchers from the University of California, Irvine, found that wastewater recycling plants emit more nitrous oxide, a greenhouse gas significantly more dangerous than carbon dioxide.
Despite this, Townsend-Small, an assistant professor of geology and geography, says that the fresh water-producing plants are still valuable resources.
"Advanced methods in wastewater treatment supplement fresh water supplies in areas where fresh water is scarce," Townsend-Small said.
"Climate change, caused at least in part by manmade greenhouse gas emissions, is expected to exacerbate freshwater scarcity in many of these areas, including the United States southwest."
Townsend-Small and the research team compared the emissions at two different wastewater treatment plants in southern California. The first plant was a standard treatment plant which removes organic carbon and then returns the treated water into a river or ocean.
The second type was a wastewater recycling plant that removes both organic carbon and nitrogen. That water is then used for irrigation of landscaping and urban greenspaces.
"Wastewater reuse for irrigation, otherwise known as 'showers to flowers', potentially reduces overall freshwater consumption in southern California, which is threatened by dwindling supply and a growing population," Townsend-Small said.
The wastewater recycling plant emitted three times as much nitrous oxide than the normal recycling plant, largely due to the density of nitrifying and denitrifying bacteria in the water.
But the researchers argue that the fresh water recycling is still a worthwhile endeavor.
"Wastewater recycling is an essential component of the urban water-resource portfolio, especially in the semi-arid, urban southwest," Townsend-Small said.
"Because drinking water in southern California is imported over very long distances, it is responsible for large energy consumption and carbon-dioxide emission rates."
"If wastewater recycling can supplement drinking water resources and not just irrigation water for landscaping, then overall greenhouse gas emissions could be reduced, as could the potential for water scarcity in southern California and beyond." Townsend-Small said.
This study appears in the September-October issue of Journal of Environmental Quality, and was supported by funding from the National Research Initiative of the U.S. Department of Agriculture and the Urban Water Research Center at the University of California, Irvine.