Here’s a conundrum.
Your state has a severe water shortage. So it invests in wastewater recycling, hoping to reduce the need to import water (the transport of which is its single most energy-intensive activity). It then passes a pioneering climate change law. But five years later, it turns out the recycling is contributing to global warming by producing lots of greenhouse gases. What do you do?
Over the past few years, Southern California has built several wastewater reclamation plants, aiming to reduce its intake of water from the Bay Area and the Colorado River. It uses the non-potable resource in public parks and other urban green spaces.
But new research says the facilities are emitting three times as much nitrous oxide (N2O) as a conventional process that sends treated sewage to a river or ocean. Although N2O is also known as "laughing gas" and "sweet air," and is a drug used by dentists and thrill-seekers, it is also greenhouse gas 298 times as potent as CO2.
Amy Townsend-Small, an assistant professor at the University of Cincinnati, who conducted the research along with a team at the University of California, Irvine, says the N2O is a by-product of a process that uses bacteria to break down effluent. Scientists already knew about the problem, but the research, published in the latest edition of the Journal of Environmental Quality, is the first to identify and quantify the problem in California.
The revelation throws up a classic environmental policy double-bind, and means that two of state’s most prized environmental initiatives could be in conflict. California’s 2006 Global Warming Solutions Act defines N2O as one of six greenhouse gases, and sets a target of reducing emissions to 1990 levels by 2020.
However, Townsend-Small says she not against the reclamation plants, believing they represent the best option for the state. For one, the plants use much less energy than desalination, an alternative used widely in the Middle East. A 2009 study by UC Berkeley researchers Jennifer Stokes and Arpad Horvath found that meeting California’s water demand through desalination would consume more than half the state’s electricity, and more than double the energy use and CO2 emissions from importing water.
Townsend-Small says the issue is that California is not using the recycled water more widely—for example, to water lawns, flush toilets, or to supplement drinking supplies.
"We have to keep recycling wastewater. It’s definitely the answer to supplying water for California. The problem is that because they use this water for irrigation only, it doesn’t actually reduce drinking water inputs. So you have this high N2O from wastewater recycling, and then high CO2 from water imports."
California law currently forbids recycled water being used for drinking, despite the fact that all water is, in some sense, recycled. Townsend-Small says it is largely people’s perception of recycled water that gets in the way of them drinking it.
She says she doesn’t know if the California will now consider N2O emissions as part of its water-planning, or include them in estimates of how the state is meeting its climate law commitments. But Townsend-Small says emissions would fall overall if California used recycled water in greater quantities.
"If we can really reduce the imports, that would reduce overall greenhouse gas emissions."
[Image: Flickr user Joe Shlabotnik]