Water treatment plant survey shows high emissions of nitrous oxide

NEW YORK, NY, May 26, 2010 — Nitrous oxide, or N2O, is a greenhouse gas considered by experts to be 300 times more powerful in its atmospheric warming effect than carbon dioxide. By far the greatest recorded sources of N2O emissions are from agricultural activities and fossil fuel combustion. But sewage breakdown by some wastewater treatment plants also emit nitrous oxide. Until recently nitrous oxide emissions from plants using microbes to breakdown toxins was estimated to be rather low. But the first large-scale survey of 12 plants across the U.S., led by Columbia scientists, shows that these waste water treatment may contribute a larger share of emissions than previously thought; it also challenges the current U.S. Environmental Protection Agency approach for assessing N2O emissions from such plants. The findings appear in the recent issue of Environmental Science & Technology (http://pubs.acs.org/journal/esthag). The study’s principal investigator, Kartik Chandran, assistant professor at the Fu Foundation School of Engineering and Applied Science, explains that nitrous oxide emissions to date have only been estimated because there has not been a consistent protocol to measure N2O from using biological nitrogen removal (BNR) — a process that uses microbes that involves bacteria to breakdown waster. To solve this problem, the Columbia team devised the first protocol to measure these emissions from full-scale water purification facilities. This protocol has been reviewed by the EPA and adopted by plants across the nation and in certain countries in Europe. Using this protocol, emissions of N2O can be measured in real-time during different phases of treatment within a single plant. To conduct this study, the Columbia team took measurements of N2O 24 hours a day for several weeks over a two year period around the nation to gain an understanding of spatial and temporal variability in N2O emissions. … As a result of the survey using the new protocol, the team found that aerobic zones generally contribute more to nitrous oxide fluxes. This is important because the EPA approximation method assumes that N2O is only emitted from anoxic zones by the process of denitrification. “Based on our actual measurements, aerobic zones contribute far more N2O than anoxic zones. This is one reason why the EPA emissions estimates are potentially underestimates, since they completely ignore aerobic zone emissions,” said Chandran. … “Until our study, everyone was following the EPA estimation method to approximate emissions,” he said. “It might very well be that wastewater treatment plants, particularly those that are not performing optimally, are a far worse contributor to global warming than we expected.” Knowing triggers for increased N2O emissions, however, explains Chandran, will make it possible to design and operate BNR plants that not only meet water quality regulations, but also minimize N2O.