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Survey of 12 wastewater treatment plants challenges current approach for assessing N2O emissions
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 break down 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 wastewater treatment plants 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.
The study’s principal investigator, Kartik Chandran, assistant professor at the Fu Foundation School of Engineering and Applied Science, explained 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). To solve this problem, the Columbia team devised the first protocol to measure these emissions from full-scale water purification facilities. 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.
Chandran studies the role of microorganisms in both natural and engineered systems. His research has shown that bacteria involved in breaking down human waste are to blame for the emission of both nitrous oxide and nitric oxide, which causes atmospheric smog. Human waste contains proteins that are eventually converted to ammonia-nitrogen. When left untreated, ammonia flows into surrounding water bodies and can lead to marine life sickness and death.
To prevent nitrogen-related impairment of water quality, biological wastewater treatment plants transform the ammonia and organic nitrogen compounds into nitrogen gas, which makes up about 79% of the earth’s atmosphere and is benign. The two-phase process of BNR in wastewater treatment plants involves nitrifying bacteria that oxidize ammonia to create nitrate (aerobic phase) while denitrifying bacteria reduces nitrate, turning it into nitrogen gas, which is then released to the atmosphere (anoxic phase).
A more accurate inventory of nitrogen emissions from wastewater treatment plants can affect policies regarding nitrous oxide and nitric oxide emissions, harmful greenhouse gases. Prior to Chandran’s study, it was not known how much N2O is emitted from plants using a BNR process, although via preliminary calculations, BNR had been implicated as a potentially dominant source. The EPA currently estimates that approximately 88 plants in the U.S. utilize this process.
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,” Chandran said. “This is one reason why the EPA emissions estimates are potentially underestimates, since they completely ignore aerobic zone emissions.”