Scientist Works to Mitigate Nitrous Oxide Emissions From Wastewater Treatment

July 7, 2009Bookmark and Share

What do tooth extraction and climate change have in common? They both involve nitrous oxide, better known as “laughing gas” for its use as an anesthetic in dental procedures. But nitrous oxide, or N2O, is also a greenhouse gas considered by some experts to be 300 times more powerful in its atmospheric warming effect than carbon dioxide. 

Bacteria that produce nitric oxide (green) and those that do not (black) [Image credit: Joon Ho Ahn]
In the above photomicrograph, the green cells represent bacteria that produce nitric oxide. The cells represented here were obtained from a nitrifying bioreactor, which has been in operation in Chandran’s labs since January 2006.  
Image credit: Joon Ho Ahn
By far the greatest source of N2O is from agricultural activities, such as soil fertilization, while emissions from fossil fuel combustion come in a distant second. Human and animal sewage also contribute to nitrous oxide emissions when they are processed in wastewater treatment plants.
“Human waste contains proteins that are eventually converted to ammonia-nitrogen,” said Kartik Chandran, an assistant professor of earth and environmental engineering at the Fu Foundation School of Engineering and Applied Science. “When left untreated, ammonia flows into surrounding water bodies and can lead to marine life sickness and death.”
Chandran studies the role of microorganisms in both natural and engineered systems. His research has shown that microbes involved in breaking down human waste are to blame for the emission of both nitrous oxide and nitric oxide (NO), which causes atmospheric smog. Currently he is working with 12 wastewater treatment plants in the U.S.—including New York City, Chicago, Washington, D.C., Los Angeles City, Los Angeles County and others—to understand and mitigate the processes by which these gases are emitted.
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 percent of the earth’s atmosphere and is benign. The two-phase process of biological nitrogen removal (BNR) in wastewater treatment plants involves nitrifying bacteria that oxidize ammonia to create nitrate while denitrifying bacteria oxidize nitrate, turning it into nitrogen gas, which is then released to the atmosphere.
In his research, Chandran has found that it is somewhere between these two steps that nitrous and nitric oxides are formed. To date, Chandran’s group has conducted full-scale N2O measurement campaigns at BNR plants and found that large-scale emissions of N2O can occur when the plants—and the bacteria—become overworked. For instance, peaks in N2O happen at the same time each day—around noon, after the majority of the population has completed their morning routines and flushed their waste to treatment plants. If the treatment plants are not designed to address these peak loads, then a significant fraction of the waste is released as nitrous oxide. Some of the release is triggered by a stress response on the part of the nitrifying and denitrifying bacteria.   
With funding from the National Science Foundation and the Water Environment Research Foundation, Chandran and his colleagues are honing in on the specific bacterial genes that are responsible for nitrous and nitric oxide formation. The ultimate goal, says Chandran, is to “engineer” the process so these genes are not expressed or over expressed.
Chandran and his colleagues will study the expression of these genes in nitrifying bioreactors, which allow them to see how the bacteria behave under different conditions and how changes in the expression of these genes correlate with the release of gaseous nitrogen
The next step is to figure out how to manipulate the activation of these genes during BNR by imposing reactor controls such as equalizing the rate of flow of wastewater or adjusting the aeration (oxygen) intensity within the bioreactor.  
Chandran hopes the results of his and his colleagues’ efforts will help further address the complex issues of climate change and increase the profile of nitrous oxide as a greenhouse gas. “This project reflects our strong commitment to sustainable development practice,” said Chandran, “and how we can bring together diverse disciplines such as engineering and molecular biology to achieve technologies that improve environmental and human health.”
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