Greenhouse Gas Emissions from Wastewater Treatment Plants Get Closer Scrutiny

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A team of scientists from Columbia University has found evidence that emissions of nitrous oxide from some wastewater treatment plants may be significantly higher than previous estimates.  Nitrous oxide is a powerful greenhouse gas that is almost 300 times stronger than carbon dioxide. As for the good news, the team also found indications that much of the problem could be resolved by tweaking operations at existing plants and introducing more efficient design standards in new plants, rather than developing expensive new technology.

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The Columbia study provides some insight into the potential for making significant but relatively inexpensive cuts in greenhouse gas emissions by working within existing infrastructure and technology.  It  also illustrates the key role that accurate measurement plays in targeting resources to achieve the biggest bang for the greenhouse gas reduction buck.  In doing so, the study supports the logic behind the U.S. EPA’s recent proposal to expand its new greenhouse gas reporting rules.

‌Wastewater Treatment and Nitrous Oxide

The nitrous oxide problem is one of those “no good deed goes unpunished” conundrums.  It arises from the use by some treatment plants of a relatively new process called biological nitrogen removal, which was designed to reduce the level of ammonia in treated wastewater, rendering it less harmful to marine life when discharged into waterways.  Under ideal conditions, the process also yields nitrogen gas, which is harmless (78.8% of the Earth’s atmosphere by volume is nitrogen).  Under less than ideal conditions, the process yields nitrous oxide.

How Much Nitrous Oxide is Too Much?

The Columbia study, spearheaded by Kartik Chandran of the Fu Foundation School of Engineering and Applied Science, was a meticulous two-year effort involving a dozen wastewater treatment plants.  Until the study, it was estimated that treatment plants account for about 1.6% of total nitrous oxide emissions globally.  This was based on the assumption that nitrous oxide production only occurs in one area of the treatment process.  Chandran’s team found that emissions occur in a second area, and at even higher levels, bringing the emission factors up to two orders of magnitude higher than previously estimated.  If left unaddressed, nitrous oxide emissions are bound to grow as more treatment plants adopt biological nitrogen removal in order to meet increasingly strict water quality standards.  The alternative would be to forgo the new process and let excess ammonia continue to pollute waterways.

Cost-Effective Solutions for Greenhouse Gas Emissions

In a recent conversation with CleanTechnica, Chandran underscored the point that solutions are at hand, for existing plants as well as new ones.  For example, closer management of flow rates could enable the process to function more efficiently, lowering the production of nitrous oxide.  Another approach involves the blowers that pump air into wastewater, which enables the bacteria to function more effectively.  At some plants, blowers have been turned down in order to save energy, which can cause the treatment process to produce more nitrous oxide.  Solar energy installations at treatment plants provide a means for running blowers and other equipment at maximum effectiveness for reducing nitrous oxide emissions, without generating more carbon emissions.  Other on-site forms of renewable energy at treatment plants, such as methane gas capture and even hydrokinetic power, are also possibilities.

Update: The Columbia study was funded by the Water Environment Research Foundation.

Image: Wastewater treatment plant via eutrophication & hypoxia on flickr.com.