In the United States, wastewater treatment facilities play a crucial role in processing trillions of gallons of water each year, ranging from what is flushed down toilets to what drains from sinks. A recent study conducted by researchers from Northwestern University and the University of Illinois Urbana-Champaign examined data from over 15,000 of these plants to uncover the hidden climate costs associated with their operations.
The findings indicate that U.S. wastewater treatment plants are responsible for emitting approximately 47 million metric tons of carbon dioxide annually. Notably, the study highlights that two greenhouse gases—methane and nitrous oxide—contribute significantly more to climate change than previously estimated, exceeding official government figures by 41%.
Understanding the role of wastewater treatment in climate change is essential for identifying effective decarbonization strategies. To calculate the total climate impact of these facilities, the researchers assessed emissions from the treatment processes themselves, as well as from the energy and chemicals required for operation and the disposal of solid waste post-treatment.
Methane emerged as the leading contributor, accounting for 41% of total emissions, which equates to about 16 million metric tons of carbon dioxide plus nitrous oxide. Wastewater undergoes several processing stages at treatment plants to ensure it is clean enough to return to the environment. A common method for breaking down sewage sludge is anaerobic digestion, where microorganisms convert waste into biogas, primarily composed of methane.
However, these anaerobic digesters can be prone to leaks. While treatment plants can utilize biogas as a renewable energy source, the methane emissions produced can negate the climate benefits. Additionally, to prevent excess nitrogen from causing algal blooms and reducing oxygen levels in water bodies, many plants employ a nitrification-denitrification process. This method, while effective, releases nitrous oxide—another potent greenhouse gas—into the atmosphere.
Moreover, the technologies currently used to extract nitrogen from the air for applications such as fertilizers are often energy-intensive. Thus, developing methods to directly harvest nitrogen from wastewater could present a dual opportunity to minimize energy consumption and reduce emissions.
The researchers are collaborating with wastewater treatment facilities to gather more comprehensive data and are refining an open-source modeling tool that these plants can use to analyze their emissions. Their research has been published in the journal Nature Water under the title “Benchmarking greenhouse gas emissions from U.S. wastewater treatment for targeted reduction.”
Dr. Tim Sandle serves as Digital Journal”s Editor-at-Large for science news and specializes in various fields, including technology, environmental issues, business, and health journalism. He is also a practicing microbiologist and an author with interests in history, politics, and current affairs.
