Hidden Climate Impact Wastewater Systems Emit More Greenhouse Gases Than Official Data Suggests

Wastewater may be a far bigger climate problem than official figures suggest. A study from Princeton University, published in Nature Climate Change, finds that countries could be underestimating methane and nitrous oxide emissions from sewage systems by up to 27 percent, exposing a significant gap in global greenhouse gas accounting.

Nations worldwide underestimate greenhouse gas emissions from wastewater systems, research from Princeton University shows. Outdated inventory methods and failure to include items like latrines and untreated sewage in national reports are main reasons.

In an article published Feb. 11 in the journal Nature Climate Change, the researchers report that nations underestimated emissions of gases including methane and nitrous oxide by amounts ranging from 19 to 27 percent. The researchers said that much of the underestimate is caused by countries relying on 2006 Intergovernmental Panel on Climate Change (IPCC) guidance to estimate wastewater gas emissions rather than incorporating updates in the 2019 refinement.

In a policy paper that accompanies the research article, Z. Jason Ren, who led the research, said that accurate estimates are a foundation for sound public reporting and practical decision-making in the wastewater sector. He urged IPCC and policymakers to take note of these differences and move countries toward the most accurate wastewater standards when developing national inventories.

“If you don’t know exactly how much emissions you have, then it’s really difficult to make effective policies or technologies or methods to reduce the emissions,” said Ren, a professor of civil and environmental engineering and the Andlinger Center for Energy and the Environment at Princeton.

Ren said most estimates rely on methods that existed in IPCC 2006 guidelines, when countries began reporting greenhouse gas emissions from wastewater systems including industrial emissions and residential sewage. Since then, he noted, science’s understanding of wastewater gas emissions has become more accurate and is better reflected in methods released in 2019 and later.

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He said that wastewater systems are among the largest emitters of nitrous oxide and methane, and they use substantial energy, concrete, and chemicals to deliver public health protection. On a CO₂-equivalent basis, the sector wide emissions are on the same order of magnitude as sectors such as aviation or commercial shipping.

The researchers said that wastewater systems offer strong options for emissions reduction because they have not been studied as much as systems that produce carbon dioxide such as transportation and power generation. While engineers have spent decades studying methods to reduce carbon dioxide emissions, Ren said, they have not spent nearly as long looking for ways to cut emissions from wastewater systems.

For the study, the researchers examined 38 countries on five continents. Twenty six of the countries were economically developed while twelve were emerging economies. The researchers compared the latest national inventory reports of these countries to determine if there were any systems, such as latrines or untreated discharges, that had been omitted from the national inventories. They also used modern evaluation techniques to determine emissions from large facilities such as sewage treatment plants and untreated sewage discharge points.

After determining shortfalls, the researchers evaluated the impact the nitrous oxide and methane would have as greenhouse gases. They determined that about 94-150 million metric tons of carbon dioxide equivalent is missing from annual global emissions data because of gaps in emissions accounting from certain wastewater systems.

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Ren said that wastewater emissions tend to have a long impact because the treatment infrastructure typically lasts for several decades or more. A municipal sewer plant built this decade might be in service at the end of the century. So, he said that technological choices and improvements made in the near future will offer benefits over a very long term.

“If you don’t have an accurate accounting, it’s hard to make good and right decisions, yet such decisions can have long lasting impacts” Ren said. “As a researcher, I want to understand the process better to put in better technologies and avoid, I don’t want to say disastrous, but very unfortunate scenarios.”

Original Article: Discrepancies in national inventories reveal a large emissions gap in the wastewater sector; Nature Climate Change; DOI:10.1038/s41558-025-02540-6

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