Sewage treatment plants are worse for the climate than expected

This article was originally published on Grist.

Wastewater treatment plants are usually overlooked when it comes to reducing greenhouse gases, but new research from Princeton University shows the plants emit twice as much methane as previously thought.

Methane is a particularly potent greenhouse gas, and the treatment plants should be part of any plan to reduce emissions, according to the study released last week.

“Sewage treatment plants are a major source of greenhouse gases in cities and we need to start treating them as such,” said Mark Zondlo, professor of civil and environmental engineering at Princeton and one of the study’s authors.

The report, published in the journal Environmental Science and Technology, is the largest conducted in the United States on methane pollution from sewage treatment plants. The scientists examined 63 facilities in California and on the East Coast. Their research showed that methane from these plants exceeded Environmental Protection Agency estimates by the equivalent of 5.3 million tons of carbon dioxide.

Scientists use carbon dioxide equivalence as a measure to standardize emissions of many different types of greenhouse gases. The previous estimate for emissions from wastewater treatment plants was 6.3 million tons of carbon dioxide, according to the EPA. The new study calculates that current emissions are now 11.6 million tons of carbon dioxide.

“We have more than a million kilometers of sewers in the US that are filled with rich organic matter that may cause methane emissions, but we know very little about their extent,” said Z. Jason Ren, professor of Civil and Environmental Engineering, another co-author.

While methane has long been a concern for scientists and environmentalists, only recently have governments focused on curbing the greenhouse gas. Reducing methane emissions as quickly as possible can drastically reduce the planet’s rate of warming.

The biggest culprit for methane emissions from wastewater treatment is a domed vessel used towards the end of the process called an anaerobic digester. The fermenter contains small microbes, like bacteria, that function without oxygen and help break down the harmful microbes in our waste. While this process naturally produces methane, scientists have historically underestimated the leaks in these supposedly airtight containers, an oversight that led to inaccurate emissions counts.

The guidelines used by the EPA were developed by the Intergovernmental Panel on Climate Change, an organization within the United Nations that publishes reports on climate change every few years. But these IPCC guidelines did not take into account the large variations in emissions from facility to facility. The Princeton researchers discovered that the most consistent factor in detecting high emissions was the use of an anaerobic digester.

“We know that urbanization will increase, we know the centralized treatment [of waste] will increase, definitely in the US, but especially in the world. So let’s try to find a way to get this right, that’s a win for the water and a win for the air,” Zondlo said.

This article originally appeared in Grist. Grist is a non-profit, independent media organization dedicated to telling stories about climate solutions and a just future. Learn more at

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