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Sewage Plants Overlooked Source of CO2

Wastewater treatment plants may be responsible for emitting up to 23 percent more greenhouse gas than previously thought because of fossil fuels in detergent-laden water from residential showers, household washing machines and industrial sites, new research shows.

Treatment plants emit greenhouse gases such as methane and nitrous oxide when they purify drain water containing detergents and personal care products derived from petroleum. International tallies of greenhouse gas emissions are underestimating the plants’ effect on the climate, however, because they do not account for carbon dioxide emissions when that water is processed, according to a study published Wednesday in the journal Environmental Science and Technology.

A sewage treatment plant in India.
Credit: Asian Development Bank/flickr

Wastewater treatment plants are responsible for an estimated 3 percent of greenhouse gas emissions globally. Countries will need to consider many different ways to cut emissions in order to meet their obligations under the Paris Climate Agreement and prevent global warming from exceeding 2°C (3.6°F). The study suggests that wastewater treatment plants hold one key to those greenhouse gas reductions.   

“With increasing interests to reduce global greenhouse gas emissions, efforts should also be made to quantify these emissions more accurately,” said study lead author Linda Tseng, an environmental science professor at Colgate University.

Intergovernmental Panel on Climate Change guidelines say explicitly that carbon dioxide emissions are not included in the panel’s tabulations for wastewater treatment plant emissions because they are thought to be derived from natural biological sources and are carbon neutral. The IPCC accounts only for methane and nitrous oxide emissions coming from decomposing human and other waste in water treatment facilities, the study says.

But Tseng’s team shows fossil fuels make up a significant portion of a wastewater treatment plant’s greenhouse gas emissions.

Tseng and her team of researchers in Australia and California used radiocarbon analyses to break down the sources of the carbon found in the wastewater samples they took from treatment plants in Australia, an industrial petrochemical plant in Southern California and a papermill in Canada.

The team found that household wastewater could be composed of up to 28 percent petroleum-derived carbon, increasing estimates of total global greenhouse gases from wastewater treatment facilities by between 13 and 23 percent.

The IPCC’s model assumes that all wastewater treatment facilities emit the same kinds of greenhouse gases, but the study found that emissions from industrial wastewater treatment facilities can differ dramatically. For example, the team found that emissions from a wastewater facility at a papermill include almost no carbon dioxide from fossil fuels, but a wastewater facility at an oil refinery contains only carbon dioxide from fossil fuels.

Wastewater processing at a sewage treatment plant.
Credit: KOMUNews/flickr

“The results of this study provide an opportunity to reduce carbon dioxide emissions, including fossil carbon dioxide, from wastewater treatment facilities,” Tseng said. “Strategies could include developing on-site carbon sequestration technology that runs on renewable energy.”

Zhiyong Ren, an associate professor of environmental and sustainability engineering at the University of Colorado-Boulder who is unaffiliated with the study, said the researchers’ use of radiocarbon analyses is a new approach to finding the source of specific emissions at wastewater treatment plants.  

“Wastewater treatment plants do contribute to global greenhouse gas emissions in multiple ways,” he said. “The majority of greenhouse gas emissions have been viewed as carbon neutral as they are from (non-fossil) carbon as the IPCC report mentioned. That’s why this study is interesting. It points out that actually it may not be true depending on the content and source of the wastewater.”

Wastewater treatment plants have the opportunity to cut emissions by capturing the carbon dioxide they produce and converting it to different kinds of fuels — technology currently being researched, said Ren, who is on a federally-funded team to convert wastewater treatment plants into energy sources.

“It will be huge to convert a whole industry from (being) a big energy consumer to an energy provider, and also potentially making it carbon neutral or negative,” Ren said.

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