Each year, U.S. wastewater treatment plants clean trillions of gallons of water, from what people flush down the toilet to drain down the sink.
Researchers from Northwestern University and University of Illinois Urbana-Champaign analysed data from more than 15,000 wastewater treatment facilities to understand the climate costs hidden within the cleaning process.
The study estimates that U.S. wastewater treatment plants emit the equivalent of approximately 47 million metric tons of carbon dioxide each year. Of these emissions, two greenhouse gases, methane and nitrous oxide, play a larger role than previously understood, exceeding current government estimates by 41%.
A clearer picture of wastewater treatment’s contribution to climate change will help pinpoint where decarbonization efforts can be most effective, according to the scientists.
Measuring the cost of cleaning
To capture the full climate cost of wastewater treatment, the researchers tallied emissions from each plant’s treatment process, together with those from producing the energy and chemicals the facilities require to operate and from the disposal of solid waste after treatment.
They found methane and nitrous oxide, potent greenhouse gases that arise from the on-site processes that plants use to clean wastewater, were the biggest contributors. Methane accounted for 41% of total emissions, the equivalent of 16 million metric tons of carbon dioxide plus nitrous oxide.
Methane: Sewage sludge’s dirty secret
At treatment plants, wastewater and the solids within it undergo multiple stages of processing to render them clean enough to return to the environment. A common way plants break down wastewater solids, or sewage sludge, is through anaerobic digestion. Inside an anaerobic digester, microorganisms feed on waste and create biogas, which is majority methane.
However, anaerobic digesters can leak. Furthermore, while treatment plants can use biogas as a renewable source of energy, methane emissions can outweigh the climate gains.
A need for innovation
Before wastewater is clean enough to be released back into surface or groundwater, treatment plants often have to remove excess nitrogen, which could otherwise lead to increased algal growth and decreased oxygen in the water, harming aquatic life.
To strip nitrogen from wastewater, many plants use a process called nitrification-denitrification. Along the way, it releases nitrous oxide — a potent greenhouse gas — and harmless nitrogen into the air.
Meanwhile, other technologies then recover nitrogen from the air for use in products such as fertiliser. These technologies are often energy-intensive, so developing a way to harvest nitrogen directly from wastewater could save energy and emissions.
What’s next?
The researchers are currently working with wastewater treatment facilities to collect more detailed data and refining an open-source modeling tool that plants can use to understand their emissions.
The research appears in the journal Nature Water, titled “Benchmarking greenhouse gas emissions from US wastewater treatment for targeted reduction.”
