Measuring cleaning costs
To capture the full climate cost of wastewater treatment, researchers counted not only emissions from each plant's treatment process, but also those from producing the energy and chemicals the plants need to operate, as well as from solid waste disposal after treatment.
They found that methane and nitrous oxide, powerful greenhouse gases produced by the on-site processes that plants use to treat wastewater, were the largest contributors. Methane was responsible for 41 percent of total emissions, equivalent to 16 million tons of CO2. And nitrous oxide made up 24 percent, equivalent to 12 million tons of CO2.
Methane: The dirty secret of sewage sludge
In wastewater treatment plants, wastewater and the solids it contains undergo several stages of processing to make it clean enough to return to the environment.
A common way plants break down wastewater solids or sewage sludge is through anaerobic digestion. In an anaerobic digester, microorganisms feed on waste and produce biogas, which consists largely of methane.
“The problem with anaerobic digesters is that they can leak, and some anaerobic digesters are prone to leaks,” Dunn said.
While wastewater treatment plants can use biogas as a renewable energy source, methane emissions may outweigh the climate benefits.
But the good news, Dunn said, is that a leak can be fixed.
“It’s something that can be addressed,” she said. “So this is an example of a low-hanging fruit opportunity that we recognized immediately.”
A need for innovation
Before wastewater is clean enough to be discharged back into surface or groundwater, wastewater treatment plants often need to remove excess nitrogen, which could otherwise lead to increased algae growth and reduced oxygen levels in the water, harming aquatic life.
To remove nitrogen from wastewater, many plants use a process called nitrification-denitrification. This releases nitrous oxide – a powerful greenhouse gas – and harmless nitrogen into the air.
Other technologies now recover nitrogen from the air for use in fertilizers, for example. These technologies are often energy intensive, so developing a way to extract nitrogen directly from wastewater could save energy and emissions.
“If we could simply avoid putting that nitrogen back into the air and use it while we have it to make a product like feed or fertilizer, then we are moving toward a circular nitrogen economy,” Dunn said.