Can wastewater help combat climate change?

In the face of a warming world, some scientists argue that directly removing carbon dioxide from the air could help limit the severity of climate change. Carbon dioxide removal (CDR) is an open field of research—no single technology has proved itself better than the rest. Now, scientists from the Institute of Marine Science and Technology at Shandong University have provided evidence for wastewater’s potential utility in the CDR tool kit (Sci. Adv. 2025, DOI: 10.1126/sciadv.ads0313).

The power of wastewater for CDR comes from the carbonate chemistry occurring below the water’s surface. When CO₂ dissolves in any body of water, the gas reacts to form carbonic acid; the weak acid dissociates. This process lowers the pH and forms a carbonate buffer system.

It takes millennia for carbon sequestered as carbonate to reemerge as CO₂. And because carbonate chemistry is partly governed by pH, scientists hypothesize that boosting alkalinity will boost water’s carbon sucking capabilities. Some researchers are already performing open-ocean field trials to test the theory.

“The pH in wastewater is much lower than in seawater,” says Jihua Liu, the researcher leading the newly published work. Through lab experiments, his team found that the lower pH of wastewater causes olivine—a mineral commonly used for ocean alkalinity enhancement—to dissolve 20 times as rapidly in wastewater as in seawater.

The group calculated how much excess carbon could be sequestered globally if wastewater treatment plants around the world added an alkalinity enhancement step to their process. In theory, it could be more than 18 Tg (18 million metric tons) and as high as 24 Tg of carbon dioxide annually. Lui’s paper compares these numbers to those of global mangrove ecosystems, which bury 16–37 Tg of carbon annually. “The carbon sequestration potential from this wastewater treatment scenario is good,” Liu says.

Others say that these numbers are likely not good enough, however. “We need to get to petagrams of carbon,” equivalent to 1 billion metric tons, to have a measurable climate impact, says Tyler Cyronak, a biogeochemist at Georgia Southern University who was not involved in the work. That said, wastewater alkalinity enhancement could be low-hanging fruit for CDR because infrastructure is already in place to do it, he adds.

In fact, some companies are selling carbon credits based on wastewater alkalinity enhancement. For example, the carbon registry, Isometric, recently verified the first CDR carbon credits for wastewater alkalinity enhancement for Crew Carbon, a company that describes its work as “engineering wastewater to remove CO₂.”

“There may be some arguments for remediating [wastewater] in this way,” James Kerry writes in an email, “but not because it will lead to true CDR.” Instead, Kerry, a marine and climate scientist at the conservation organization, Ocean Care, considers alkalinity addition more like environmental remediation. It’s irresponsible to dump acidic wastewater into the ocean; the pH should be adjusted first, he says.

Kerry was not involved in the new wastewater study and thinks Liu’s team probably overestimates the true carbon sequestration potential of wastewater alkalinity enhancement. The lab experiment did not replicate how water flows through many wastewater treatment plants, Kerry notes This means olivine is likely to dissolve more slowly in the real world, thus minimizing alkalinity enhancement and the technique’s CDR potential. “The comparison to mangroves is ironic, because remediating and restoring these natural ecosystems would have greater climate benefits as well as numerous other co-benefits,” Kerry adds.