Reducing the acidity of the huge volumes of wastewater discharged into the oceans could be a relatively straightforward way to remove carbon dioxide from the atmosphere and slow climate change. But early tests have spurred controversy over potential environmental consequences.
at the University of Maryland says about 100 trillion litres of wastewater are discharged into the oceans each year. Chemicals are often added to make it less acidic in order to meet environmental standards before it is discharged – but lowering the acidity even more could boost CO2 storage in the sea.
The ocean naturally absorbs vast amounts of CO2 from the air, becoming more acidic as it does so. Over time, the CO2 forms stable minerals as alkaline rocks dissolve in the water, freeing the water up to absorb more CO2 from the air. One of the main approaches to amping up this process is known as “ocean alkalinity enhancement” – this could involve removing CO2 from sea water and then returning it to the ocean, or adding alkaline minerals or chemicals directly to the ocean. Using wastewater instead wouldn’t require building new infrastructure or using energy to pump sea water, and could lower legal obstacles to altering ocean chemistry.
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The total potential CO2 removal using wastewater is still uncertain, but at large scales the approach could theoretically remove more than a billion tonnes of CO2 each year, says at Planetary Technologies, a Canadian company that has conducted small tests with wastewater in the UK and Canada. It is now working with Li’s group on a test planned for this May in the Chesapeake Bay in Maryland and Virginia, the largest estuary in the US.
Li and his colleagues analysed how this could work in the specific conditions of the bay. They simulated adding an alkaline chemical called magnesium hydroxide to different stages of the wastewater treatment process to estimate the effect on the amount of CO2 removed.
On average, wastewater from the plant they studied normally has a pH of around 7; the US Environmental Protection Agency requires that wastewater has a pH between 6 and 9 when it is discharged. The researchers found that adding the alkaline chemical to the water after it was treated would have the largest CO2 removal effect. Adding the chemical at the beginning of the treatment process would remove less CO2 from the bay, but would reduce CO2 emissions from the wastewater treatment process itself.
If the first approach were implemented at the four major wastewater treatment plants around the bay, Li estimates that about 60,000 tonnes of CO2 would be removed each year. These results were presented at the American Geophysical Union Ocean Sciences conference in New Orleans, Louisiana, in February.
A test of this will take place at a wastewater treatment facility in Norfolk, Virginia, over the course of a week, and will involve measuring the change in acidity in the bay as well as monitoring environmental effects. “We want to make sure there’s no negative environmental impact,” says Li.
at Ocean Visions, a US non-profit focused on marine CO2 removal, says using wastewater could have the advantage of getting projects underway faster. But it could have trouble reaching the scale of other methods for altering the ocean’s carbon cycle, such as fertilising plankton blooms by adding iron, which boosts the amount of CO2 the plankton absorb through photosynthesis.
Altering wastewater may also attract controversy, as have other approaches to the rapidly growing field of marine CO2 removal. In 2022, Planetary Technologies’s tests using wastewater in Cornwall, UK, provoked protests from community members who said it amounted to untested “marine geoengineering” that risked harming ecosystems.
In a recent , the UK Environment Agency found that the company’s tests came with “very low risk” to the environment. Burt says he hopes the report will act as a “reset” as they pursue future tests in Cornwall. “We’re aiming to do it again, but the details of it are very much still to be figured out.”
Article amended on 26 February 2024
We clarified where a test of the wastewater treatment will occur
