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Treating wastewater with alkaline minerals can improve COâ‚‚ removal and storage

Carbon dioxide removal (CDR) strategies use a wide range of techniques to capture CO₂ from the air and store it durably, offering a frontier solution for counteracting the increasing levels of the greenhouse gas in our environment. Increasing the alkalinity of wastewater by treating it with alkaline minerals can substantially boost the CO₂ sequestration abilities, finds appearing in Science Advances.

CO₂ dissolves in ocean water to form carbonic acid, which, being a weak acid, dissociates into hydrogen ions and bicarbonate ions, with the former increasing the acidity of the water.

Ocean alkalinity enhancement (OAE), the process of treating wastewater with alkaline minerals before discharging it into the ocean, makes the ocean water less acidic, thus lowering the amount of CO₂ at the sea's surface. This helps the ocean absorb more CO₂ from the air, and the researchers of this study estimate that OAE can potentially help sequester roughly 18 teragrams—18 trillion grams—of CO₂ per year globally.

A common practice in wastewater-based OAE is using the wastewater treatment plants as alkalinity factories. This not only improves CO₂ absorption in coastal areas but also helps mitigate the ocean acidification problem caused by the dumping of untreated wastewater.

Existing studies have resorted to treating wastewater with alkaline materials for targeted purposes, such as using dolomite (calcium magnesium carbonate) to recover phosphorus, alkaline silicate minerals to capture CO₂ in its solid carbonate form or olivine (magnesium-iron silicate) for improving methane quality for biogas production from sewage sludge.

Even though most CO₂ emissions arise from aerobic or oxygen-rich environments, most wastewater-based OAE studies have looked into anaerobic or oxygen-free settings. Also, there is a limited understanding of how well OAE performs in wastewater treatment in terms of alkalinity enhancement and stable carbon storage.

To change this, the researchers of this study added different quantities of olivine-rich rock to artificial urban wastewater and treated it with biologically activated sludge, under aerobic conditions at ambient temperature. In each case, they measured for changes in total alkalinity of wastewater, the amount of dissolved inorganic carbon and water quality indicators.

They found that the addition of olivine significantly increased the total alkalinity of wastewater and helped capture CO₂ as dissolved inorganic carbon (DIC) in the effluent. Also, the rate of alkalinization through olivine dissolution in this aerobically treated wastewater was approximately 20.5 times faster than in seawater.

An estimation based on data from wastewater treatment plants near the ocean revealed that the global carbon sequestration potential of coastal wastewater-based OAE is estimated to be 18.8 ± 6.0 teragrams of CO₂ per year.

The region between the latitudes 20°N and 60°N holds 79.3% of the global potential, and the European Union, the United States, and China—the top three economies—together account for over half of the global total.

The researchers note that their results showcase how wastewater treatment plants can play a significant role in carbon sequestration by enhancing ocean alkalinity, offering a dual benefit for climate mitigation and water quality improvement.