Wastewater alkalinity enhancement for carbon emission reduction and marine CO₂ removal

Abstract Wastewater alkalinity enhancement is a promising approach for ocean alkalinity enhancement due to its potential to deliver strong bases with minimum secondary precipitation and its potential use of the global network of wastewater treatment plants (WWTPs). WWTPs are also significant sources of CO 2 due to organic matter oxidation, and integrating alkalinity addition into treatment processes may both reduce in-plant CO 2 emissions and increase downstream CO 2 uptake. This study presents a modeling framework that combines a modern activated sludge model-based WWTP simulator with an integrated hydrodynamic-biogeochemical-carbonate chemistry model of coastal oceans. We evaluate the effects of adding alkalinity either upstream (UpAdd) of the biological treatment stage or downstream at the discharge location (DnAdd) on WWTP carbon emission reduction and marine CO 2 removal. The carbon emission from WWTPs decreases with increasing alkalinity dosage in UpAdd and can be eliminated at a dosage level that is feasible to implement. However, carbon uptake in the surrounding oceanic water is much reduced due to elevated dissolved inorganic carbon in the discharge water. DnAdd does not affect CO 2 emissions from WWTPs but enhances carbon uptake in the ocean, with the net oceanic uptake of atmospheric CO 2 increasing with increasing dosage level. Across all tested dosage levels, total CO 2 removal, including emission reduction at the WWTPs and enhanced carbon uptake in the ocean, is 30% greater in UpAdd than in DnAdd . WWTP treatment tanks have much higher p CO 2 than in the ocean, and aeration of process tanks enhances the gas transfer. The upstream alkalinity addition leads to sharp declines in p CO 2 in the treatment tanks and large reductions in carbon emission from the WWTPs. These results have implications for developing strategies to reduce global carbon emission and enhance oceanic carbon burial using WWTPs as a delivery mechanism.