Wastewater treatment-induced carbonate chemistry alteration and its implications for ocean alkalinity enhancement

Vollständiger Abstract

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Abstract Municipal wastewater treatment plants (WWTPs) are important components of urban carbon cycling and have been proposed as potential platforms for ocean alkalinity enhancement (OAE). However, carbonate-system transformations across full-scale treatment processes remain poorly constrained. Here, we present a process-resolved snapshot of total alkalinity (TA), dissolved inorganic carbon (DIC), pH, pCO₂, dissolved CH₄, and dissolved N₂O across five sequential treatment units at a full-scale coastal WWTP in Taiwan. From the influent channel to the chlorination unit, TA and DIC decreased by 94.5% and 84.0%, respectively, accompanied by acidification and persistent CO₂ supersaturation. These patterns were consistent with alkalinity consumption during nitrification and DIC transformation associated with aeration and other internal treatment processes. Dissolved CH₄ declined markedly following aeration, whereas N₂O reached its maximum concentration in the aeration basin and remained elevated in the final effluent. CO₂ exchange estimates obtained using conventional wind-based and limited-fetch parameterizations differed substantially from apparent DIC balances between consecutive treatment stages. This divergence highlights the strong methodological uncertainty associated with estimating CO₂ exchange in confined and mechanically aerated treatment units. Accordingly, the results are interpreted primarily in terms of observed concentration changes and carbonate-system transformations rather than as definitive emission estimates. The marked chemical differences among treatment stages provide empirical constraints for evaluating where and how alkalinity addition might interact with wastewater treatment processes. Repeated observations, direct gas-flux measurements, and alkalinity mass balances will be required to determine the net carbon-removal potential of wastewater-based OAE.