Under the pressing mandate of the global carbon neutrality strategy, the repurposing of municipal wastewater treatment plants (WWTPs) from traditional emission sources into pivotal nodes for carbon dioxide removal (CDR) represents a frontier in environmental science and engineering. This review provides a comprehensive synthesis of wastewater alkalinity enhancement (WAE), an emerging technology designed to utilize WWTPs as engines for driving atmospheric CO2 into stable oceanic carbon sinks. WAE fundamentally relies on the unique biogeochemical environment within WWTPs—characterized by high partial pressures of CO2 (pCO2) and relatively low pH generated by microbial respiration. These conditions create a thermodynamically favorable reactor for the rapid dissolution of alkaline materials, such as silicate minerals (e.g., olivine) or industrial hydroxides. The subsequent elevation of effluent total alkalinity, upon discharge into coastal zones, shifts the marine carbonate equilibrium, facilitating the conversion of dissolved CO2 into stable bicarbonate (HCO3–) and carbonate (CO32–) ions, thereby enhancing the ocean’s capacity for long-term carbon storage. A critical dimension of WAE implementation is its seamless integration with core biological treatment processes. This review systematically analyzes the coupling mechanisms between alkalinity enhancement and biological nitrogen removal, specifically focusing on nitrification–denitrification and anaerobic ammonium oxidation (anammox) pathways. Nitrification is an alkalinity-consuming process that often results in system acidification and process instability. We elucidate how WAE serves as an in situ “chemical buffer”, replenishing consumed alkalinity to maintain optimal pH conditions for sensitive functional microbes (e.g., nitrifiers and anammox bacteria). This synergy not only stabilizes treatment performance but also reduces the reliance on external chemical dosing, effectively harmonizing pollution control with carbon sequestration. From an engineering perspective, we provide a comparative evaluation of two primary deployment strategies: upstream addition (UpAdd) and downstream addition (DnAdd). UpAdd, which involves dosing alkaline agents directly into biological reactors, favors “source reduction” by capturing biogenic CO2 before it is vented to the atmosphere. Model-based analyses suggest that this approach yields higher net carbon removal efficiency. Conversely, DnAdd, which targets the final effluent, aims at “terminal sink enhancement” by maximizing alkalinity export to the ocean. While DnAdd offers greater operational flexibility and minimal interference with existing biological processes, it has a greater risk of inducing “runaway precipitation” of calcium carbonate in receiving waters, which could negate carbon sink benefits. Moving beyond the inorganic carbon cycle, this review incorporates the novel concept of “gray carbon”—defined as the recalcitrant dissolved organic carbon (RDOC) in wastewater effluents. We discuss the potential of WAE to influence the microbial carbon pump (MCP) effect, thereby modulating the transformation of labile organic matter into inert “gray carbon” that persists in the ocean for millennia. This perspective expands WAE into a dual-pathway technology capable of enhancing both inorganic and organic marine carbon sinks. Despite its promise, the large-scale deployment of WAE faces significant hurdles, including the management of secondary mineral precipitation, the potential ecological toxicity of trace metals released from mineral weathering, and the lack of robust monitoring, reporting, and verification (MRV) frameworks. However, the technology also offers substantial co-benefits, such as enhanced phosphorus removal, mitigation of coastal acidification, and the potential for resource recovery from mineral precipitates. Future research priorities must focus on large-scale pilot demonstrations, comprehensive life-cycle assessments (LCAs), and the development of standardized MRV protocols. Through interdisciplinary collaboration, WAE has the potential to develop into a safe, scalable, and cost-effective component of the global portfolio for ocean-based negative carbon emissions.
Tong et al. (Thu,) studied this question.