Pathways Toward Carbon-Neutral Municipal Wastewater Treatment Plants: Process Reconfiguration, Resource Recovery, and Sustainability Assessment

Municipal wastewater treatment plants (WWTPs) are essential for protecting public health, however, their contribution to greenhouse gas (GHG) emissions has often been overlooked. Achieving carbon-neutral operation requires more than incremental improvements in energy efficiency; it calls for a rethinking of process design, energy flows, and resource recovery strategies. This review examines recent developments across several key pathways, including carbon capture through A-B configurations, energy recovery via anaerobic digestion, and low-carbon nitrogen removal based on autotrophic processes such as partial nitritation–anammox. Emerging technologies, such as microalgal and bioelectrochemical systems, are also reviewed, although their large-scale applicability remains uncertain. Particular attention is given to the trade-offs introduced by advanced treatment for micropollutant removal, which can significantly increase energy demand if not carefully integrated. Beyond individual technologies, the paper highlights the importance of system-level optimization, life-cycle assessment, and data-driven control strategies. A staged roadmap is proposed to distinguish near-term improvements from longer-term transitions. Rather than presenting a single solution, the review emphasizes that feasible pathways depend strongly on local conditions, including influent characteristics, climate, and energy mix.