Controversial challenges in the transition to energy- and carbon-neutral municipal wastewater management

Conventional municipal wastewater treatment plants are predominantly designed around biological redox processes, which require substantial energy inputs and inevitably emit greenhouse gases (e.g., CO₂, CH₄, and N₂O), rendering them significant contributors to global carbon emissions. Transitioning toward energy- and carbon-neutral municipal wastewater management therefore demands multidimensional system optimization. Although considerable efforts have been devoted to developing carbon-neutral wastewater treatment technologies, several critical yet controversial issues remain under debate in the ongoing pursuit of carbon neutrality. This Review therefore seeks to provide insights by in-depth discussion of below key issues: (1) Energy generation and compensation as a dual approach toward carbon-neutral municipal wastewater treatment; (2) Unresolved issues of wastewater-derived GHG emissions in the transition toward carbon neutrality, (3) emerging opportunities enabled by next-generation hybrid and integrative treatment strategies, and (4) the role of carbon trading and taxing in municipal wastewater treatment. By clarifying key controversies and technological frontiers, this review aims to inform the development of a rational and actionable roadmap toward carbon-neutral municipal wastewater treatment.