Here we delve into the challenges and innovative strategies for achieving net-zero emissions in the wastewater sector, a notable source of global greenhouse gases. Unlike other infrastructure sectors, wastewater management involves complex and variably quantifiable emissions across all scopes, making standardization difficult. This study provides a global overview of the sector’s emissions profiles by leveraging literature mining, data analysis and case studies. It emphasizes the substantial variability in emissions, identifies key emission sources and locations, and advocates for tailored monitoring and mitigation strategies. It highlights the potential emissions shifting across scopes due to the adoption of new technologies and accounting practices, and it argues for a holistic analysis for optimization and integration to ensure a net benefit of the overall reductions in carbon footprints. This study underscores the urgency of rethinking current practices to align with ambitious mid-century net-zero targets, emphasizing the critical role of accurate emissions quantification and comprehensive decarbonization strategies.

The ongoing paradigm shift in sewage management from pollution control to resource recovery is an integral part of a circular economy. Efforts have traditionally concentrated on the recovery of organic carbon as bioenergy, and nitrogen and phosphorus as nutrients. In the past decade, technologies have been developed to recover high-value commodities. Simultaneously, innovative processes have been proposed to produce chemical agents that can be immediately reused in the sewage system to facilitate sewage management. This Review evaluates these latest developments and the underpinning scientific discoveries, identifies the key challenges, and outlines a roadmap towards greener and more sustainable sewage management.

This chapter provides a review of the models available for estimating the production and emission of methane from wastewater collection and treatment systems. The details of a number of mechanistic models as well as the simplified empirical models have been summarized. Their limitations have been identified and general methods for calibration and validation have been presented.