A new systematic review published in Engineering offers a comprehensive look at the greenhouse gas (GHG) emissions resulting from combined sewer overflows (CSOs) and explores strategies for mitigating these emissions in pursuit of carbon neutrality. The study, titled “Environmental Frontiers for Water–Energy Nexus—Review: A Systematic Review of Greenhouse Gas Emissions Derived From Combined Sewer Overflows and Synergistic Control Strategies Toward Carbon Neutrality,” was conducted by researchers from Tongji University in China.
The review highlights the significant environmental risks associated with CSOs, which occur when stormwater runoff exceeds the capacity of combined sewer systems, leading to the discharge of untreated wastewater into natural water bodies. This phenomenon is particularly concerning as it contributes to GHG emissions, including carbon dioxide (CO₂), methane (CH₄), and nitrous oxide (N₂O), which are critical for achieving carbon neutrality in urban water systems.
The researchers utilized the life-cycle assessment method expansion approach to analyze the formation and discharge processes of CSOs, identifying the underlying mechanisms driving GHG emissions. They emphasized the complexity and uncertainty in the spatial distribution of these emissions, advocating for the development of standard monitoring and estimation methods. The study identifies factors influencing GHG emissions within urban drainage systems (UDS) and defines interactive GHG emission boundaries and accounting frameworks related to CSOs.
Key findings of the review include the identification of emission hotspots from CSOs, such as riverine ecosystems, where substantial GHG emissions occur during overflow events. The study also highlights the indirect emissions resulting from increased electrical and chemical consumption due to stormwater overflows. For instance, the dramatic increase in flow during wet weather puts pressure on wastewater collection and transportation systems, leading to higher energy use and GHG emissions. The review underscores the need for integrated, low-carbon wastewater management solutions.
The review also examines various strategies for mitigating GHG emissions associated with CSOs. These strategies can be categorized as “Front of the pipe” mitigation, which involves preventing stormwater from entering sewer systems through methods like green infrastructure and low-impact development; “In the pipe” monitoring, which utilizes real-time control technologies to optimize the operation of existing infrastructure; and “End of the pipe” treatment, which includes nature-based and technological compact treatment methods to intercept and process CSOs before they are discharged.
The study emphasizes the potential of data-driven technologies, such as machine learning and deep learning, in enhancing the management and mitigation of CSOs. These technologies can provide more accurate predictions and real-time monitoring of CSO events, leading to more effective control strategies. The review calls for an integrated CSO control framework that combines advanced control processes, comprehensive data-driven technologies, and large-scale urban facilities to achieve sustainable GHG management.
Despite the progress made in understanding and managing CSO-related GHG emissions, the review identifies several challenges and limitations. These include the need for clearer emission boundaries, improved data monitoring and collection, and the development of comprehensive process models for carbon emissions. The study also highlights the importance of addressing the trade-offs between the costs and benefits of different CSO control strategies.
The review provides valuable insights into the policies, dynamics, and control strategies for GHG emissions from CSOs. It underscores the importance of integrating data-driven technologies with practical urban water system management to achieve carbon neutrality. As urban areas continue to face the challenges of population growth and climate change, the findings of this study offer a roadmap for sustainable and low-carbon urban water management.

The paper “A Systematic Review of Greenhouse Gas Emissions Derived From Combined Sewer Overflows and Synergistic Control Strategies Toward Carbon Neutrality,” is authored by Yilin Xu, Cheng Ye, Zuxin Xu, Wenhai Chu. Full text of the open access paper: https://doi.org/10.1016/j.eng.2025.03.027. For more information about Engineering, visit the website at https://www.sciencedirect.com/journal/engineering.