Carbon emission characteristics and reduction technologies in urban wastewater systems: A review

The urban wastewater system emits greenhouse gases (GHGs), including CO₂, CH₄, and N₂O, throughout the processes of collection, transportation, treatment, and discharge. These emissions account for 1%–2% of total societal GHGs emissions. During decarbonization efforts, CH₄ and fossil-derived CO₂ are the primarily emissions, while N₂O is predominantly released during nitrogen removal processes. Specifically, N₂O emissions are most significant during the aerobic stage, whereas CH₄ emissions dominate in the anaerobic stage. Septic tanks contribute roughly one-third of the total carbon emissions from the entire wastewater system, highlighting the need for improved field monitoring and accounting methods to better understand and manage these emissions. This study provides a comprehensive analysis of the current status and underlying mechanisms of carbon emissions at each stage of the urban wastewater system. It also reviews recent advancements in carbon emission reduction technologies. Additionally, the research identifies five key directions for low-carbon development in wastewater treatment: energy recovery, low-carbon nitrogen removal, energy saving and carbon reduction measures, biological carbon sequestration, and intelligent carbon management. These strategies pave the way for reducing both pollution and carbon emissions in urban wastewater systems.