Impact of GHG mitigation measures in sanitation service chains: A focus on septic tanks and sewers

Sanitation service chains (SSCs) often consist of a complex mix of different components, frequently involving the coexistence of non-sewered sanitation (e.g., septic tanks) and sewered sanitation. Poorly-maintained components within these chains can lead to substantial, yet potentially avoidable greenhouse gas (GHG) emissions. In this study, we developed a model for estimating the impact of GHG mitigation measures along SSCs that feature overlapping and poorly maintained non-sewered and sewered sanitation, taking the interdependencies of the GHG emissions of these components into account. To this end, we employed mass balance, empirical emission equations, and a carbon footprint estimation model to estimate GHG emissions by component at baseline and under four mitigation scenarios using an example SSC in Hanoi. The results showed that the SSC is predominantly methane-emitting, with poorly-maintained septic tanks and sewers being the primary contributors to the GHG emissions. Annual septic tank emptying was also identified as an effective strategy for reducing GHG emissions and it accounted for a 31-38 % decline in total emissions relative to baseline emission level. Scenario comparison further showed that removing septic tanks and upgrading sewers, even though associated with a slight increase in N₂O emissions from the wastewater treatment plant, offer the greatest long-term mitigation potential, yielding 15-24 % lower emissions than annual emptying septic tanks with sewer upgrades. Additionally, if septic tanks are not removed, they will remain the primary source of GHG emissions even after upgraded sewer and centralized treatment is established. However, in cases where septic tank removal poses social challenges, frequent emptying remained a robust and immediately applicable mitigation option. Overall, this study provides a framework for identifying and quantifying major GHG emission reduction strategies for complex SSCs. Additionally, the results obtained indicated that managing septic tanks and sewers are important climate action strategies for ensuring sustainable city-wide inclusive sanitation.