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

Sanitation service chains (SSC) in many cities in low- and middle-income countries are complex and comprise poorly managed non-sewered and sewered sanitation technologies that emit greenhouse gases (GHG). In this study, we aimed to estimate the impact of GHG mitigation measures along SSCs where both non-sewered and sewered sanitation were widely used, and to account for the interdependencies of SSC components with respect to GHG emissions. Using an SSC in Hanoi, we employed a mass balance approach, empirical emission equations, and a carbon footprint estimation model to estimate GHG emissions by component at baseline and four mitigation scenarios. At baseline, the SSC emitted 3,698–5,147 ton CO2e/year, with CH4 accounting for 78–85% of the total emissions. Infrequently emptied septic tanks were responsible for 44–60% of the total emissions, followed by poorly maintained sewers (23–32%) and a wastewater treatment plant (WWTP, 17–24%). The results indicated that annual emptying of septic tanks alone contributed to a 31–38% reduction in GHG emissions compared to the baseline scenario. Further, scenario comparison showed that removing septic tanks alongside sewer improvement led to 15–24% lower GHG emissions compared to annual septic tank emptying with sewer improvements, despite a slight increase in the N2O emissions at the WWTP. Therefore, if not removed, septic tanks will remain a main source of GHG emissions even after a centralized sanitation is established. However, the removal of septic tanks, which are often privately owned, may pose significant social challenges, thus requiring further careful consideration. In the meantime, frequent emptying of existing septic tanks with effective fecal sludge management provides an option for partial mitigation of GHG emissions.