Improving Methane Emission Estimates from Nonsewered Wastewater during Storage in Septic and Holding Tanks

This study quantified methane (CH₄) emissions from nonsewered sanitation (NSS) in septic and holding tank containments for onsite storage of wastewater. It further examined how the measured emissions and corresponding parameters relate to the Intergovernmental Panel on Climate Change (IPCC) approach, which uses Tier 1 default parameters and simplified assumptions that do not capture the variability of NSS containment conditions. Paired with questionnaires to residents, samples were collected from household containments in rural Southern Coastal British Columbia (BC), Canada (n = 16), and urban Kampala, Uganda (n = 17). Gas samples and 22 physicochemical parameters were quantified in situ or in a laboratory. Median net cumulative CH₄ concentrations were slightly higher in BC (20,150 ppm) than in Kampala (14,950 ppm), despite less favorable conditions for biological activity. Scum depth exhibited the strongest correlation with CH₄ concentrations (BC r_s = 0.76; Kampala r_s = 0.88). Compared to Tier 1 IPCC estimates for Canada (18.0 g CH₄ capita^-1 day^-1) and Uganda (11.1 g CH₄ capita^-1 day^-1), median emissions based on CH₄ measurements were 30.8 and 12.6 g CH₄ capita^-1 day^-1 for Southern Coastal BC and Kampala, respectively, when normalized by the number of household residents, and 82.3 and 85.8 g CH₄ m^-2 day^-1 when normalized by the surface area of the containment. Methane correction factors (MCFs) calculated in this study were different from those recommended by the IPCC. These results reveal important limitations in the IPCC methodology for scaling point-source measurements using current Tier 1 defaults, including the difficulty of reliably estimating organic loading values and population equivalents, and the inadequacy of IPCC MCF values to represent highly variable and context-specific conditions in containments.