Challenges to Accurate Estimation of Methane Emission from Septic Tanks with Long Emptying Intervals

[Image: see text] Septic tanks in low- and middle-income countries are often not emptied for a long time, potentially resulting in poor pollutant removal efficiency and increased greenhouse gas emissions, including methane (CH(4)). We examined the impact of long emptying intervals (4.0–23 years) on the biochemical oxygen demand (BOD) removal efficiency of 15 blackwater septic tanks and the CH(4) emission rates of 23 blackwater septic tanks in Hanoi. The average BOD removal efficiency was 37% (−2–65%), and the average CH(4) emission rate was 10.9 (2.2–26.8) g/(cap·d). The emptying intervals were strongly negatively correlated with BOD removal efficiency (R = −0.676, p = 0.006) and positively correlated with CH(4) emission rates (R = 0.614, p = 0.001). CH(4) emission rates were positively correlated with sludge depth (R = 0.596, p = 0.002), but against expectation, negatively correlated with BOD removal efficiency (R = −0.219, p = 0.451). These results suggest that shortening the emptying interval improves the BOD removal efficiency and reduces the CH(4) emission rate. Moreover, the CH(4) emission estimation of the Intergovernmental Panel on Climate Change, which is a positive conversion of BOD removal, might be inaccurate for septic tanks with long emptying intervals. Our findings suggest that emptying intervals, sludge depth, and per-capita emission factors reflecting long emptying intervals are potential parameters for accurately estimating CH(4) emissions from septic tanks.