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
(CH4). 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 CH4 emission
rates of 23 blackwater septic tanks in Hanoi. The average BOD removal
efficiency was 37% (−2–65%), and the average CH4 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 CH4 emission
rates (R = 0.614, p = 0.001). CH4 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 CH4 emission rate. Moreover,
the CH4 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 CH4 emissions from
septic tanks.