In 2020, the population served by on-site sanitation worldwide, including septic systems and pit latrines, exceeded for the first time than that relying on sewer connections. (1) Furthermore, since 2010, more people have reportedly been relying on septic systems than on improved latrines. (1) In Southeast Asia, septic systems are used by a majority of the population (i.e., 90% in Vietnam, (2) 84% in the Philippines, (3) and 79% in Indonesia (4)). While septic systems are preferable over open defecation, they can potentially emit substantial amounts of greenhouse gases (GHGs), such as methane (CH4). (5−8) Thus, assessing the GHG emissions from septic systems is crucial to achieving climate change mitigation. (9−11) The Intergovernmental Panel on Climate Change (IPCC) approach has been widely used to estimate CH4 emission rates (g CH4/(cap·d)) based on biochemical oxygen demand (BOD). (12) The method is based on three parameters: (i) region- or country-specific per-capita BOD (g BOD/(cap·d)), (ii) maximum CH4 production capacity (0.6 g CH4/g BOD), and (iii) CH4 correction factor or BOD removal efficiency of septic tanks (40–72%). Different from the IPCC, in Hanoi, BOD removal efficiencies of 10–50% have been reported. (13) Applying the suggested BOD removal efficiency from the IPCC might lead to a considerable estimation error in CH4 emissions from septic systems with long emptying intervals. Therefore, the goal of this article is to investigate whether the suggested BOD removal efficiency can be used to estimate the CH4 emission and, if not, what alternative indicators can be used.

A septic system is usually constructed in either of the following two ways: (i) with two components, namely, a septic tank and a soil treatment unit (e.g., leach, infiltration, or drain fields) or (ii) with only a septic tank without a soil treatment unit. The septic system of type ii has to be connected to a sewerage for further treatment. However, in low- and middle-income countries, type (ii) septic tanks are frequently found and they are not always connected to sewerage but discharged to open environments. (14,15) In this study, we focused on the septic system of type (ii) which we from here onward call septic tanks. In low- and middle-income countries in Southeast Asia, septic tanks often receive only blackwater (i.e., blackwater septic tanks), while graywater is directly discharged to a combined sewer or a drain channel... (16)

In 2010, the wastewater sector accounted for 8% of the global anthropogenic CH4 emissions, following enteric fermentation (28%), agriculture (20%), oil and gas (18%), and landfills (10%). (48) From 1990 to 2005, global CH4 emissions from wastewater were estimated to have increased by about 35% and are predicted to increase by 28% in 2030. (49) The major contributors to emitting CH4 in the wastewater sector are the low- and middle-income countries in Asia and Africa regions, (49) where septic systems are prevalent. However, the quantification of CH4 emissions and thus the implementation of mitigation strategies within this sector pose significant challenges.