Temperature effect on performance and methane emissions of highly controlled replicate septic tanks

Septic tanks are widely used for decentralized wastewater treatment but remain poorly characterized with respect to greenhouse-gas emissions, particularly under variable temperature regimes. Understanding how temperature influences treatment performance and methane production is essential for improving both emission inventories and environmental sustainability through tailored mitigation strategies. This study used a unique set of twelve full-scale, replicate septic tanks fed with real domestic wastewater to isolate the effect of four controlled temperature conditions (ambient, insulated, 20 °C, 30 °C). Continuous monitoring of headspace gas, dissolved methane and water quality enabled a complete carbon balance across gaseous, liquid, and sludge pathways. Higher temperatures enhanced organic degradation and established stronger anaerobic conditions, increasing methane production from ~42% under insulated conditions to ~54% at 30 °C. Critically, temperature governed methane partitioning: at 30 °C, desorption was favored and ~70% of the methane accumulated in the headspace, whereas at lower temperatures (≤20 °C), a greater fraction (often >80% of methane) remained dissolved and was discharged with the effluent. This behavior reveals that dissolved methane can represent a substantial fraction of total methane generated in septic tanks, yet it is typically not quantified separately in standard emission assessments. By jointly accounting for gaseous and dissolved pathways, our results show that total methane release from septic tanks may be higher than estimates based solely on headspace measurements. These findings highlight the importance of explicitly considering dissolved-phase dynamics when refining emission factors and developing mitigation strategies.