Greenhouse gas emissions from on-site sanitation systems: A systematic review and meta-analysis of emission rates, formation pathways and influencing factors.

Onsite sanitation systems (OSS) are significant sources of greenhouse gases (GHG) including carbon dioxide (CO 2), methane (CH 4) and nitrous oxide (N 2 O). While a handful of studies have been conducted on GHG emissions from OSS, systematic evaluation of literature on this subject is limited. Our systematic review and meta-analysis provides state-of-the- art information on GHG emissions from OSS and identifies novel areas for investigation. The paper analyzes GHG emission rates from different OSS, the influence of various design, operational, and environmental factors on emission rates and proffers mitigation measures. Following the Preferred Reporting Items for Systematic reviews and Meta-analysis (PRISMA) guidelines, we identified 16 articles which quantified GHG emissions from OSS. Septic tanks emit substantial amounts of CO 2 and CH 4 ranging from 1.74 to 398.30 g CO 2 /cap/day and 0.06–110.13 g CH 4 /cap/day, respectively, but have low N 2 O emissions (0.01–0.06 g N₂O/cap/day). CH 4 emissions from pit latrines range from 0.77 to 20.30 g CH 4 /cap/day N 2 O emissions range from 0.76 to 1.20 gN 2 O/cap/day. We observed statistically significant correlations (p < 0.05) between temperature, biochemical oxygen demand, chemical oxygen demand, dissolved oxygen, storage period, and GHG emissions from OSS. However, no significant correlation (p > 0.05) was observed between soil volumetric water content and CO 2 emissions. CH 4 emissions (expressed as CO 2 equivalents) from OSS estimated following Intergovernmental Panel for Climate Change (IPCC) guidelines were found to be seven times lower (90.99 g CO 2 e/cap/day) than in-situ field emission measurements (704.7 g CO 2 e/cap/day), implying that relying solely on IPCC guidelines may lead to underestimation of GHG emission from OSS. Our findings underscore the importance of considering local contexts and environmental factors when estimating GHG emissions from OSS. Plausible mitigation measures for GHG emissions from OSS include converting waste to biogas in anaerobic systems (e.g. biogas), applying biochar, and implementing mitigation policies that equally address inequalities in sanitation service access. Future research on GHG from OSS should focus on in-situ measurements of GHGs from pit latrines and other common OSS in developing countries, understanding the fate and transport of dissolved organics like CH 4 in OSS effluents and impacts of microbial communities in OSS on GHG emissions. Addressing these gaps will enable more holistic and effective management of GHG emissions from OSS. [Display omitted] • Onsite sanitation systems (OSS) are major sources of greenhouse gas (GHG) emission in the environment • Emissions from septic systems vary between 1.74 and 398.30 g/cap/day for CO 2 , and 0.06 and 110.13 g/cap/day for CH 4. • CH 4 emissions from OSS estimated following IPCC guidelines are seven times lower than field-measured emissions. • Local contexts and environmental factors should be considered when assessing GHG emissions from OSS. • Future GHG research on OSS should prioritize in-situ measurements of pit latrines and other OSS in developing nations. [ABSTRACT FROM AUTHOR]