Whole-system analysis reveals high greenhouse-gas emissions from citywide sanitation in Kampala, Uganda

Biological decomposition of human faeces produces methane, nitrous oxide and carbon dioxide when wastewater or faecal sludge is stored long enough for microbial digestion. Existing national and global greenhouse-gas inventories underestimate emissions from sanitation because they largely emphasize wastewater treatment plant technologies and end-of-pipe discharge, while largely omitting on-site containment and the full sanitation-service chain. IPCC 2006 and 2019 guidance provides methane correction factors for some technologies, but remains insufficient to model mixed urban systems that combine on-site and off-site services and to estimate nitrous oxide beyond treatment plants. This study proposes and applies a whole-system framework to quantify both direct and indirect emissions from all stages of a citywide sanitation system—containment, emptying/transport, and treatment—covering both on-site and off-site pathways. Kampala, Uganda (population ~2.25 million; 78% on-site, 22% sewered) is used as a case to demonstrate the method and to reveal the relative contributions of different sanitation pathways to total city emissions.

Most published work on sanitation-related greenhouse gases focuses on wastewater treatment technologies and discharge, including life-cycle and plant-scale assessments. Only a small number of studies estimate emissions from on-site containment or septic systems. IPCC 2006 guidelines introduced methane correction factors (MCFs) for select wastewater options and certain pit latrine and septic configurations, with limited guidance for nitrous oxide and an assumption of lower methane emissions for low- and middle-income countries. The 2019 refinement adds some options but still lacks sufficient granularity for full city-level modelling of mixed on-/off-site systems managed imperfectly. Additional literature highlights methane from sewers and the need for system-level approaches that consider the entire sanitation chain.

The analysis quantifies three emission categories along the sanitation-service chain for both on-site (pits, septic tanks, containers) and off-site (sewers) systems: (a) direct emissions of CH4 and N2O from stabilization in containment and treatment; (b) operational CO2 from energy/fuel used for trucking, pumping and aeration; and (c) embedded (embodied) carbon in infrastructure materials. Emissions from post-treatment reuse/disposal and any offsets were excluded. City sanitation flows were mapped using the Kampala excreta flow diagram (SFD) with 2018 population of ~2.25 million. Direct methane from containment was modelled with IPCC equations using updated, locally derived MCFs based on field data of dissolved oxygen and oxidation-reduction profiles in pits (Nakagiri et al.). Equation 1 calculates total methane from each system using population, COD load per capita, percent COD reduction in situ (assumed 70%), and emission factor EF = Bo × MCF with Bo = 0.25 kg CH4/kg COD. Nitrous oxide from containment and treatment was estimated via IPCC methodology using per-capita nitrogen excretion (assumed 4.672 kg N/cap/year) and emission factors informed by field conditions (nitrification at surface, denitrification at depth). Direct emissions from treatment plants were computed using a modified IPCC formula (Reid et al.) incorporating total organics in wastewater, process-specific MCFs, fractions removed as effluent or sludge, and no methane recovery. Operational emissions were estimated from fuel/electricity use: trucking emissions via number of trips, distance and vehicle emission factors; pumping/aeration via electricity and diesel consumption with grid/diesel emission factors. Embedded carbon was estimated analytically from standard designs and material quantities for toilets, sewers and treatment plants, applying standard emission factors and amortizing over assumed design lives. Kampala’s sanitation system characteristics, flows and treatment allocations were compiled from multiple sources. Two main plants (Lubigi and Bugolobi) dominate treatment; assumptions included 80% of treated wastewater and 33% of treated faecal sludge handled at Bugolobi, with remaining at Lubigi. Failures at containment were allocated primarily to infiltration (negligible emissions) and partially to flush-outs into drains during floods; illegal dumping and upstream discharges were assumed to enter open drains. Sewer methane emissions were not included due to limited sedimentation data and low sewer coverage reaching treatment.

- Total emissions: Sanitation in Kampala produces an estimated 189 kt CO2e per year, potentially more than half of city-level emissions. - Emission contributions by pathway (approximate shares): 49% from prolonged storage in sealed anaerobic pits/tanks; 4% from direct discharges from pits/tanks to open drains or ground; 2% from illegal dumping; 6% from sewer leakage/non-delivery; <1% from truck transport; ~1% from transport in sewers; 7% from wastewater/faecal sludge bypassing treatment; 31% from treatment plants without methane capture. - Containment per-capita emissions (population-weighted averages): 58.62 kg CO2e/cap/year from methane and 15.13 kg CO2e/cap/year from nitrous oxide; total containment (including embedded carbon) 76.18 kg CO2e/cap/year. - Treatment emissions: Total direct emissions from treatment are ~59 kt CO2e/year, split between wastewater and faecal-sludge processes; operational emissions at treatment ~2.9 kt CO2e/year; embedded emissions from treatment ~0.06 kt CO2e/year. - Transport and infrastructure: Trucking faecal sludge generates ~0.52 kt CO2/year; wastewater pumping ~0.024 kt CO2/year; embedded carbon in sewer network ~0.97 kt CO2/year. - Per-capita emission rates by system element (kg CO2e/cap/year): containment 76.18; transport of faecal sludge 0.85; treatment of faecal sludge 53.96; transport in sewers (embedded) 4.06; treatment of wastewater 180.79 (140.27 CH4; 24.34 N2O; 16.16 operational; 0.02 embedded); unsafe discharges 33.85 (22.84 CH4; 11.02 N2O). - On-site vs off-site dynamics: For on-site systems with road-based transport, emissions are dominated by methane from anaerobic conditions in pits/tanks, open drains during dumping, and treatment. For off-site systems, treated excreta have higher emission rates due to anaerobic treatment with no methane capture. - Embedded carbon for containment systems totals ~3.7 kt CO2/year; operational emissions for containment are negligible.

The whole-system analysis demonstrates that sanitation-related greenhouse-gas emissions in a rapidly urbanizing LMIC city are dominated by direct methane emissions from anaerobic storage in on-site containment and from treatment processes lacking methane capture. Nitrous oxide is non-negligible, particularly where aerobic surfaces overlay anaerobic depths in pits and in treatment. Emissions from transport and embedded carbon are small relative to direct biological emissions. By mapping emissions to the excreta flow pathways, the study clarifies where mitigation can be most impactful: reducing storage times and anaerobic conditions in on-site systems, preventing flush-outs and illegal dumping to drains, minimizing sewer leakage, increasing the share of excreta safely reaching treatment, and introducing methane capture or less methane-intensive treatment processes. The findings indicate that sanitation may represent a major fraction of citywide emissions, implying that municipal climate strategies and national inventories should explicitly incorporate sanitation-service chain emissions rather than focusing solely on wastewater treatment endpoints.

This study provides the first end-to-end, citywide greenhouse-gas emission profile covering direct, operational, and embedded emissions across both on-site and off-site sanitation pathways. Applying a framework to Kampala reveals total emissions of ~189 kt CO2e/year, with dominant contributions from anaerobic storage in pits/tanks and from treatment processes without methane recovery. The work highlights large opportunities for mitigation through improved system management, reduced leakage and bypass, and adoption of methane capture or alternative treatment technologies. The framework and updated, field-informed emission factors for typical on-site containment systems can inform more accurate national and global inventories. Future research should expand empirical measurements (especially for sewers and on-site systems), refine emission factors across diverse contexts, characterize temporal dynamics (e.g., wet/dry seasons, flood events), and evaluate mitigation scenarios and offsets from safe reuse.

- Scope excludes downstream disposal/reuse and potential offsets from resource recovery, likely under- or overestimating net emissions depending on reuse practices. - Sewer methane emissions during conveyance were excluded due to limited sedimentation data and low coverage reaching treatment; this may underestimate off-site pathway emissions in other contexts. - Several parameters rely on IPCC default or literature values (e.g., Bo, COD, nitrogen excretion, emission factors), and on city-level assumptions (e.g., allocation of failures to infiltration vs drains, treatment plant allocation), introducing uncertainty. - Treatment plants lack methane capture, and plant operational status (e.g., digesters) may vary over time; results are sensitive to such operational conditions. - Results are specific to Kampala’s 2018 service profile and may not generalize without adaptation to different infrastructural, hydrogeological, and behavioral contexts.