The accurate estimation of greenhouse gas (GHG) emissions is crucial for effective climate change mitigation strategies. Current global estimates often underestimate the contribution of sanitation systems, particularly in rapidly growing urban areas of low- and middle-income countries (LMICs). This underestimation arises from the complexity of sanitation service chains in these settings, which often involve a mix of on-site and off-site sanitation systems with varying levels of efficiency and management. These systems are characterized by a complex interplay of direct emissions from biological decomposition of human waste (methane, nitrous oxide, and carbon dioxide) and indirect emissions from energy consumption in sanitation operations and the embodied carbon in infrastructure. The existing literature largely focuses on wastewater treatment technologies and discharge, with limited attention to emissions from on-site sanitation systems like septic tanks and pit latrines. Furthermore, there is a lack of studies that comprehensively assess both direct and indirect emissions across the entire sanitation service chain, from waste containment to treatment and disposal. The Intergovernmental Panel on Climate Change (IPCC) models include sanitation emissions under 'waste' and link them to agricultural emissions due to manure use. However, the current IPCC methodologies lack sufficient detail to accurately model emissions in cities with mixed sanitation systems and imperfect management, leading to unreliable national emission estimates. This research aims to address these gaps by developing a comprehensive framework for estimating GHG emissions from an entire city-wide sanitation system, encompassing both on-site and off-site services and considering all stages of the service chain. The study uses Kampala, Uganda, as a case study due to its readily available data and its characteristic mix of on-site and sewered sanitation, providing a valuable representation of challenges faced in many LMIC cities.

Existing literature on GHG emissions from sanitation systems primarily focuses on wastewater treatment technologies and discharge, neglecting the significant contributions of on-site systems and the entire service chain. Studies have examined emissions from specific technologies such as wastewater treatment plants (WWTPs) using various methods, including life cycle assessments (LCAs) and process-based models. While some research has investigated emissions from on-site systems like septic tanks and pit latrines, these studies often lack the scope and detail required for comprehensive city-wide assessments. The IPCC guidelines for National Greenhouse Gas Inventories provide emission factors for common wastewater treatment options and some on-site systems, but these factors are often based on limited data and may not accurately reflect the diversity of systems and conditions in LMICs. Furthermore, the existing frameworks generally do not consider the entire sanitation service chain, including collection, transport, and treatment, potentially leading to significant underestimation of total emissions. This study aims to bridge these knowledge gaps by considering the entire sanitation service chain and employing a more comprehensive approach to emission estimation.

This study adopted a whole-system approach to estimate GHG emissions from Kampala's sanitation service chain. The methodology involved three main steps: (1) data collection and system characterization, (2) emission factor development, and (3) emission quantification. **Data Collection and System Characterization:** The study relied on existing data sources, including the Kampala Capital City Authority (KCCA) and several research studies. These sources provided information on the city's population, sanitation infrastructure (on-site and off-site systems), waste flows (excreta flow diagram or SFD), treatment plant operations, and waste transportation. The SFD for Kampala characterized the different pathways of faecal waste flow, from household containment to treatment or final disposal, quantifying the proportion of waste managed safely and unsafely. Key data inputs included population size, sanitation technology distribution, waste collection and transportation practices, and treatment plant characteristics and operations. **Emission Factor Development:** The study utilized existing IPCC methodologies and adapted them to account for the specific conditions in Kampala. This involved the development of site-specific emission factors for methane and nitrous oxide from various on-site sanitation systems (pit latrines, septic tanks) and treatment plants based on factors like waste storage time, moisture content, and aerobic/anaerobic conditions. Data on chemical oxygen demand (COD), dissolved oxygen (DO), oxidation-reduction potential (ORP) were obtained from relevant field studies to improve emission factor accuracy. **Emission Quantification:** Emissions were quantified for each stage of the sanitation service chain: containment, emptying and transport, and treatment. For on-site systems, emissions from storage in pits and tanks, along with emptying and transportation (trucking), were estimated using population-weighted averages. For off-site systems (sewered sanitation), emissions from sewers and treatment plants were calculated. The study incorporated both direct emissions from the biological decomposition of waste and indirect emissions associated with operational energy consumption and embodied carbon in infrastructure. Specific equations were used to model emissions from methane, nitrous oxide and carbon dioxide across different system components, relying on the measured or estimated values of relevant parameters obtained from the data analysis. Finally, the pathway-based emission rates were combined with population data from the SFD to build a total emission profile for the city.

The study found that sanitation in Kampala generated a total of 189 kt CO2e per year. This is a substantial amount, potentially representing more than half of the city's total GHG emissions. The analysis revealed that the dominant sources of emissions were: (1) long periods of faecal waste storage in sealed anaerobic tanks (49% of total emissions), (2) uncollected methane emissions at treatment plants (31%), (3) wastewater bypassing treatment (7%), (4) sewer leakage (6%), (5) discharge from tanks and pits directly to open drains (4%), and (6) illegal dumping of faecal waste (2%). The per capita annual emissions rates were highest for wastewater treatment and typical on-site containment systems. The emissions from different excreta pathways were significantly influenced by the handling and treatment of waste. The analysis highlighted that a significant portion of waste escapes safe management, with substantial leakage and untreated discharge contributing to high emissions. The results underscore the need to improve sanitation infrastructure and management to significantly reduce the city's carbon footprint.

The findings highlight the significant and often overlooked contribution of sanitation to GHG emissions in rapidly growing cities of LMICs. The high emissions from long storage in anaerobic tanks suggest that improved sanitation technologies and more frequent emptying are crucial for emission reduction. The substantial emissions from treatment plants underscore the need for improved treatment plant efficiency and methane capture technologies. The significant proportion of wastewater bypassing treatment emphasizes the importance of upgrading sewer networks and ensuring efficient wastewater collection. The results emphasize the importance of integrating sanitation planning and management with broader climate change mitigation efforts. This study's whole-system approach provides a more accurate and comprehensive assessment than previous studies and can serve as a model for estimating sanitation emissions in other similar urban settings. The detailed assessment allows for the identification of specific emission hotspots, enabling targeted interventions to reduce GHG emissions from the sanitation sector.

This study presents the first comprehensive analysis of citywide sanitation GHG emissions, using Kampala, Uganda as a case study. The results reveal substantially higher emissions than previously estimated, with a significant portion attributable to on-site sanitation and treatment plant inefficiencies. This highlights the urgent need for improved sanitation infrastructure, management practices, and technology to mitigate these emissions. The study's framework provides a valuable tool for future research and policy development, supporting more accurate global GHG emission estimates and targeted interventions to reduce the environmental impact of urban sanitation in LMICs. Further research should focus on validating the methodology in other cities and exploring the cost-effectiveness of different mitigation strategies.

The study relied on existing data, which may have limitations in terms of accuracy and completeness. The accuracy of emission factor estimates could be improved with more comprehensive field measurements. The modelling approach simplified certain aspects of the sanitation system, such as the heterogeneity of on-site sanitation systems. Future studies could incorporate more detailed spatial information and incorporate uncertainty analysis to improve the robustness of the results. The study did not consider emissions downstream of treatment, such as from sludge disposal or effluent reuse, potentially underestimating potential offsets.