Greenhouse gas emissions from container-based sanitation systems in East African cities: a case study in Nairobi, Kenya

The escalating trend in greenhouse gas (GHG) emissions has become a critical factor in exacerbating the global effects of climate change (UNEP 2021). The consistent rise in GHG emissions poses a significant threat to the goals outlined in the Paris Agreement (United Nations 2015), necessitating immediate and decisive action to avert a substantial deviation.

As of 2023, a staggering 40% of the world population still lacks access to safely managed sanitation systems, representing almost half of the global population (WHO & UNICEF 2024). This deficiency is a substantial impediment to achieving Sustainable Development Goal (SDG) 13, addressing climate action, and SDG 6.2, focusing on sanitation (United Nations General Assembly 2015). The urban slums of Nairobi, characterised by informal settlements and inadequate sanitation and hygiene facilities (Auerbach 2016; Richmond et al. 2018), are at the forefront of experiencing the adverse impacts of climate change, including flooding, heavy rainfall, and droughts (Twinomuhangi et al. 2021).

Climate change is disrupting the national economies of many low- and middle-income countries (LMICs) (United Nations General Assembly 2015) in various ways, and the destruction of the sanitation infrastructure is the most worst among others (Howard & Bartram 2010). In the CBS system, the faecal waste is hygienically and safely emptied in a sealed container as soon as it fills, reducing the chances of the excreta getting mixed with water during flooding (Mara & Evans 2018; World Bank 2019).

The container-based sanitation (CBS) system is an innovative approach that originated in Bolivia in 1998 and gained global recognition through organisations such as SOIL in Haiti, Sanergy in Kenya, and Clean Team in Ghana (World Bank 2019). The CBS system provides toilets with removable containers to safely dispose of or reuse faecal sludge, effectively separating faeces and urine for more efficient treatment. Within the CBS system, faeces and urine are stored in separate containers for a short duration (3 days to a week) (Auerbach 2016). No water is used for flushing, but rather, cover materials such as sawdust, ash, or lime are sprinkled on the faeces after each defaecation to facilitate drying (Russel et al. 2019; World Bank 2019; Russel & Montgomery 2020). This results in dry conditions at the containment of the CBS system, leading to a slower degradation speed and expected lower GHG emissions. Notably, the World Health Organization (WHO) and the United Nations Children's Fund (UNICEF) classified CBS as a safely managed sanitation system (Russel et al. 2019; World Bank 2019; Russel & Montgomery 2020).

Sanergy's Fresh Life Toilets (FLTs) are examples of CBS systems in East Africa (Riungu 2021). FLT, classified as a dry toilet, segregates urine and faeces in separate cartridges, which are replaced once full. The CBS system sanitation chain comprises containment, emptying, transportation, treatment, and disposal (Russel et al. 2019; McNicol et al. 2020). The containment involves the separation of the human waste from users and placing it in removable cartridges (World Bank 2019). The cartridges are then emptied, and the content is consolidated and transported to the treatment plant located in Kinanie, approximately 40 km from Nairobi (Riungu 2021). Treatment methods typically involve aerobic composting or the use of black soldier fly larvae (Mertenat et al. 2019).

The adoption of CBS systems presents a range of benefits, including reduced GHG emissions, improved privacy, enhanced security, decreased odour, and lower water usage compared to traditional sanitation systems (Reade 2016). Studies have indicated that the CBS system can reduce GHG emissions by up to 79% when replacing pit latrines (Trimmer et al. 2020).

Sanitation systems produce 3% of global man-made methane and 2% nitrous gas (Russel & Montgomery 2020). Pit latrines alone produce 1–2% of global methane (Reid et al. 2014), and generally by 2020 about 4.7% of anthropogenic methane results from non-sewer sanitation systems (NSSSs) (Cheng et al. 2022) Climate-positive sanitation solutions can reduce GHG emissions (Russel et al. 2019). In Kampala, poor sanitation management results in 189 kg CO₂/year GHG emissions (Johnson et al. 2022). The CBS system reduces emissions through innovative methods such as black soldier fly treatment (Mertenat et al. 2019; Parodi et al. 2020). Estimated emissions from CBS system containment are 15.4 kg CO₂eq/capita/year for methane and 21.9 kg CO₂eq/capita/year for nitrous oxide (Johnson et al. 2022). However, direct quantification of GHGs from CBS system containment is lacking, and IPCC methodology may underestimate emissions by over 53% (Shaw et al. 2021). Moreover, a recent study found that the IPCC estimated CH₄ emissions are over seven times the real field measurement (Manga & Muoghalu 2024). Further study is needed to provide accurate field-observed results. Alternative approaches, such as the Container-Based Sanitation Alliance (CBSA) Calculator and flux chamber methodologies, are gaining traction (Huynh et al. 2021; Reddy et al. 2022; Russel et al. 2019). This study addresses the gap in GHG emissions quantification along the CBS system containment, which is crucial for reducing GHG emissions and informing effective climate mitigation strategies.

All the data used in this study were obtained through field measurements and from a peer-reviewed article by Johnson et al. (2022) in Kampala. The data from Johnson et al. (2022) were used, based on the assumption they were comparable as they were obtained from the model that uses data from one of the East African Cities, Kampala.

The flow velocity and gas flow volumes were measured using a Smart Anemometer (Testo 405i, United States) with a detection level of 0.00001 m³/s. For this study, the device was inserted through a hole in a 50 mm diameter pipe to measure the flow rate, the volume flows, and the temperature of the gases exiting the flux chamber through the pipe.

The division of the containment was done to allow measurement of the actual activity happening at the consolidation centre as the collected 20-L containers could have been taken directly to the treatment site. At each of these points, GHG emissions measurements were performed separately.

Measurement was carried out in the morning between 8 a.m. and 12 noon, afternoon (12 noon to 3 p.m.), and in the evening between 3 p.m. to 5:30 p.m. every day for 10 days. However, at the emptying point of the containment, measurements were taken only early in the morning as all the containers were emptied and taken back to the toilets.

The storage, after the consolidation centre was temporary, and the faeces took 3 days to a week before transportation to the treatment plant. This strategy was necessary to reduce the costs of transportation. All the sets of data collected were checked for outliers using Microsoft Excel version 2021.

At this point, faeces samples brought in the containers using a tuk-tuk (tricycle) or handcart to the consolidation centre were sampled for measurements and the rest were consolidated immediately. The containers and the polyethylene sheets inside were opened and the flux chamber setup was immediately fixed on the top. The polyethylene sheet was used to reduce the exposure risk while handling the faecal matter. The flux chamber base was modified (Figure 2(b)) with a lid that fitted the opening of the faeces container and the edges between the flux chamber and the container were made airtight.

The temporary storage point allowed faeces accumulation in 160-L barrels before transportation to the treatment plant. Sanitation waste (faeces) consolidated in each barrel weighed about 98 kg (Figure 5(b)). The barrel was opened only during filling, and it remained covered in the storage shade until transportation to the treatment plant.

Upon connecting the GA 5000, initial readings fluctuated but eventually stabilised, and the recorded values are presented in Table 1. Conversely, the G200 displayed high initial readings that subsequently decreased and normalised to a constant value. The smart anemometer automatically recorded and printed flow velocity and volume flow readings. Notably, the smart anemometer readings were predominantly zero, except for temperature, which was consistently recorded and noted each time when the data were collected from the G200 and GA 5000.

Details of the quantity and origin of the sawdust used were not recorded. However, the amount of the faecal matter (including sawdust collected with it) from each of the 20-L containers was weighed and recorded. This quantity was used to estimate the number of persons using the toilet based on the data from Rose et al. (2015) and Oxfam WASH (2017). The quantity of the faeces from the 160-L barrel at the temporary storage was also weighed, however, it was not used to estimate the total number of users as it was purely the consolidated faecal matter from the 20-L containers.

Upon determination of the gas concentration in mg/m³, the quantity of gas in g/day was calculated by multiplying the volume flows from the smart anemometer with the gas concentration while converting the seconds to days. As the measurement of the emissions was executed during the day only, an assumption was made to distribute the quantity of the gases throughout 24 h. This was done by multiplying the resulting emission by 28,8000 s (measurement for morning, afternoon, and evening for points of capture and temporary storage) and by 84,6000 s (measurement done in the morning only at the end of emptying). The resulting values were divided by the quantity of the faeces in kilograms measured from each container (20 and 160 L) at the point of capture, emptying and temporary storage (Table 2). The average quantity of faeces obtained during the study per container of 20 L was 5.4 kg.

The emission value obtained for each gas emitted was converted to carbon dioxide equivalent values using the conversion factor obtained from the IPCC Guidelines 2019 (IPCC 2019). The global warming potential conversion factors considered were 34 for methane, 34 for nitrous oxide, and 1 for carbon dioxide (GWP) (Vallero 2019).

At the same time, very little nitrous oxide (0.34 g/day/kg of faeces after summing from morning, afternoon, and evening hours) was detected at the point of capture, resulting from little biodegradation activities (Andersen et al. 2010). However, the quantity of nitrous oxide was higher than that of methane due to existing aerobic conditions at the surface of the container (Johnson et al. 2022).

The quantity of methane (below the detection level) and nitrous oxide (0.34 g/day/kg of faecal) was low due to little biodegradation (Table 3) resulting from high aeration (Osada et al. 2000). This is further supported by the theoretical evidence that shows little decomposition of the sanitation waste during emptying arising from the little time (2–3 days) the waste spends in the containment (Hellmann et al. 1997; Beck-Friis et al. 2001; Andersen et al. 2010; World Bank 2019). The fact that fresh sawdust covered the faeces in the containers, there is a higher chance the sawdust could have caused a reduction in the moisture content (Semiyaga et al. 2018) resulting in low emissions.

However, at the point of temporary storage, methane was detected due to the presence of anaerobic conditions (Nakagiri et al. 2017), resulting from consolidating the faeces in the 160-L barrels before transportation. The unit of measurement for the total quantity of the gases was in g/day/kg of faecal matter to accommodate the differences in the sizes of the containers. The unit was then converted to kg/CO₂eq/day/kg of faecal matter to cater to global warming potential (GWP).

This study calculated methane and nitrous oxide emissions from the CBS system containment, totalling 16 kgCO₂ eq/year/capita (Table 4), significantly lower than the 37 kgCO₂ eq/year/capita estimated by Johnson et al. (2022). The discrepancy may be attributed to varying environmental conditions, such as temperature, pressure, and aeration (Huynh et al. 2021), and the use of modified emission factors by Johnson et al. (2022) that are based on IPCC assumptions (Shaw et al. 2021) rather than actual system performance.

Notably, the study's emission values are 73% lower for methane (0.40 kgCO₂ eq/year/capita vs. 15 kgCO₂ eq/capita) and 32% lower for nitrous oxide (15 kgCO₂ eq/year/capita vs. 22 kgCO₂ eq/capita) compared to the estimate values by Johnson et al. (2022). The lower emission values may also be because the study does not account for urine-derived emissions, a significant source of nitrous oxide (Reid 2020). Given the limitations and uncertainty in sanitation system emission estimates (Doorn et al. 2006), further research is necessary to confirm these findings and provide more accurate data.

Carbon dioxide (79 kgCO₂ eq/year/capita) being biogenic was not considered in estimating the total emissions (Table 4) for the comparison with IPCC methodology since the method does not include it. This is because faecal waste at the containment originates from plant tissues and this process allows for the natural carbon cycle flow (Brown et al. 2008; Shaw et al. 2021; Mehmood et al. 2022). Carbon dioxide also has a GWP of only 1 compared to methane (34) and nitrous oxide (300) in the sanitation system (IPCC 2019) and thus, is considered only to have a minimum impact (Shaw et al. 2021; Johnson et al. 2022).

This study provides new insights into GHG emissions from CBS systems. Methane and nitrous oxide are identified as primary emissions, with nitrous oxide production dominating due to aerobic conditions. Although carbon dioxide is present, its biogenic nature and minimal GWP render it to be a minor contributor. Our findings show significantly lower emissions – 73% less methane and 32% less nitrous oxide – than previous estimates. These results underscore the importance of accurate on-site measurements and improved techniques to inform sustainable strategies for mitigating GHG emissions in urban sanitation.

All relevant data are included in the paper or its Supplementary Information.

The authors declare there is no conflict.