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

In order to maximise the potential for comparability with established global estimates GHG emission rates were built up for each emission category from established IPCC methodology wherever possible. All emissions were converted to carbon dioxide equivalent (CO₂e) using the 100-year global warming potential (GWP) of each gas (34 for methane, 298 for nitrous oxide)¹⁸.

Methane emission factor for typical sanitation containment and treatment systems

The IPCC estimates methane emissions for sanitation systems from chemical oxygen demand based on Eq. 1. Emission factors are derived from Eq. 2, summed for the population segment using each type of sanitation system.

where; CH₄ = total methane emissions from a given element of the system (kgCH₄/year), P = population using the system, COD = chemical oxygen demand from the excreta of each person (kg COD/cap/year), PR_COD = percentage reduction of chemical oxygen demand whilst in situ (0–1), EF = emission factor for each containment technology (kgCH₄/kg COD)

where; EF_C = emission factor for each containment technology, B₀ = maximum methane-producing capacity kgCH₄/kg COD, MCF_C = methane correction factor for each containment technology

We used Eqs. 1 and 2 to model estimates of direct emissions from typical sanitation systems based on updated methane correction factors (MCF_c). MCF_c varies from 0 (for a fully aerobic environment) to 1 (for a fully anaerobic environment)^19,20. We developed new models for the types of latrines commonly found in Kampala based on field data provided by Nakagiri et al²¹. As inputs, we assumed a typical value for the COD of raw feaces (upstream of the toilet) of 71 kg COD/capita/day²², and a typical value for PR of 70%^22,23. The value of B₀ is 0.25 kg CH₄/kg COD¹².

The methane-forming reaction, methanogenesis, occurs under obligate anaerobic conditions. A low dissolved oxygen (DO) level is a good indicator for higher rates of methane emission. DO falls when the loading is high, and correspondingly when dilution rates are low. DO will also tend to be lower at depth in static flow systems (i.e., within pit latrines or stagnant water bodies)²¹. DO also appears to fall in dry seasons and rise during the rains²⁴. Consistent with DO, low oxidation reduction potential (ORP) of less than +50 mV indicates anoxic condition. Further, low ORP between −199 and −51 mV indicates acidic environment, ideal for methane formation²¹. Almost all pit latrines surveyed by Nakagiri et al.²¹ were within low DO and acidic ORP. In sludges, within pits and tanks, or in wastewater and faecal sludge treatment plants, higher moisture content and acidic environment are associated with enhanced methanogenesis. Thus, lined/sealed containers, waterlogged toilets, water borne piped sewerage and anaerobic, high load or saturated treatment processes are all likely to be associated with higher methane emissions.

To establish values for MCF_c, the physical characteristics of sludge inside containers are required, particularly the extent of aerobic and anaerobic conditions at different depths (see also Supplementary Method 1). Nakagiri et al.²¹ examined the physical properties of sludge cores taken from a number of pits in Kampala. These data were combined with citywide sanitation data from Musabe²⁵ to produce emissions profiles for a set of ‘typical types’ of containers in the city using the IPCC method^11,13. Details of the determination of MCF_c and EF for methane are in Supplementary Tables 1, 3 with a summary of the results shown in Table 2.

Methane emissions from treatment plants were calculated using a modified IPCC formula that is based on Reid et al.²⁰

where methane emissions are expressed in kg CH₄/year and are summed for each treatment plant. U = effective population (the population equivalent of excreta from direct inflow to the process plus effluent from previous, usually drying, process), EF = emission factors (kg CH₄/kg COD) = B₀ × MCF, B₀ = Maximum methane producing capacity kg CH₄/kg COD by process in the local context, MCF = methane correction factor, TOW = total organics in wastewater per year (kg COD/ year), L = proportion of organic component removed as effluent, S = proportion of organic component removed as sludge, R = proportion of methane recovered through capture processes

Detailed calculations are presented in the Supplementary Information.

Nitrous-oxide emission factors for typical sanitation containment and treatment systems

Nitrous oxide is produced during both nitrification and denitrification. Nitrification occurs at the surface facilitating the escape of nitrous oxide gas, and is therefore the more significant process. During denitrification nitrous oxide formed in an anaerobic zone may be dissolved into a liquid phase or converted to dinitrogen (N₂) before it can escape as a gas²⁶. The rate of nitrous oxide emission is therefore dependent on the extent to which aerobic conditions exist at the surface and anaerobic conditions below the surface. These can be impacted by both system design and operational conditions.

Nitrous oxide emissions are calculated based on Eq. 4 summed for the population segment using each type of sanitation system^11,13,21:

where N₂O = total N₂O emissions (kg N₂O/year), P = population using each sanitation facility (cap), N_I= nitrogen influent from urine and faeces (kg N/cap/year), EF = emission factor for each sanitation facility (kg N₂O-N/kg N), (frac{44}{28}) = conversion factor for N₂O–N into kg N₂O.

For containment, we used field-study-derived data^21,25 to generate modelled estimates for emission factors. We assumed a production of 4.672 kg N/capita /year in faeces and urine combined for Kampala (based on a reported value of 12.8 g/cap/day)²⁷. For treatment processes we used the standard emission factors provided by IPCC^11,13. Details of the resultant emission factors for nitrous oxide are in the Supplementary Tables 2, 4 with a summary of the results shown in Table 3.

Operational emissions (trucking)

Operational emissions were calculated on the basis of fuel use for trucking faecal sludge (see also Supplementary Method 4). We used data from truck operations to estimate typical transport distances²⁸ and combined this with estimate of emissions factors for typical trucks, based on work conducted on the transport sector in South Africa²⁹. The emissions from faecal sludge trucking were calculated using Eq. 5 summed for all known trucks operating in Kampala²⁸.

where CO_2,T = total CO₂ emissions from the transport of FS (kgCO₂/year), N_T = number trips made per year, DT = average distance travelled per trip (vehicle km), EF_V = emission factor for each type of vehicle within the FSM fleet (kgCO₂/vkm)

Data on truck journeys are summarised in Supplementary Table 8, which also shows the resultant total CO₂ emissions obtained by applying Eq. 5.

Operational emissions (pumping and aerating wastewater in sewers and treatment plants)

Emissions associated with electricity or fuel usage (e.g., diesel) were calculated using Eqs. 6 and  7 for electricity and diesel respectively summed for each pumping station and/or treatment plant.

where CO_2,el = CO₂ emissions associated with electricity usage (kgCO₂/year), C_el = electricity consumption (MWh/year), EF_el = emission factor (tCO₂e/MWh/year)

where CO_2d = CO₂ emissions associated with diesel usage (kgCO₂/year), C_d = diesel consumption (l/year), EF_d = emission factor (kg CO₂e/l diesel)

We used data on electricity and fuel usage in sewer and wastewater treatment operations and applied Eqs. 6 and 7 to obtain total operational emissions for wastewater operations. Supplementary Method 7 provides more details in the method and the results are broken down on Supplementary Table 14.

Embedded carbon in construction material

We used analytical estimation to model emissions associated with embedded carbon. Full details of the approach are in Supplementary Method 2 for containment, 3 for sewerage, and 6 for treatment plants. Quantities of materials in sanitation structures (toilets, sewers, treatment plants etc.) were estimated based on standard designs and information on the design of toilets in Kampala from Nakagiri, et al.³⁰. Standard emission factors were applied^31,32,33. Typical estimates of infrastructure design life were used to create an annual value. A summary of the emission factors used is shown in Supplementary Table 5 and details of the system-wise calculations are in Supplementary Tables 6, 7, 13.

Sanitation system in Kampala

In order to create the emission profile for the city sanitation system of Kampala, we used data from Nakagiri et al.²¹, Kimuli et al.²⁴, Musabe²⁵, Schoebitz et al.³⁴, McConville et al.³⁵ and Lwasa¹⁷. The section below draws on all these sources.

According to the most recent estimate of excreta flows in Kampala, close to half ends up in the environment untreated³⁴. Around one fifth of the population have sanitation connected to sewers; around a third of wastewater is treated, while two-thirds end up in drains or other water bodies. The remaining population primarily use onsite sanitation systems that are either unlined or lined pit latrines, or so-called septic tanks, many of which are shared. Two-thirds of the population, and many of the people who rely on onsite systems, live in informal low income settlement in low lying areas with high water table, and it is widely reported that most onsite systems are regularly inundated with surface water or flooded with ground water. Of the excreta collected in onsite sanitation systems, about one third remains safely stored in pit latrines and one third are stored in tanks and pits that are located in areas where there is significant risk of groundwater pollution. The remaining third are collected in tanks and pits that are emptied on average once every three years. During flood events there is evidence that many toilets located near to drains are flushed out, using a ‘foot valve’ or vertical gate at the bottom of the tank that can be lifted manually. A graphical summary of the sanitation system is shown in Fig. 1.

There are two major treatment plants, Lubigi and Bugolobi. The Lubigi plant comprises a series of waste stabilization ponds (anaerobic followed by facultative ponds) followed by drying beds for wastewater sludge. Faecal sludge from onsite sanitation is delivered to settling/thickening tanks; liquids are co-treated with wastewater in the stabilisation ponds, and solids in the drying beds. The faecal sludge treatment plant was reportedly already at design capacity of 400 m³ faecal sludge per day within the first months of operation²⁸. Lubigi receives 3,000 m³ wastewater daily out of the 5,000 m³ design capacity³⁵.

Bugolobi wastewater treatment plant consists of settling tanks with supernatant going to trickling filters, solids going to digesters (if operational) followed by drying beds²⁸. While Bugolobi was not designed to co-treat faecal sludge, it nonetheless receives about 200 m³ faecal sludge per day. The plant receives 13,000 m³ wastewater daily out of the 32,000 m³ design capacity³⁵.

The remaining three wastewater treatment plants in Kampala, Naalya, Ntinda and Bugolobi Flats have negligible capacity of 1,175 m³/d¹⁴, approximately 3% of the capacity of Lubigi and Bugolobi combined (37000 m³). Based on the available data we therefore assume that of the excreta that are treated, 80 percent of wastewater and 33 percent of faecal sludge are treated at Bugolobi with the balance treated at Lubigi.

Emissions profile

To produce an emission profile across the entire system, the unit emissions rates calculated as described above were mapped onto the actual sanitation service profile for Kampala using the excreta-flow diagram or SFD for the city³⁴. The process is described in Supplementary Methods 8. Peal et al.¹⁶ note that significant system failures occur in typical urban sanitation systems in Subsaharan Africa. This confirms the findings of Schoebitz et al.³⁴. Many system failures result in discharges to the open stormwater drainage network. Because the drains are sometimes dry we used the mean of the emission rates for untreated waste discharged to open drains in the wet and dry seasons to estimate methane and nitrous oxide emissions caused by flows to open drains (see ‘No facility’ emission rates in Supplementary Table 4). We assumed that all illegal dumping and discharges upstream of the treatment plants went to open drains. However, failures at containment were divided. Shoebitz et al. report that most ‘failed’ containment results in infiltration to the groundwater that is assumed to have negligible impact on emissions³⁴. A quarter of failures at containment are assumed to result in pits and tanks being flushed out to drains during flood events.

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