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 (CO2), methane (CH4) and nitrous oxide (N2O). 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 CO2 and CH4 ranging from 1.74 to 398.30 g CO2/cap/day and 0.06-110.13 g CH4/cap/day, respectively, but have low N2O emissions (0.01-0.06 g N₂O/cap/day). CH4 emissions from pit latrines range from 0.77 to 20.30 g CH4/cap/day N2O emissions range from 0.76 to 1.20 gN2O/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 CO2 emissions. CH4 emissions (expressed as CO2 equivalents) from OSS estimated following Intergovernmental Panel for Climate Change (IPCC) guidelines were found to be seven times lower (90.99 g CO2e/cap/day) than in-situ field emission measurements (704.7 g CO2e/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 CH4 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.

Urban Sanitation: New Terminology for Globally Relevant Solutions?

Progress toward Sustainable Development Goals for global access to safe sanitation is lagging significantly. In this Feature, we propose that misleading terminology leads to errors of categorization and hinders progress toward sanitation service provision in urban areas. Binary classifications such as "offsite/onsite" and "sewered/nonsewered" do not capture the need for "transport to treatment" or the complexity of urban sanitation and should be discarded. "Fecal sludge management" is used only in the development context of low- or middle-income countries, implying separate solutions for "poor" or "southern" contexts, which is unhelpful. Terminology alone does not solve problems, but rather than using outdated or "special" terminology, we argue that a robust terminology that is globally relevant across low-, middle-, and upper-income contexts is required to overcome increasingly unhelpful assumptions and stereotypes. The use of accurate, technically robust vocabulary and definitions can improve decisions about management and selection of treatment, promote a circular economy, provide a basis for evidence-based science and technology research, and lead to critical shifts and transformations to set policy goals around truly safely managed sanitation. In this Feature, the three current modes of sanitation are defined, examples of misconceptions based on existing terminology are presented, and a new terminology for collection and conveyance is proposed: (I) fully road transported, (II) source-separated mixed transport, (III) mixed transport, and (IV) fully pipe transported.

Characteristics of sanitation and hygiene facilities in a slum community in Kampala, Uganda

Background

This study assessed characteristics of sanitation and hygiene facilities in a slum community in Kampala, Uganda.

Methods

We conducted a household-based cross-sectional study among 395 households in Kasubi slum using a semi-structured questionnaire and observational checklist to collect data.

Results

Almost 98.0% (387/395) of households owned a sanitation facility and 77.0% (298/387) shared it with other households. The most common type of sanitation facility was a pit latrine with slab (66.9% [259/387]). Most (90.5% [305/337]) latrines had a door or shutter, a roof (92.9% [313/337]) and a depth >1.5 m (68.2% [229/337]). Overall, 21.3% (84/395) and 65.6% (259/395) of households had improved and functional sanitation facilities, respectively. Only 16.5% (65/395) of the households had a hand-washing facility. Student-led (adjusted prevalence rate [PR] 2.67 [95% confidence interval [CI] 1.83–3.94]) and households that owned their house (adjusted PR 2.17 [95% CI 1.33–3.53]) were 2.67 and 2.17 times more likely to have improved sanitation facilities, respectively. Households that owned their house (adjusted PR 1.90 [95% CI 1.18–3.05]) were 1.9 times more likely to possess a hand-washing facility.

Conclusions

The coverage of improved sanitation and hygiene facilities was low. The majority of households were using a shared pit latrine with a slab that had no hand-washing facility. Sanitation and hygiene interventions should prioritize improving sanitation and hygiene facilities.

Re-conceptualizing sustainable urban sanitation in Uganda: why the roots of 'Slumification' must be dealt with

Background

Country-wide urbanization in Uganda has continued amidst institutional challenges. Previous interventions in the water and sanitation sector have not addressed the underlying issues of a poorly managed urbanization processes. Poor urbanisation is linked to low productivity, urban poverty, unemployment, limited capacity to plan and offer basic services as well as a failure to enforce urban standards.

Methods

This ethnographic study was carried out in three urban centres of Gulu, Mbarara and Kampala. We explored relationships between urban livelihoods and sustainable urban sanitation, using the economic sociology of urban sanitation framework. This framework locates the urbanization narrative within a complex system entailing demand, supply, access, use and sustainability of slum sanitation. We used both inductive and deductive thematic analysis.

Results

More than any other city in Uganda, Kampala was plagued with poor sanitation services characterized by a mismatch between demand and the available capacity for service provision. Poor slum sanitation was driven by; the need to escape rural poverty through urban migration, urban governance deficits, corruption and the survival imperative, poor service delivery and lack of capacity, pervasive (urban) informality, lack of standards: ‘to whom it may concern’ attitudes and the normalization of risk as a way of life. Amidst a general lack of affordability, there was a critical lack of public good conscience. Most urbanites were trapped in poverty, whereby economic survival trumped for the need for meeting desirable sanitation standards.

Conclusions

Providing sustainable urban livelihoods and meeting sanitation demands is nested within sustainable livelihoods. Previous interventions have labored to fix the sanitation problem in slums without considering the drivers of this problem. Sustainable urban livelihoods are critical in reducing slums, improving slum living and curtailing the onset of slumification. Urban authorities need to make urban centres economically vibrant as an integral strategy for attaining better sanitation standards.

Supplementary Information

The online version contains supplementary material available at 10.1186/s12889-021-11029-8.

Navigating Multidimensional Social-Ecological System Trade-Offs across Sanitation Alternatives in an Urban Informal Settlement

Urban growth in low- and middle-income countries has intensified the need to expand sanitation infrastructure, especially in informal settlements. Sanitation approaches for these settings remain understudied, particularly regarding multidimensional social-ecological outcomes. Guided by a conceptual framework (developed in parallel with this study) re-envisioning sanitation as a human-derived resource system, here we characterize existing and alternative sanitation scenarios in an informal settlement in Kampala, Uganda. Combining two core research approaches (household survey analysis, process modeling), we elucidate factors associated with user satisfaction and evaluate each scenario's resource recovery potential, economic implications, and environmental impacts. We find that existing user satisfaction is associated with factors including cleaning frequency, sharing, and type of toilets, and we demonstrate that alternative sanitation systems may offer multidimensional improvements over existing latrines, drying beds, and lagoons. Transitioning to anaerobic treatment could recover energy while reducing overall net costs by 26-65% and greenhouse gas emissions by 38-59%. Alternatively, replacing pit latrines with container-based facilities greatly improves recovery potential in most cases (e.g., a 2- to 4-fold increase for nitrogen) and reduces emissions by 46-79%, although costs increase. Overall, this work illustrates how our conceptual framework can guide empirical research, offering insight into sanitation for informal settlements and more sustainable resource systems.

Costs and Willingness to Pay for Pit Latrine Emptying Services in Kigali, Rwanda

Kigali, Rwanda lacks a centralized sewer system, which leaves residents to choose between on-site options; the majority of residents in informal settlements use pit latrines as their primary form of sanitation. When their pits fill, the pits are either sealed, or emptied; emptying is often done by hand and then dumped in the environment, putting the residents and the broader population at risk of infectious disease outbreaks. In this paper, we used revealed and stated preference models to: (1) estimate the demand curve for improved emptying services; and, (2) evaluate household preferences and the willingness to pay (WTP) for different attributes of improved emptying services. We also quantify the costs of improved service delivery at different scales of production. The study included 1167 households from Kigali, Rwanda across 30 geographic clusters. Our results show that, at a price of US$79 per pit, 15% of all the pits would be emptied by improved emptying services, roughly the current rate of manual emptying. Grouping empties by neighborhood and ensuring that each truck services an average of four households per day could reduce the production costs to US$44 per empty, ensuring full cost coverage at that price. At a lower price of US$24, we estimate that the sealing of pits might be fully eliminated, with full coverage of improved emptying services for all pits; this would require a relatively small subsidy of US$20 per empty. Our results show that households had strong preferences for fecal sludge (FS) treatment, formalized services (which include worker protections), and distant disposal. The results from the study indicate a few key policies and operational strategies that can be used for maximizing the inclusion of low-income households in safely managed sanitation services, while also incorporating household preferences and participation.

Drinking Water Supply, Sanitation, and Hygiene Promotion Interventions in Two Slum Communities in Central Uganda

Poor water, sanitation, and hygiene (WASH) continue to contribute to the high prevalence of diarrhoeal diseases in low-income countries such as Uganda particularly in slums. We implemented a 3-year WASH project in two urban slums in Uganda with a focus on safe drinking water and improvement in sanitation. The project implemented community and school interventions in addition to capacity building initiatives. Community interventions included home improvement campaigns, clean-up exercises, water quality assessment, promotion of drinking safe water through household point-of-use chlorination, promotion of hand washing, and support towards solid waste management. In schools, the project supported health clubs and provided them with "talking compound" messages. The capacity building initiatives undertaken included training of youth and community health workers. Project evaluation revealed several improvements in WASH status of the slums including increase in piped water usage from 38% to 86%, reduction in use of unprotected water sources from 30% to 2%, reduction in indiscriminate disposal of solid waste from 18% to 2%, and increase in satisfaction with solid waste management services from 40% to 92%. Such proactive and sustainable community interventions have the potential to not only improve lives of slum inhabitants in developing countries but also create lasting impact.

Waste treatment and energy production from small-scale wastewater digesters

Three tubular anaerobic digestion (AD) systems were installed in Haiti to treat black water (toilet-based wastewater), including a three cell 36m³ clinic digester (CD), a two cell 2m³ hotel digester (HD), and a three-cell 3m³ farm digester (FD) for worker use. During digestion, total coliforms were reduced by 99.1%, E. coli by 98.5%, and chemical oxygen demand (COD) by 93.6%. Nutrients in the effluent averaged 99.4mg/L NH₄⁺ and 10.6mg/L PO₄^2-, producing an effective organic fertilizer. Average biogas production in CD was 108L/d, with 65.4% CH₄. Survey participants (n=573) were willing to pay $0.10-0.30 per use for sanitation facilities. Seventy-two percent of the rural population surveyed in Cange, Haiti lacked access to improved sanitation due to financial constraints. The economic analysis calculated an investment cost for a shared toilet AD systems of $16-$47 (USD) per person based on daily use at design capacity.

Decision-making on shared sanitation in the informal settlements of Kisumu, Kenya

Unlike most quantitative studies that investigate decision-making on investing in sanitation, this study adopted a qualitative approach to investigate decision-making on shared sanitation in the informal settlements of Kisumu city, in Kenya. Using a grounded theory approach, landlords and tenants were interviewed to identify sanitation decisions, individuals involved in decision-making and factors influencing decision-making. The results indicate that the main sanitation decisions are on investment, emptying, repair and cleaning. Landlords make investment, emptying and repair decisions, while tenants make cleaning decisions. Absentee landlords are less involved in most decision-making compared to live-in landlords, who rarely consult tenants in decision-making. Tenants make decisions after consultations with a third party and often collectively with other tenants. Sanitation interventions in informal settlements should thus, target landlords and tenants, with investment efforts being directed at landlords and maintenance efforts at tenants.

Estimating the Cost and Payment for Sanitation in the Informal Settlements of Kisumu, Kenya: A Cross Sectional Study

Lack of sanitation facilities is a common occurrence in informal settlements that are common in most developing countries. One challenge with sanitation provision in these settlements is the cost and financing of sanitation. This study aimed at estimating the cost of sanitation, and investigating the social and economic dynamics within Kisumu's informal settlements that hinder provision and uptake of sanitation facilities. Primary data was collected from residents of the settlements, and using logistic and hedonic regression analysis, we identify characteristics of residents with sanitation facilities, and estimate the cost of sanitation as revealed in rental prices. Our study finds that sanitation constitutes approximately 54% of the rent paid in the settlements; and dynamics such as landlords and tenants preferences, and sharing of sanitation facilities influence provision and payment for sanitation. This study contributes to general development by estimating the cost of sanitation, and further identifies barriers and opportunities for improvement including the interplay between landlords and tenants. Provision of sanitation in informal settlements is intertwined in social and economic dynamics, and development approaches should target both landlords and tenants, while also engaging various stakeholders to work together to identify affordable and appropriate sanitation technologies.

A two-step crushed lava rock filter unit for grey water treatment at household level in an urban slum

Decentralised grey water treatment in urban slums using low-cost and robust technologies offers opportunities to minimise public health risks and to reduce environmental pollution caused by the highly polluted grey water i.e. with a COD and N concentration of 3000-6000 mg L(-1) and 30-40 mg L(-1), respectively. However, there has been very limited action research to reduce the pollution load from uncontrolled grey water discharge by households in urban slums. This study was therefore carried out to investigate the potential of a two-step filtration process to reduce the grey water pollution load in an urban slum using a crushed lava rock filter, to determine the main filter design and operation parameters and the effect of intermittent flow on the grey water effluent quality. A two-step crushed lava rock filter unit was designed and implemented for use by a household in the Bwaise III slum in Kampala city (Uganda). It was monitored at a varying hydraulic loading rate (HLR) of 0.5-1.1 m d(-1) as well as at a constant HLR of 0.39 m d(-1). The removal efficiencies of COD, TP and TKN were, respectively, 85.9%, 58% and 65.5% under a varying HLR and 90.5%, 59.5% and 69%, when operating at a constant HLR regime. In addition, the log removal of Escherichia coli, Salmonella spp. and total coliforms was, respectively, 3.8, 3.2 and 3.9 under the varying HLR and 3.9, 3.5 and 3.9 at a constant HLR. The results show that the use of a two-step filtration process as well as a lower constant HLR increased the pollutant removal efficiencies. Further research is needed to investigate the feasibility of adding a tertiary treatment step to increase the nutrients and microorganisms removal from grey water.