Faecal-oral infections are a major component of the disease burden in low-income contexts, with inadequate sanitation seen as a contributing factor. However, demonstrating health effects of sanitation interventions - particularly in urban areas - has proved challenging and there is limited empirical evidence to support sanitation decisions that maximise health gains. This study aimed to develop, apply and validate a systems modelling approach to inform sanitation infrastructure and service decision-making in urban environments by examining enteric pathogen inputs, transport and reduction by various sanitation systems, and estimating corresponding exposure and public health impacts. The health effects of eight sanitation options were assessed in a low-income area in Dhaka, Bangladesh, with a focus on five target pathogens (Shigella, Vibrio cholerae, Salmonella Typhi, norovirus GII and Giardia). Relative to the sanitation base case in the study site (24% septic tanks, 5% holding tanks and 71% toilets discharging directly to open drains), comprehensive coverage of septic tanks was estimated to reduce the disease burden in disability-adjusted life years (DALYs) by 48-72%, while complete coverage of communal scale anaerobic baffled reactors was estimated to reduce DALYs by 67-81%. Despite these improvements, a concerning health risk persists with these systems as a result of effluent discharge to open drains, particularly when the systems are poorly managed. Other sanitation options, including use of constructed wetlands and small bore sewerage, demonstrated further reductions in local health risk, though several still exported pathogens into neighbouring areas, simply transferring risk to downstream communities. The study revealed sensitivity to and a requirement for further evidence on log reduction values for different sanitation systems under varying performance conditions, pathogen flows under flooding conditions as well as pathogen shedding and human exposure in typical low-income urban settings. Notwithstanding variability and uncertainties in input parameters, systems modelling can be a feasible and customisable approach to consider the relative health impact of different sanitation options across various contexts, and stands as a valuable tool to guide urban sanitation decision-making.

Public health benefits are often a key political driver of urban sanitation investment in developing countries, however, pathogen flows are rarely taken systematically into account in sanitation investment choices. While several tools and approaches on sanitation and health risks have recently been developed, this research identified gaps in their ability to predict faecal pathogen flows, to relate exposure risks to the existing sanitation services, and to compare expected impacts of improvements. This paper outlines a conceptual approach that links faecal waste discharge patterns with potential pathogen exposure pathways to quantitatively compare urban sanitation improvement options. An illustrative application of the approach is presented, using a spreadsheet-based model to compare the relative effect on disability-adjusted life years of six sanitation improvement options for a hypothetical urban situation. The approach includes consideration of the persistence or removal of different pathogen classes in different environments; recognition of multiple interconnected sludge and effluent pathways, and of multiple potential sites for exposure; and use of quantitative microbial risk assessment to support prediction of relative health risks for each option. This research provides a step forward in applying current knowledge to better consider public health, alongside environmental and other objectives, in urban sanitation decision making. Further empirical research in specific locations is now required to refine the approach and address data gaps.

Sustainable Development Goal target 6.2 calls for universal access to adequate and equitable sanitation, setting a more ambitious standard for ‘safely managed sanitation services’. On-site sanitation systems (e.g., septic tanks) are widely used in low- and middle-income countries (LMICs). However, the lack of indicators for assessing fecal exposure risks presents a barrier to monitoring safely managed services. Furthermore, geographic diversity and frequency of disasters require a more nuanced approach to risk-informed decision-making. Taking Indonesia as an example, the purpose of this paper is to provide insights into current status and practices for on-site sanitation services in the contexts of LMICs. Using a dataset from a national socio-economic survey (n = 295,155) coupled with village census (n = 83,931), we assessed (1) household sanitation practices across Indonesia stratified by city-level population density and meteorological factors, (2) factors associated with septic tank emptying practice, and (3) inequalities in potential fecal exposure as measured by population density and WASH access by wealth quintile. We found a high reliance on on-site sanitation facilities (80.0%), almost half of which are assumed to be ‘uncontained’ septic tanks and one in ten facilities discharging untreated waste directly into the environment. The most densely populated areas had the highest rates of septic tank emptying, though emptying rates were just 17.0%, while in the lowest population density group, emptying was rarely reported. Multivariate regression analysis demonstrated an association between flooding and drought occurrence and septic tank emptying practice. Higher groundwater usage for drinking among poorer households suggests unsafe sanitation may disproportionally affect the poor. Our study underscores the urgent need to strengthen the monitoring of on-site sanitation in LMICs by developing contextualized standards. Furthermore, the inequalities in potential fecal exposure require greater attention and tailored support mechanisms to ensure the poorest gain access to safely managed sanitation services.

Significant progress is needed, in both large cities and small towns, to meet the ambitious targets set at international and national levels relating to universal access to safely managed sanitation. There has been increased recognition in the urban sanitation sector that in rapidly growing cities, there is unlikely to be a single centralized sanitation solution which can effectively deliver services to all demographics, and that heterogeneous approaches to urban sanitation are required. At the same time, due to competing investment priorities, there is a greater focus on the need for sanitation investments to address multiple objectives. However, calls for more informed sanitation planning and a more dynamic and disaggregated approach to the delivery and management of sanitation services have had limited impacts. This is in part due to the complexity of the drivers for sanitation investment, and the difficulties involved in identifying and addressing these multiple, often conflicting, goals. This paper examines three potential drivers of citywide sanitation decision-making – public health, sustainability and economic performance – via the three proxies of contamination, climate change and costs. It examines the importance of each driver and proxies, how they are considered in investment decisions, the current state of knowledge about them, and priority aspects to be included in decisions. At present, while public health is a common driver for improving sanitation, there are significant gaps in our understanding of fecal contamination spread and exposure, and how to select sanitation solutions which can best address them. Climate change is sometimes seen as a low priority for the sanitation sector given the immediacy and scale of existing challenges and the uncertainty of future climate predictions. However, potential risks are significant, and uninformed decisions may result in greater costs and increased inequalities. Cost data are sparse and unreliable, and it is challenging to build robust cost-effectiveness analyses. Yet these are needed to compare citywide options based on least-cost over their full life cycle. This paper provides insights into how existing evidence on contamination, climate change and costs can inform decisions on sanitation investments and help chart a sustainable way forward for achieving citywide services.

The level of risk that onsite sanitation systems (OSS) pose to groundwater quality remains uncertain. The link between contamination and OSS can only be proved if the source, pathway, and receptor are investigated and confirmed when assessing contamination. The literature on the connection between OSS and groundwater contamination has been reviewed several times but with limited assessments of the extent to which the literature confirms that the source of contamination is an OSS, that a pathway has been identified, and that the receptor is groundwater. A systematic review was conducted on published studies and supports previous work that concluded that the removal and transport of contaminants from OSS to groundwater is complex and varies significantly according to local conditions. This variability means simple siting guidelines based on horizontal separation are not reliable. Though not always recognised in the literature, formation of a biological layer is important for removal of microbial pathogens. This layer takes months to form which impacts the performance of OSS that are new or subject to highly variable loading. Under ideal conditions of an unsaturated zone comprising fine material, faecal indicator bacteria can be reduced to detection limits within 10 metres distance. However, ideal conditions are very often not present. Multiple studies showed the presence of viruses in the absence of faecal indicator bacteria. Ingress of contaminated surface water into faulty boreholes/wells and contamination of wells from users are both often significant yet not adequately covered in the included literature. The review established that it is extremely difficult to eliminate the risk of groundwater contamination when OSS effluent is discharged into the subsurface. It is unsafe to assume that contamination can be prevented in areas with high OSS density, and further research is needed to determine the critical density threshold for different ground conditions.

Ensuring sustainability of sanitation infrastructure assets and services over the long-term is crucial for achieving safe sanitation for all. Co-management is an emerging approach that balances state and citizen responsibility for services, with applicability to community-scale (or decentralized) sanitation systems in a city-wide context. In Indonesia more than 30,000 of these systems are typically managed solely by communities, however, due to challenges in technical, social and financial aspects, commonly fall to disrepair. This paper presents qualitative research comprising document review, interviews and co-design workshops with local government and community management groups that developed a model for co-management of community-scale systems. The co-management model articulated four minimum responsibilities for local government: monitoring and corrective action; provision of technical and institutional support to community groups; formalization of fee collection; and funding of large costs for rehabilitation and expansion. This model was developed and tested in two case study locations, and through this process, was deemed appropriate, acceptable and feasible for both local government and community management groups. Related changes to Indonesia’s national program guidelines were also identified to clearly articulate local government’s role. The agreed co-management approach aligns with the human right to sanitation by supporting local governments fulfill their legal mandate for services, promotes professionalized sustainable management arrangements, ensuring community-scale systems can contribute effectively to future citywide solutions.