Table 1.

Contributions and limitations of existing tools and approaches to estimate pathogen flows and health risks and inform decision making.

Approach	Description	Assessment of Approach or Tool (in Relation to Estimating Pathogen Flows and Health Risks to Inform Decision-Making)
		Contributions	Limitations
Shit Flow Diagram (SFD) [25,26]	- Advocacy and decision-support tool that assesses the fate of excreta in urban areas based on secondary data and stakeholder interviews or primary field research - Produces a graphic representation of the proportion of population’s excreta that is considered “safely” or “unsafely” managed in terms of service outcomes along the sanitation service chain - Similar: Urban Sanitation Status Index [27], and Faecal waste flow calculator [28]	- Relatively simple diagram for identifying the major service failures and advocating improvements across the service chain - Considers multiple flow paths of both solid (sludge) and liquid (effluent) waste - Reports available from 50 countries - Standardised definitions of sanitation types - Aligned with Sustainable Development Goals	- The approach does not specifically identify health/pathogen hazards as the approach does not consider the volume of excreta flows, or pathogen concentrations or exposure, and hence it is not a risk assessment tool - Definition of “safe” is based on achieving a service standard as agreed with city stakeholders rather than “safe” in terms of actual or relative health risk
Material Flow Analysis (MFA)	- Systematic assessment of stocks and flows of material within a defined system in space and time [29] - Applications for sanitation include assessment of nutrient flows [30] and E. coli [31]	- Quantifies flow and load considering changes (i.e., treatment) and interconnected pathways, can therefore quantify the effect on the entire system if one part is changed - Potential for varied scale and complexity of analysis	- Quantifies the system inputs and outputs but not the impact/consequence or spatial aspects - Limited data in developing countries, however there is potential to use “expert judgement” to fill data gaps [30]
Quantitative Microbial Risk Assessment (QMRA)	- Method to quantitatively assess scientific data in the context of estimated health outcomes due to the potential or actual exposure to infectious microorganisms [32] - Four step risk assessment process: hazard identification, dose–response assessment, exposure assessment and risk characterization	- Considers the difference in pathogens’ infectivity and health effects and the different frequency, volume and proportion of population exposed through different pathways - Increasingly applied in low-income countries, including field-based studies on exposure from wastewater reuse or drainage channels [5,21,33,34,35]	- Often based on limited samples of indicator organisms due to cost and difficulty of measuring pathogens in low-income countries - Does not generally include systematic analysis of how or where pathogens enter the environment, and therefore proposed improvements are often limited to reducing exposure through behaviour change or physical barriers rather than preventing pathogen entry to the environment - Other potential limitations of QMRA included in the Discussion section
Sanipath [7]	- Assesses exposure to faecal contamination in urban neighbourhoods based on questionnaires, field surveys and environmental microbiology samples - Produces an exposure risk profile (percentage population exposed and monthly average E. coli dose) for multiple exposure pathways associated with inadequate sanitation	- Detailed assessment of behaviours to understand site-specific child and adult exposure including dose and frequency - Data available on environmental contamination and exposure behaviours in five cities - Findings in Ghana highlighted widespread pathogen contamination in public areas, with high E. coli concentration in drains and highest exposure risk in fresh produce	- Exposure is based on E. coli only, which may not be representative of other pathogen types - Difficult to compare findings (i.e., Prioritise between high dose and low % population exposed vs low dose and high % population) - Results are not linked to the source of pathogens; proposed improvements therefore focus on exposure risk mitigation rather than prevention of pathogens entering the environment - In environments with high pathogen concentrations from multiple sources, limited sampling may not capture all risk pathways
Sanitation Safety Planning (SSP) [24]	- Risk assessment approach to systematically identify and manage health risk along the sanitation chain and guide investment based on health risks - Participatory approach to include actors from different sectors to identify risks and agree on improvements and regular monitoring - Applicable to all sanitary systems, however it was developed for the implementation of the Guidelines for Safe Use of Wastewater, Excreta and Greywater	- Draws on local knowledge to identify health hazards and exposure pathways - Promotes a multi-barrier approach with a focus on achieving pathogen log reduction - Step-by-step guidance to identifying and assessing hazards and highlighting the numerous pathways of exposure to various user groups	- Risk assessment of likelihood and severity is subjective and may not be informed by sufficient evidence - Control measures focus on disease transmission routes rather than source of pathogens in the environment due to a strong focus on wastewater and excreta reuse in agriculture - Difficult to quantify effectiveness of control measures [36]
Rapid Participatory Sanitation SystemRisk Assessment (RPSSRA) [20]	- Draws on local community knowledge and their perception of their environment to derive risk scores based on a set of pre-defined indicators - Participants rank the risk for their neighbourhood from a set of defined conditions for 14 indicators	- Identifies local behaviours, status of services and contextual factors that influence exposure risks - Provides a rapid and resource-efficient assessment of behaviours and exposure with results from small-group discussions validated with detailed survey	- Risks are not health focused and the predefined indicators are site specific and subjective - Traffic light assessment may limit differentiation between exposures (i.e., most indicators were ranked high risk)
BORDA risk mapping [37]	- Guide to planning sanitation based on mapping existing services, environment and health data to identify priority and challenging areas - Water, wastewater and waste options identified based on population density, road width and income	- Spatial analysis of priority locations due to multiple hazards; informs option selection based on physical and economic factors - Applied in Dar es Salaam, Tanzania	- Requires detailed spatial data - Assumes risks to be higher where there are overlapping hazards but does not consider exposure, which limits health risk assessment
Technology options assessments (various)	- The EAWAG Sanitation Compendium [1] presents a complete overview of individual technologies, their advantages and disadvantages, and how they can be linked together in a systems approach; although no explicit approach for comparison or relating to existing pathogen flow situation in an urban environment is provided. Various other tools compare individual technology options through an indicator approach, including NESTAFF [38] and TAF [39]	- Detailed description of individual technologies, including their performance against criteria or indicators	- Health aspects, if included, are typically qualitative and limited to a broad assessment of whether an exposure or health risk exists - Do not typically consider how proposed solutions relate to or build from existing sanitation conditions and services, or recognise that a single sanitation solution in such contexts is unlikely
Microbial Exposure and Health Assessments in Sanitation Technologies and Systems [22]	- Assessment of the health risks of each technology in the EAWAG Sanitation Compendium based on risk ratio for diarrhoea infection or helminthiasis	- Assessment of system performance to remove all four classes of pathogens - Considers system efficiency, robustness, and different exposure pathways and risks for worker, farmer, community	- Unclear how the risk ratio (infection per 10,000 p/y) is calculated - Focus on individual technologies rather than the combined risk of a system and how it is influenced by local context
Saniplan [40]	- Excel-based decision support tools, developed for India, considering infrastructure and service improvements and financial planning	- Considers entire sanitation service chain and based on current service performance (access, service quality, efficiency, finance)	- Suggested improvements based on comparison of key performance indicators (e.g., % households with improved sanitation) rather than health risks
Sanitech [41]	- Tool for assessing options in Indian cities, based on spatial data, physical constraints and cost	- Comparison based on cost, coverage and environmental treatment performance	- Health risks not considered in selection or comparison of improvement options
Citywide planning tools (various)	- Various planning tools focus on the steps for implementing planning, including Community-Led Urban Environmental Sanitation Planning (CLUES) [42], Sanitation 21 [43], Performance Assessment System (PAS) [44], Citywide Sanitation Strategy [45], City Sanitation Plan [46]	- Highlight the importance of a participatory approach and of considering local conditions and service status, typically along entire sanitation service chain - Considers other services (drainage, waste) - Governance and finance focused	- Health often not included in criteria used for comparison of options (Citywide Sanitation Strategy, PAS) - Guidance typically does not inform decision on different technical solutions or the extent to which different options achieve overall objectives