Shit also flows uphill: Exploring neglected non-hydraulic flows of faecal pathogens

Motivation.
Exposure to faecal pathogens is a driver of preventable disease worldwide, with an estimated 3.4 billion people still lacking access to safely managed sanitation (UNICEF, 2025). The sanitation chain protects public health only when it achieves the full separation of faeces from human contact. Failures in Containment, Transport and Treatment release pathogens into the environment, from where they travel along multiple routes to human exposure (Peal et al., 2014).
Purpose.
In the face of such failures, interventions planned without a robust source-pathway-exposure framework risk becoming fragmented and misdirected, with resources channelled into meeting indicators rather than achieving meaningful public health benefits.
Approach and Methods.
The approach was mixed methods and exploratory, combining literature review, fieldwork and modelling. Evidence for source-pathway-exposure linkages for faecal pathogens in the urban context was synthesised. The F-diagram was critically assessed for its usefulness in representing these complex flows, and a new model was proposed: the novel Amplified F-diagram. This diagram was then applied using data collection in Delhi: 180 microbial samples tested for faecal coliforms, 259 household survey participants, over 2000 behaviour observations, six focus group discussions and twelve key informant interviews. The Amplified F-diagram was used to consider how different intervention focus areas would influence public health outcomes.
Findings.
Faecal pathogens are highly mobile across urban environments, creating exposures for communities remote from their source. This underscores the need for systemic city-wide approaches to sanitation interventions. Both empirical and modelled data demonstrate that preventing failures in the Transport stage of the sanitation chain is the most critical for protecting public health. In Delhi, faecal pathogens on the Fresh Produce were traced primarily to herbs grown on urban farms, while Solid Waste had more localised contamination pathways. These non-hydraulic pathways are neglected in both research and in intervention design.
Contribution and Implications.
By conceptualising and analysing source-pathway-exposure linkages, this research expands the traditional F-diagram into a model that more accurately represents urban realities. The Amplified F-diagram provides decision-makers with clearer evidence on which interventions are most likely to disrupt transmission and reduce disease risk. The contribution is therefore both conceptual and empirical: developing a new framework for thinking about urban faecal flows and applying it to case study data from Delhi.