Spatial representation of faecal pollution in unsewered urban catchments

In many secondary cities in Bangladesh and other economically developing regions in Asia, Africa and Latin America, urban sanitation is dependent on individually constructed and maintained decentralised sanitation technologies, e.g., septic tanks operating in the absence of a city-wide support system. In such urban areas, wastewater is transported through a network of storm drains which were not designed for this purpose. The release of wastewater runs the risk of imperfect containment and high risk of exposure to faecal pathogens. Effective methods to identify the sources and movement pathways of faecal matter within cities are currently lacking. Here, a Sanitation Infrastructure and Faecal Flow (SanIFFlow) approach is introduced, representing a novel methodology that utilises open-source data to map the sanitation infrastructure and the faecal matter sources and movement pathways. This approach is first demonstrated through a prototype sub-catchment model within Rajshahi city, Northwest Bangladesh. The sub-catchment model identifies and characterises the sources, pathways, and movement of faecal matter. To refine and validate the method, an uncertainty analysis was conducted, supplemented by a field study, to assess the reliability of the approach. Sensitivity analysis identified five key factors influencing the spatial pattern of faecal flow: septic tank emptying, soak pit use, sludge removal from drains, variations in faecal matter production, and the absence of toilets in some buildings. While each factor might have a negligible impact individually, in combination the factors showed almost 50% faecal matter cannot reach the outlet point. Further insights from the uncertainty analysis and fieldwork suggest that, although the sub-catchment model has potential for individual building level sanitation management, the existing ward-level management system, being the smallest administrative unit in the case study city, calls for a model at that spatial scale as a more practical approach. Building upon this, the SanIFFlow approach has been deployed to develop a city-scale model built from ward-level subunits, tailored for practical application in unsewered cities like Rajshahi. This approach holds promise for global applicability, given the widespread availability of open-source data.