Pathogen pathways and urban planning

Themes: Health impact Planning Research Sanitation Tools and guidance Urban Sanitation Research Initiative

Improving public health is one of the primary reasons for investing in sanitation infrastructure like sewer networks or sludge treatment plants. This makes sense: minimising contact with faeces almost certainly means that fewer people will get sick from diseases like diarrhoea or cholera. Tools such as Shit Flow Diagrams provide an overview of a city’s whole sanitation management system, showing how much untreated poop leaks back into the environment at each stage (containment, emptying, transport, treatment and reuse/disposal). And the SaniPath research project has shed very useful light on ways in which people are exposed to faecal pathogens in urban environments.

But to date no-one has developed a comprehensive modelling approach which pulls together the different elements (release of faeces into the environment, transport through the environment, eventual human exposure) into a single model. There are very considerable practical challenges in developing a model of this type: but it could be very useful indeed to municipal planners and other sanitation decision-makers as they try to work out the most cost-effective way of improving sanitation in their city.

In a recently completed study carried out under the Urban Sanitation Research Initiative, researchers with the Institute of Sustainable Futures at the University of Technology Sydney have made a first step towards addressing this, by developing the skeleton outline of a model that could be used to  predict the effect of different sanitation interventions on pathogen flows through urban environments, and thus to predict effects on disease risk. It’s an early concept-stage study, but eventually an approach of this type could provide the basis for building a practical model that helps urban policymakers and funders definitively link specific sanitation interventions to health outcomes.

The model is a source-flux-exposure model: it takes into account the source of pathogens (like poorly functioning septic tanks or leaky sewers), how pathogens move across the city (through things like open drains or water bodies, and including die-off), and when and where humans are exposed to pathogens (if food is washed in dirty water or people bathe in a contaminated river, for example). The basic idea is to be able to work out what would happen to the health of a city’s population (expressed through disability-adjusted life years [DALYs] lost) following investments into different sanitation improvements.

For an overview of this project’s aims, methods and findings, see our new Policy Brief.

Or if you want to the whole complicated shebang, the research has just been published, with open access, in the International Journal of Environmental Research and Public Health.

Private water sellers collect untreated water from Lake Naivasha, Kenya

A city’s Head of Sanitation, for example, could be all too aware that half of the city lacks access to decent sanitation facilities: but how can she make the most difference with the resources available to her? Using a model of this type, the Head of Sanitation could see the impact on DALYs if, for example, her department were to fund the expansion of the sewer network to 25% of the population, while covering open drains in the remaining areas.

If data on volumetric flows of faecal sludge and the pathogen load of excreta are available (and that’s a Big Ask in many cities), a model of this type could produce useful real-world results for city sanitation planners and managers. But all of us (the researchers, WSUP, and probably you the reader) are very aware that developing a useful model will be challenging, particularly because of data availability issues. This research has been early work using dummy data for a hypothetical urban area. The next step will be to develop and apply the model in more detail, in a real city: so watch this space!

But even at this early stage, we think this research is useful to encourage people to start thinking more carefully about the complexity of faecal pathogen pathways in urban environments. If you think that just building toilets will resolve faecal-oral diseases rates in your city: stop and think again!

Credits: This research was carried out by Cynthia Mitchell, Freya Mills and Juliet Willetts (University of Technology Sydney) and Susan Petterson (Water and Health Pty Ltd.).

Full findings are reported in a journal article entitled “Faecal pathogen flows and their public health risks in urban environments: A proposed approach to inform sanitation planning”, published in the International Journal of Environmental Research and Public Health

Collecting water in Kisumu, Kenya

Featured image: An open sewer in Kibera, Nairobi. Credit: Frederic Courbet