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Using the Shit/Excreta Flow Diagrams – SFDs- for modelling future scenarios in Kumasi, Ghana

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posted on 09.10.2020, 13:21 by Lara Fernández Martínez
The high population density of the cities does not allow families to safely abandon onsite sanitation facilities. This creates a need for a sanitation service chain to safely manage the faecal waste. Hence, Shit/Excreta Flow Diagram (SFD) are being developed as an analysis tool, which illustrates excreta pathways along the sanitation service chain in a city. The main objective of this study is to use the SFD methodology to model four possible future scenarios in Kumasi and predict the changes in excreta flow patterns. Four different scenarios have been defined using the population growth rate, the number of public toilets, number of private toilets and the treatment plant capacity as main variable parameters. First, a “baseline scenario” was formed, in order to analyse the potential change in the SFD if there is no investment in the next years and the population continues to increase. Afterwards two more scenarios were studied regarding the on-going projects or those that are about to start in the city. Finally, a combination of second and third scenarios was defined to illustrate the total change in the SFD if all projects are implemented. For each scenario a list of questions has been proposed to define the minimum data collection required from secondary data and interviews. Once the scenarios were produced, bottlenecks were identified throughout the sanitation service chain in Kumasi: Private toilets: are not expected to significantly increase in the next year, and considering the high population growth rate this creates a high dependency on the public toilets. Public toilet capacity: the current number of public toilets cannot meet the future demand. The new toilets that will be constructed next years can meet the demand up to 2022. Trucks capacity: If the number of trucks remains constant, the trucks will not be enough after 2017. Treatment plant capacity: The treatment plant will work over capacity and its efficiency will be reduced. Changes from one scenario to another can be observed comparing the SFDs but only interventions that target a high percentage of population have visual impact. Additionally, trends and changes within the same scenario are not easily observed (regarding only the SFDs). For that reason trend graphs have been used to analyse and discuss the results, disclosing: When there are installations that are full but not emptied the percentage of people who safely managed their excreta in the SFDs increases. This situation cannot represent a risk for the environment because the faecal sludge (FS) is contained. However, the FS is not being managed. Additionally, SFDs do not show what is happening with those people who relied on these full installations, e.g. are they to come back to practise OD? Finally it was observed that SFDs show the percentage of FS that is treated in the plants, but analysing only the SFD it is not clear if the treatment plant is working under or over capacity.



  • Architecture, Building and Civil Engineering

Research Unit

  • Water, Engineering and Development Centre (WEDC)