posted on 2020-10-09, 13:21authored byLara 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.