27. How do I change flows or volumes into proportions of the population?

The SFD Graphic shows the percentage of the population whose excreta is either safely managed or unsafely managed. Generally, data on sanitation services at the household level are expressed in terms of the percentage of population, but further along the sanitation chain data are more commonly expressed as volumes per day; for instance wastewater flows and faecal sludge deliveries to a treatment plant. Local expert opinion and the views of local stakeholders can help in agreeing a method for converting these volumetric flows to population-based estimates. The factors that may need to be considered include:

  • Volume of excreta produced per person per day
  • Volume of water used per day for flushing water-based systems
  • Accumulation rate of faecal sludge in tanks and pits per day
  • Exfiltration/leaching rate of pits per day
  • Volume of leakage per day from sewers etc.

Remember to state clearly in the SFD Report the method used and any assumptions made.