5.5 Basic calculations in water supply
A critical factor in the sustainability of a water supply system is ensuring that the volume of water provided is sufficient to meet current and future demand. Table 5.1 shows the water supply requirements for towns of different sizes in Ethiopia according to the Growth and Transformation Plan II.
Table 5.1 Water supply requirements for urban areas in Ethiopia (note that for categories 1–4, the water should be available at the premises). (MoWIE, 2015)
|Urban category||Population||Minimum water quantity (litres per person per day)||Maximum fetching distance (m)|
|2 Big city||100,000–1,000,000||80||–|
|3 Large town||50,000–100,000||60||–|
|4 Medium town||20,000–50,000||50||–|
|5 Small town||<20,000||40||250|
The water needs of a town can be estimated from the size of the population and the water requirements of users such as schools, health facilities and other institutions within it. The guidelines for the water supply requirement of different categories of towns, shown in Table 5.1, may be used to estimate the minimum quantity of water that should be supplied for a given population.
Consider a town with a population of 60,000. What would be the minimum amount of water required?
From Table 5.1, the minimum amount of water needed per person would be 60 litres a day. So, with a population of 60,000, the daily total supply would need to be:
60 litres x 60,000 = 3,600,000 litres, or 3600 m3.
There will be institutions in the town with particular water requirements. Table 5.2 shows the requirements of some of these in Ethiopia.
Table 5.2 Water requirements for various types of institutions in Ethiopia. (Adapted from Kebeda and Gobena, 2004)
|Institution||Water requirement (litres per person per day)|
Once the consumers’ total water requirement has been calculated, an allowance should be added for leakage losses, and for water use by the water utility itself (for washing of tanks, etc.). This allowance could, for example, be 15%.
The water will have to be stored in service reservoirs. As you learned in Study Session 1, service reservoirs have to hold a minimum of 36 hours’ or 1.5 days’ water supply.
Box 5.1 shows a calculation of water requirement and service reservoir size for a hypothetical town.
Box 5.1 Water provision for a small town
Imagine a town with a population of 5000 people, and a health centre that treats 100 people a day.
The minimum water requirement per day for the population (using the guidelines in Table 5.1) will be 40 litres x 5000 = 200,000 litres, or 200 m3.
The water requirement for the health centre would be 135 litres x 100 = 13,500 litres, or 13.5 m3. The total water requirement each day would be 200 + 13.5 = 213.5 m3.
Allowing for 15% leakage and water usage by the water utility, each day the required volume of treated water supplied would be:
213.5 m3 x 1.15 = 245.5 m3. This could be rounded up to 246 m3.
The service reservoir would need to hold a minimum of 36 hours’ of supply (1.5 days). This means that the service reservoir size would be:
246 m3 x 1.5 = 369 m3. This could be rounded up to 370 m3.
The water requirement would therefore be 246 m3 per day, and the minimum service reservoir capacity required would be 370 m3. This volume could be held in one service reservoir or shared between two, located in different parts of the town.
These simple calculations are included here to give you an idea of the approach that would be taken to planning a new water supply system. In practice, the process would require many different engineering, economic and environmental considerations involving a team of experts.