Some communities do not need storage tanks because the flow from the source is sufficient all year long. The storage tank is used for temporary water storage. In many cases, community members think that storage tanks are for the drier season. It is important to show that storage will only last for a day or two and that a much larger tank would be needed to store months worth of water. In many cases, the tank serves to buffer daily water usage. The tank fills during the night when water usage is low and empties during the day.
Design
The tank site should be a flat area free of debris. If the tank is located on a hillside, create drainage trenches around the future tank area to avoid future erosion from rain runoff.
Each tank is designed on a site-by-site basis. An engineer should be consulted for approval of the final design.
To size a tank, you will need to create a daily water usage chart of the community. Look at daily water usage by conducting surveys in a few representative homes. What are the daily activities and how would that affect water usage (cooking, washing, etc.)? Create a chart that shows approximate water usage based on the hour of the day. Look at what peak water usage will be.
Measure the flow during the dry season of your water source. Calculate gallons/day. Create spreadsheet that shows daily demand for community minus the input from your source. For peak demand, there will need to be X number of gallons to meet future community needs. This should be the approximate size of the tank. Double-check to make sure tank will fill at night to meet those needs during dry season.
A more detailed description of tank sizing and potential designs is located in the fabulous book, A Handbook of Gravity Flow Water Systems, Thomas D. Jordan Jr.
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