Maintaining Storage Tanks

The tank should be cleaned in a quarterly maintenance program. The valve to the community should be closed to avoid sediment flowing into the system during cleaning. Open the cleanout valve and drain the tank. The tank should be scrubbed with plastic brushes. A plastic broom can also be used.

Inspect the tank for organic growth. If there is any growth such as algae, the algae should be removed, the tank thoroughly scrubbed in that area and painted. If the tank does not have a smooth concrete surface (inside and outside) the tank should be spackled with a concrete mix (See concrete section of website). The outside tank should be painted as well. If the tank does not have paint, clean the outside of the tank thoroughly. An outdoor high gloss paint should be used.

Building Storage Tanks

In general, you should work with an engineer or someone who has experience designing and building storage tanks. 

Here are some photos of the construction of a octagonal deposit tank built by a fellow volunteer.  While the shape of the deposit tank can vary, the following photos are a great overview of formwork and the construction process.

	Clear the construction area of debris.  Make sure the area is level.
	Pour the foundation slab.  Dig away along the perimeter of the foundation to create a “key” where the slab will be slightly thicker.
	Pour the foundation slab with your rebar that will be used to reinforce the sides of the tank.  Using rebar, you can tie a horizontal piece to align/true the vertical pieces of rebar exiting the foundation.
	Use posts to stake the formwork.  Make sure the posts are strong enough and embedded deep enough to keep formwork in place under the pressure of the concrete poured into the form.
	Internal supports can be used to align formwork and ensure that the formwork stays in place during the first pour.
	Using boards as shown helps keep even spacing of your form to ensure an even wall thickness throughout the structure.
	The formwork from the first pour can be reused in a “lift” for the second pour.  The second pour should be made as soon as the first pour sets (1-2 days) so that the concrete from the second pour bonds to the concrete of the first pour.
	Use posts to keep the form elevated. Sand bags can be used to anchor the posts in place.
	Add a coat of plaster mix (1 part cement to 3 parts sand) to the tank walls.  A smooth coat on the inside and outside will reduce future organic growth on the walls.
	Setup the formwork table for the ceiling concrete slab.
	Posts are used to support the table form and are removed after the ceiling slab has set.
	Rebar layout to tie the slab to the rest of the tank structure and provide reinforcement to span the unsupported distance of the ceiling.  A form is used to provide an access door to the inside of the tank for future cleaning and maintenance.

Here are some photos of additional tank features worth considering.

	Here is an example of a built in ladder and hand hold as part of the tank structure.
	The tank should have an access hole from the top for cleaning and maintenance. The weight of the lid alone should deter children from tampering with the system
	The tank should have an inlet, overflow, exhaust vent, outlet, and cleanout (not shown). Note: This tank contains two chambers so there is redundant pipework. This tank also needs to be cleaned and painted.

Designing Storage Tanks

Why is it Needed
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.

Use WHO’s 30 gallon/day/person (generous) to calculate total water usage across the community.  Conduct a census of the community to look at total population.  Consider using a multiplier to base the design on a future population.

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.