DESIGN AND CONSTRUCTION
An opening large enough to provide easy access of a person into the cistern should be left at the top. This opening needs to have a watertight cover with a lock to reduce the risks of contamination or accidents. An overflow pipe should also be provided. The cistern must be watertight. Inlets and outlets should be screened and valves should permit control of water flow.
Provide positive ventilation when anyone is working in a cistern. There may be hazardous gases present or insufficient oxygen. A water sealant should be applied to concrete tank surfaces.
Elevated tanks can be fabricated from concrete, metal, or plastic. The weight of these tanks is considerable. One gallon of water weighs 8.3 pounds, and each cubic foot of water weighs 62.4 pounds. Concrete weighs about 150 pounds per cubic foot. Wind loads may also be a problem on exposed elevated tanks.
Elevated tanks should be placed on structurally sound towers. They can also be designed as part of a building. When a cistern is elevated, the amount of pressure developed will depend upon the height of the water surface. About one pound of pressure is developed for each 2 1/2 feet the water surface is above the water outlet. To achieve a satisfactory rate of flow, a head of at least 20 feet of elevation is usually necessary.
Friction causes pressure losses as water flows through a pipe. There is less loss in a large pipe than in a small pipe. It is best to use at least 1 1/4 inch pipe for main supply lines.
Elevated tanks fabricated from plastics and fiberglass-reinforced plastic may have a shorter lifetime than metal or concrete tanks. If possible, tanks fabricated from synthetic materials should be located in shaded areas to reduce the damaging effects of ultraviolet radiation. Wooden cisterns are generally not satisfactory, particularly when they are used below ground, because they are difficult to keep sealed and allow pollution and ground water to enter through their cracks.
The excavation for below-ground cisterns should be large and deep enough to permit the laying of the foundation and walls. Underground tanks should be made from concrete to reduce problems of wall deterioration because of contactwith the soil.
The walls of concrete cisterns should be four to six inches thick. A concrete mix of 5 gallons of water, 2 1/4 cubic feet of sand, and 3 cubic feet of gravel per sack of cement should be used. Use one inch diameter or smaller gravel. Make sure that the water is clean. Portland cement must be dry and free of lumps. Sand should be clean and well graded; that is, with particles of many sizes.