Cisterns are tanks that store water for a variety of purposes, such as irrigation, fire suppression and drinking.
- Potable water may be stored in cisterns for such purposes as drinking, bathing and dishwashing. Treated water may arrive from public water sources or filtered from rainwater in a catchment system.
- Non-potable water can be stored for uses such as irrigation and washing cars.
- Many communities have invested in large cisterns for fire-suppression purposes, some of which can hold more than three times as much water as a typical fire truck. The town of Littleton, New Hampshire, for instance, approved funding for the purchase of 21 cisterns, each with a capacity of 10,000 gallons, at a cost of $41,000 per tank.
Cisterns must be made from sturdy materials to support an immense water weight. One gallon of water weighs 8.3 pounds (3.7 kg), and each cubic foot of water weighs 62.4 pounds (28.3 kg). Wind loads may also take a toll on exposed elevated tanks. Some common materials used in cistern construction include:
- reinforced concrete. Often the best investment, reinforced concrete is durable and may help neutralize water acidity.
- reinforced concrete block. These have a tendency to leak at their joints.
- metal. However, metal may corrode.
- fiberglass. Fiberglass may have strength problems when buried. If above ground, fiberglass cisterns should be located in shaded areas to reduce the damaging effects of ultraviolet radiation.
- wood. Wooden cisterns are generally not satisfactory, particularly when they are used below ground, because they are difficult to keep sealed.
Cisterns, along with all their components and accessories, should undergo regular inspections. Replacement or repair of the unit as a whole, and any of its constituent parts and accessories, should subsequently be undertaken, if needed. The primary concern of a cistern inspection is to detect leaks, which can allow water to escape or contaminants to enter the tank. In addition, the following elements may be inspected:
- roof catchment, to ensure that no particulate matter or other parts of the roof are entering the gutter and downspout. Rainwater picks up dust, soot, bird droppings, leaves and other foreign materials that add objectionable organisms, color and odor to the water. For this reason, inspect to make sure that overhanging trees are not part of the catchment system;
- gutters and downspouts should be inspected to assure that no leaks or obstructions are occurring;Above-ground cistern connected to a roof gutter
runoff/overflow pipe, to check that overflow is draining in a non-erosive manner;
- any accessories, such as a rain diverter, soaker hose, linking kit or additional guttering; and
- cistern drains should not be interconnected with waste or sewer lines, as this may allow backflow contamination.
Maintenance requirements for cisterns are relatively few if they supply non-potable water. Cisterns designed for the drinking water supply have much higher maintenance requirements, such as biannual testing for water quality and filtering systems. The following maintenance guidelines can be followed for most types of cisterns:
- Before a cistern is used, it should be cleaned and disinfected. After cleaning out any dirt and other debris accumulated during construction, scrub the interior with a bleach-water solution.
- Make sure that there is ample ventilation for the workers inside the cistern. After this treatment, hose down the interior until the chlorine odor disappears.
- A cistern needs to be cleaned at least every five years. This might be needed more often where blowing dust, leaves and fireplace or stove ash fall on the roof. Inspecting and cleaning the gutters, downspout and filter will help to keep the cistern cleaner.
- Keep manhole covers tight.
- Repair leaks promptly with sealants. Portland cement paints and epoxy resins are available to seal cracks in concrete.
Although usually located underground, cisterns may be placed at ground level or on elevated stands either outdoors or within buildings. For safety and efficiency considerations, cisterns should be placed:
- away from sewage lines or other sources of contamination;
- as far away from trees as possible, as tree roots can crack cistern walls;
- in areas that are sloped to drain surface water away from the cisterns;
- near their catchments;
- in firm ground to avoid settling, which can crack cistern walls; and
- away from sources of heat.
In summary, cisterns are water storage tanks that must be maintained and inspected, especially if they supply potable water.