A cistern (Middle English cisterne, from Latin cisterna, from cista, box, from Greek kistê, basket) is a receptacle for holding liquids, usually water. Often cisterns are built to catch and store rainwater. They range in capacity from a few litres to thousands of cubic metres (effectively covered reservoirs).
Some cisterns sit on the top of houses or on the ground higher than the house, and supply the running water needs for the house. They are often supplied not by rainwater harvesting, but by wells with electric pumps, or are filled by manual labor or by truck delivery. Very common throughout Brazil, for instance, they were traditionally made of concrete walls (much like the houses, themselves), with a similar concrete top (about 5 cm. thick), with a piece that can come out for water filling and be re-inserted to keep out debris and insects. Modern cisterns are manufactured of plastic (in Brazil with a characteristic bright blue color, round, in capacities of about 10k and 50k liters). These cisterns differ from water tanks in the sense that they are not completely enclosed and sealed with one form, rather they have a lid made of the same material as the cistern, which is removable by the user.
To keep a clean water supply, the cisterns must be kept clean. It is recommended to inspect them regularly, keep them well-enclosed, and to occasionally empty them and clean them with an appropriate dilution of chlorine and to rinse them well. Well water must be inspected for contaminants coming from the ground source. City water has up to 1ppm (parts per million) chlorine added to the water to keep it clean, and in many areas can be ordered to be delivered directly to the cistern by truck (a typical price in Brazil is BRL$50, USD$20 for 10k liters). If there is any question about the water supply at any point (source to tap), then the cistern water should not be used for drinking or cooking. If it is of acceptable quality and consistency, then it can be used for (1) toilets, and housecleaning; (2) showers and handwashing; (3) washing dishes, with appropriate sanitation methods, and for the highest quality, (4) cooking and drinking. Water of non-acceptable quaility for the before mentioned uses may still be used for irrigation. If it is free of particulates but not low enough in bacteria, then boiling may also be an effective means to prepare the water for drinking.
Many greenhouses use cisterns to help meet their water needs, especially in the USA. Some countries or regions, such as Bermuda and the U.S. Virgin Islands have laws that require rainwater harvesting systems to be built alongside any new construction, and cisterns can be used in these cases. Other countries, such as Japan, Germany and Spain, also offer financial incentives or tax credit for installing cisterns.