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A cistern (; , ; ) is a waterproof receptacle for holding liquids, usually water. Cisterns are often built to catch and store rainwater. To prevent leakage, the interior of the cistern is often lined with hydraulic plaster. (1992). 9789652210135, Israel Exploration Society. ISBN 9789652210135
Cisterns are distinguished from by their waterproof linings. Modern cisterns range in capacity from a few liters to thousands of cubic meters, effectively forming covered .
Early examples of ancient cisterns, found in Israel, include a significant discovery at Tel Hazor, where a large cistern was carved into bedrock beneath a palace dating to the Bronze Age. Similar systems were uncovered at Ta'anakh. In the Iron Age, underground water systems were constructed in royal centers and settlements throughout ancient Israel, marking some of the earliest instances of engineering activity in urban planning.
The Ancient Rome impluvium, a standard feature of the domus house, generally had a cistern underneath. The impluvium and associated structures collected, filtered, cooled, and stored the water, and also cooled and ventilated the house.
One particularly unique modern utilization of cisterns is found in San Francisco, which has historically been subject to devastating fires. As a precautionary measure, in 1850, funds were allocated to construct over 100 cisterns across the city to be utilized in case of fire. The city's firefighting network, the Auxiliary Water Supply System (AWSS) maintains a network of 177 independent underground water cisterns, with sizes varying from 75,000 US gallons (280,000 L) to over 200,000 US gallons (760,000 L) depending on location with a total storage capacity of over 11 million U.S. gallons (42 million liters) of water. These cisterns are easily spotted at street level with manholes labeled CISTERN S.F.F.D surrounded by red brick circles or rectangles. The cisterns are completely separate from the rest of the city's water supply, ensuring that in the event of an earthquake, additional backup is available regardless of the condition of the city's mainline water system.
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 by Water well with , or are filled manually or by truck delivery, rather than by rainwater collection. Very common throughout Brazil, for example, they were traditionally made of concrete walls (much like the houses themselves), with a similar concrete top (about 5cm/2 inches thick), with a piece that can be removed for water filling and then reinserted to keep out debris and insects. Modern cisterns are manufactured out of plastic (in Brazil with a characteristic bright blue color, round, in capacities of about 10,000 and 50,000 liters (2641 and 13,208 gallons)). These cisterns differ from in the sense that they are not entirely 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 cistern must be kept clean. It is important to inspect them regularly, keep them well enclosed, and to occasionally empty and clean them with a proper 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. 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) , and housecleaning; (2) showers and handwashing; (3) washing dishes, with proper sanitation methods, and for the highest quality, (4) cooking and drinking. Water of non-acceptable quality for the aforementioned 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 method to prepare the water for drinking.
Many rely on a cistern to help meet their water needs, particularly in the United States. Some countries or regions, such as Flanders, Bermuda and the U.S. Virgin Islands, have strict laws requiring that rainwater harvesting systems be built alongside any new construction, and cisterns can be used in these cases. In Bermuda, for example, its familiar white-stepped roofs seen on houses are part of the rainwater collection system, where water is channeled by roof gutters to below-ground cisterns. Other countries, such as Japan, Germany, and Spain, also offer financial incentives or tax credit for installing cisterns. Cisterns may also be used to store water for firefighting in areas where there is an inadequate water supply. The city of San Francisco, notably, maintains fire cisterns under its streets in case the primary water supply is disrupted. In many flat areas, the use of cisterns is encouraged to absorb excess rainwater which otherwise can overload sewage or drainage systems by heavy rains (certainly in urban areas where a lot of ground is surfaced and doesn't let the ground absorb water).
At the beginning of the flush cycle, as the water level in the toilet cistern tank drops, the flush valve flapper falls back to the bottom, stopping the main flow to the flush tube. Because the tank water level has yet to reach the fill line, water continues to flow from the tank and bowl fill tubes. When the water again reaches the fill line, the float will release the fill valve shaft and water flow will stop.
The rainwater falling on the rooftops is directed through pipelines or gutters and stored in the cistern. The cistern is covered with a lid to avoid evaporation. Each cistern has a capacity of 16,000 liters. Water collected in it during 3–4 months of the rainy season can sustain the requirement for drinking, cooking, and other basic sanitation purposes for rest of the dry periods. By 2016, 1.2 million rainwater harvesting cisterns were implemented for human consumption alone. After positive results of P1MC, the government introduced another program named "One Land, Two Water Program" (Uma Terra, Duas Águas, P1 + 2), which provides a farmer with another slab cistern to support agricultural production.
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