A rainwater tank (sometimes called a rain barrel in North America in reference to smaller tanks, or a water butt in the UK) is a water tank used to collect and store rainwater runoff, typically from rooftops via pipes. Rainwater tanks are devices for collecting and storing harvested rain. A rainwater catchment or collection (also known as "rainwater harvesting") system can yield 1,000 litres (260 US gal) of water from 1 cm (0.4 in) of rain on a 100 m2 (1,100 sq ft) roof. Rainwater tanks are installed to make use of rainwater for later use, reduce mains water use for economic or environmental reasons, and aid self-sufficiency. Stored water may be used for watering gardens, agriculture, flushing toilets, in washing machines, washing cars, and also for drinking, especially when other water supplies are unavailable, expensive, or of poor quality, and when adequate care is taken that the water is not contaminated and is adequately filtered. Underground rainwater tanks can also be used for retention of stormwater for release at a later time and offer a variety of benefits. In arid climates, rain barrels are often used to store water during the rainy season for use during dryer periods. Rainwater tanks may have a high (perceived) initial cost. However, many homes use small scale rain barrels to harvest minute quantities of water for landscaping/gardening applications rather than as a potable water surrogate. These small rain barrels, often recycled from food storage and transport barrels or, in some cases, whiskey and wine aging barrels, are often inexpensive. There are also many low cost designs that use locally available materials and village level technologies for applications in developing countries where there are limited alternatives for potable drinking water. While most are properly engineered to screen out mosquitoes, the lack of proper filtering or closed loop systems may create breeding grounds for larvae. With tanks used for drinking water, the user runs a health risk if maintenance is not carried out.
Contamination and maintenance If rainwater is used for drinking, it is often filtered first. Filtration (such as reverse osmosis, ultraviolet sterilization, or ultrafiltration) may remove pathogens. While rainwater is pure, it may become contaminated by particulate matter in the air as it falls or during collection. While rainwater does not contain chlorine, contamination from airborne pollutants, which settle onto rooftops, may be a risk in urban or industrial areas. Many water suppliers and health authorities, such as the New South Wales Department of Health, do not advise using rainwater for drinking when there is an alternative mains water supply available. However, reports of illness associated with rainwater tanks are relatively infrequent, and public health studies in South Australia (the Australian state with the highest rainwater usage rate) have not identified a correlation. Rainwater is generally considered fit to drink if it smells, tastes and looks fine; however, some pathogens, chemical contamination and sub-micrometre suspended metal may produce neither smell nor taste and may not be visible. Australian standards may differ greatly from other places in the world where rainwater is commonly used for drinking water. In the United States, rainwater is being increasingly used throughout the country for various purposes. In the semi-arid western state of New Mexico, for instance, many residents in the Taos and Santa Fe areas in particular use rainwater either for landscaping purposes or even all household uses (including potable indoor water). The "smells, tastes, and looks fine" standard used in the above paragraph is not an absolute indicator of rainwater safety. Most people who are rainwater users for potable purposes in the US make certain that their water is safe through filtration, ultraviolet sterilization, and testing. Certain paints and roofing materials may cause contamination. In particular, a Melbourne Water publication advises that lead-based paints never be used. Tar-based coatings are also not recommended, as they affect the taste of the water. Zinc can also be a source of contamination in some paints, as well as galvanized iron or zincalume roofs, particularly when new, should not collect water for potable use. Roofs painted with acrylic paints may have detergents and other chemicals dissolve in the runoff. Runoff from fibrous cement roofs should be discarded for an entire winter, due to leaching of lime. Chemically treated timbers and lead flashing should not be used in roof catchments. Likewise, rainwater should not be collected from parts of the roof incorporating flues from wood burners without a high degree of filtration. Overflows or discharge pipes from roof-mounted appliances such as air conditioners or hot-water systems should not have their discharge feed into a rainwater tank. "Copper Poisoning", a 2010 news article, linked copper poisoning to plastic tanks. The article indicated that rainwater was collected and stored in plastic tanks and that the tank did nothing to mitigate the low pH. The water was then brought into homes by copper piping. The copper was released by the highly acidic rainwater and caused poisoning in humans. While the plastic tank is an inert container, the collected acid rain could and should be analysed and pH adjusted before being brought into a domestic water supply system. The solution is to monitor stored rainwater with swimming pool strips, cheap and available at swimming pool supply outlets. If the water is too acidic, the state, county or local health officials may be contacted to obtain advice, precise solutions and pH limits, and guidelines as to what should be used to treat rainwater to be used as domestic drinking water. Maintenance includes checking roofs and rain gutters for vegetation and debris, maintaining screens around the tank, and occasionally desludging (removing sediment by draining and cleaning the tank of algae and other contaminants). Rainwater tanks which are not properly sealed (secured at the top) may act as breeding grounds for mosquitoes. Several options for dealing with the mosquito problem are:
Flushing all water once a week Using a small amount of cooking oil to suffocate the larvae (the water is still suitable for landscape use after this) Adding Bacillus thuringiensis israelensis to the water, a natural pest-control agent. Adding mosquito-eating fish, such as the mosquitofish
Tanks
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