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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 rain water runoff, typically from rooftops via rain gutters. A rainwater catchment or collection (also known as "rainwater harvesting") system can yield 2358 litres (623 gal) of water from 2.54 cm (1 in) of rain on a 93 m2 (1,000 sq ft) roof. Rainwater tanks are devices for collecting and maintaining harvested rain. Rainwater tanks are installed to make use of rain water 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 described in more detail below. 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.〔Camilli, Luis (2000), (Rainwater Harvesting: constructing a cistern and gutter system using village technologies and materials ), Maji Safi Rainwater Harvesting Manual〕 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 rain water is pure it may become contaminated during collection or by collection of particulate matter in the air as it falls. While rain water does not contain chlorine, contamination from airborne pollutants, which settles 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;〔(【引用サイトリンク】title=Buying guide: Rainwater tanks )〕 However some pathogens, chemical contamination and sub-micrometre suspended metal may produce neither smell, taste and 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 USA 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〔 "Aluminium and cadmium can be an impurity in zinc galvanised iron roofs (Gromaire et al., 2001, Van Metre and Mahler, 2003) and therefore found in collected tank rainwater. Indeed, in Study 2, the tank with high concentration of Cd collected water from a galvanised iron roof."〕 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.〔(【引用サイトリンク】title=Copper poisoning linked to plastic water tanks )〕 The water was then brought into homes by copper piping. The copper was released by the high acid rainwater and caused poisoning in humans. It is important to note that, 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.〔The Sunday Age. November 25, 2007. "These flies! Will mozzies be next?". Stephen Cauchi quoting Luke Simpkin, Museum Victoria.〕 Several options for dealing with the mosquito issue are: 1. Flushing ALL water once a week 2. Using a small amount of cooking oil to suffocate the larvae (the water is still ok for landscape use after this) 3. Adding the bacillus Bt to the water. This bacteria will not harm animals. 4. Adding mosquito eating fish Another way to store rainwater without worry of contamination by mosquitos is to use underground storage tanks. Underground tanks keep the water too cool (@55 degrees F) for mosquito larvae and also are dark, preventing both mosquito, bacterial, and algae growth. A very good article by Richard Hill goes into depth about the benefits of underground rainwater storage.〔http://www.whitewaterlimited.com/BacterialActivityinHarvestedRainWater.pdf〕 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Rainwater tank」の詳細全文を読む スポンサード リンク
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