Rainwater tank

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 pipes. Rainwater tanks are devices for collecting and maintaining harvested rain. A rainwater catchment or collection (also known as "rainwater harvesting") system can yield of water from of rain on a roof. 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. 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.

Rainwater tanks may be constructed from materials such as plastic (polyethylene), concrete, galvanized steel, as well as fiberglass and stainless steel which are rust and chemical resistant. Full tanks are usually installed above ground, and are usually opaque to prevent the exposure of stored water to sunlight, to decrease algal bloom. Smaller tanks, such as the plastic ( barrel) are also used in some cases. Larger tanks are commonly used where there is no access to a centralised water supply. Companies recommend a tank for a house supporting two people (if compost toilets are placed) and if the region receives at least of precipitation a year. If it receives less (between , two or three of these 300 gallon tanks can be placed so that more rain can be gathered at times when it does rain. Also affecting tank size is predicted rainfall and rainfall variability; the higher prices for larger tanks; intended use of rainwater and typical consumption for these uses; the area of roof draining into the tank; security of supply desired. Most rainwater catchment tanks used throughout the world are composed of virgin polyethylene, a substance which in the US is both FDA and NSF approved for potable water storage. Other types of tanks used for rainwater storage include fiberglass, galvanized metal, stainless steel, and concrete. Each type of tank has positive and negative aspects. Polyethylene tanks, when placed above ground, can be subject to algae growth as well as the possibility of a short life (about 20 years) due to normal UV exposure in sunlight. The very strong fiberglass tanks must undergo a specific coating process to be brought up to potable grade. Galvanized tanks must either be lined or coated both for potability as well as to prevent the inevitable rusting at any welded seams. Uncoated galvanized tanks will leach zinc into the stored water and are not recommended in most instances certainly not for water stored for human consumption. Concrete tanks leach a more benign substance, lime, into stored water and many are used around the world for rainwater storage. One method of harvesting rain water has been is modular, scalable systems which are installable underground. These came as an evolution of a geosynthetic applications called Infiltration Tanks, which when stacked provide a void space volume which allows for the storing of water. Improved and more cost effective industrial design now allow for theoretically limitless storage of water underground.

In Australia, the brand TransTank International manufactures portable water tanks from specially formulated polyethylene for transport and static applications. In some cities, installation of rainwater tanks may be mandatory, or may help a new building be approved. For example, in Victoria, Australia, new houses which have rainwater tank connected to all flush toilets are given an additional 1 star of the required 5 star House Energy Rating. Some governments subsidise purchases of rainwater tanks or provide rebates in areas where they are considered an important means of water supply augmentation. In the United States, Santa Fe County, New Mexico requires a rainwater collection system on all new construction with greater than 2,500 square feet, mostly for landscaping purposes and to prevent over reliance on wells but in some instances because ground water is prohibitively expensive to obtain, if even available. Rainwater to supplement drinking water supplies may be seen as an alternative to other water supply options such as recycling or seawater desalination. Tanks are often perceived to have environmental costs that are comparatively lower than other water supply augmentation options. Rainwater collection can be made compatible with centralised water supply by tapping it using an electropump. Widespread use of rain barrels also changes the amount of rainwater reaching the ground in a particular area and draining into streams. Depending on the climate, this either helps prevent erosion, sedimentation, and/or pollution, and can reduce the strain on stormwater drainage systems; or it could cause rivers to dry up and ponds to stagnate if the water is diverted to a different watershed. If collected water is used in the same watershed in which it is collected, rainwater collection actually can stabilize flow in rivers and provide more regular and filtered groundwater transfer into ponds.

In the State of Colorado, United States, the installation of rainwater collection barrels is subject to the Constitution of the State of Colorado, state statutes and case law. This is a consequence of the system of water rights in the state; the movement and holding of rainwater is inextricably linked with ownership of water rights and is enshrined in the constitution of the State of Colorado. The use of water in Colorado and other western states is governed by what is known as the prior appropriation doctrine. This system of water allocation controls who uses how much water, the types of uses allowed, and when those waters can be used. This is often referred to as the priority system or "first in time, first in right." Since all water arriving in Colorado has been allocated to "senior water right holders" since the 1850s, rainwater prevented from running downstream may not be available to its rightful owner. In 2009, legislation in Colorado was enacted that permits capture of rain water for residential use subject to strong limitations and conditions. To be permitted, a residence may not be connected to a domestic water supply system serving more than three single family dwellings. The permit must be purchased from the State Engineer's office and is subject to water usage restrictions.

Rainwater tanks or drums may be used inside a house to provide thermal mass for a trombe wall (or water wall). Rainwater Hog modular tanks invented by Sally Dominguez to fit within building structure were used in the Modabode House of the Future floor and on the foyer wall of the Department of Sustainability building in Anglesea, Victoria, harnessing the higher value of the stored rainwater to add effective thermal mass to the enclosed spaces. Specially designed rainwater tanks can also be embedded in or under the concrete slab of a building (stab tank). A house in Cape Schanck, Victoria, Australia, uses an internal rainwater tank to provide cooling to the living room in summer. During winter the tank is drained and wrapped in an insulating jacket. The tank also provides structural support to the roof, and excess water is used for domestic use including drinking.