Rainwater tank

A rainwater tank (also known as a rain barrel in the US or a water butt in the UK) is a water tank which is used to collect and store rain water runoff, typically from rooftops via rain gutters.

Rainwater tanks are installed to make use of soft rain water for home use, reduce mains water use, and aid self-sufficiency. In arid climates, rain barrels are often used to store water during the rainy season for use during dryer periods. Tanks can also be used for retention of stormwater.

Stored water is used for flushing toilets, in washing machines, watering gardens, washing cars, and sometimes for drinking, if adequate care is taken that the water is not contaminated.

Disadvantages of a rainwater tank include the initial upfront cost, especially where water charges are otherwise fixed. Occasional maintance to the tank is also required, or the user runs a health risk if maintenance is not carried out.

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 pollution, and can reduce the strain on stormwater drainage systems; or it could cause rivers to dry up and ponds to stagnate.

Contamination and maintenance
If rainwater is used for drinking, it is often filtered first. Filtration (such as reverse osmosis or ultrafiltration) may remove pathogens which may be present in rainwater (such as cryptosporidium and giardia), or for chemical contamination in some areas. 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 ; However some pathogens, chemical contamination and sub-micrometre suspended metal may produce neither smell, taste and not be visible to the eye.

Certain paints and roofing materials which 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 galvanised 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. 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.

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).

Tanks
Rainwater tanks may be constructed from materials such as plastic (polyethylene), concrete, and galvanized steel, as well as fibreglass which is rust and chemical-resistant. Tanks are usually installed above ground, and are usually opaque to prevent the exposure of stored water to sunlight, to avoid algal blooms.

Tanks are also covered and have screen inlets to exclude insects, debris, animals and bird droppings. Tanks often come with a plastic inner lining to both increase the life of the tank and protect the water quality.

Apart from rooftops, tanks may also be set up to collect rainwater from concrete patios, driveways and other impervious surfaces.

Sizes typically range in capacity from around 400 to 100,000 litres (100 to 25,000 US gallons). Smaller tanks, such as the plastic 208-liter (55-gallon barrel) are used in some cases. Larger tanks are commonly used where there is no access to a centralised water supply. 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.

Water supply augmentation
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.

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. Where stormwater feeds natural creeks and streams, there are concerns about rainwater harvesting systems reducing the amount of run-off entering natural bodies of water.

Rainwater collection can be made compatible with centralised water supply by tapping it using an electropump.

Colorado law
In the State of Colorado, USA, the installation of rainwater collection barrels is an offense. 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.

Calculating rainwater collection
Rainwater collection from roofs can be calculated using the following formulas:
 * 1 inch of rain on a 1000 square feet roof yields 623 gallons of water; or
 * 1 millimeter of rain on a 1 square meter surface yields 1 litre of water.

External links and references

 * Rainwater Tanks (PDF), NSW Health
 * WSUD Engineering Procedures: Stormwater. Robin Allison, Matt Francey. 2005 (Melbourne Water). Published by CSIRO Publishing.
 * Water Storage: Tanks, Cisterns, Aquifers, and Ponds For Domestic Supply, Fire and Emergency Use- Includes How to Make Ferrocement Water Tanks Book by Art Ludwig
 * Standard Methods for the Examination of Water and Wastewater (standardmethods.org)

Building water barrels

 * Build your own water barrel
 * Building a water barrel system
 * Example and instructions for closed barrel system
 * Constructing residential rain barrel systems--both closed and open
 * Rain barrel construction - Great Lakes Green Initiative

Initiatives

 * Rain Barrel Initiative in Pittsburgh, Pennsylvania
 * Waterwise - Reducing Water in the UK house

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