For years, we have been letting water run off into rivers and streams and back to the ocean. Meanwhile, we are pumping it out of the ground faster than it can recharge.

All green building designs must minimize the amount of runoff from impervious surfaces on a property. Rainwater falling on these surfaces should be collected and channeled to a recharge area, so it can go back into the ground, or should replace municipal water used for irrigation, cleaning, process cooling, or even drinking, if properly purified.

Rainwater has been collected and used in the Caribbean islands and other communities and homes for centuries. It has traditionally been collected at downspouts from a residential roof and directed to storage tanks or cisterns. Many green buildings are using this age-old technology to reduce their water consumption.

Rainwater catchment or collection systems offer several advantages, in addition to saving water and money.

• The water has no minerals and is therefore “soft” and better for washing and watering plants.
• Rainwater collection systems provide building owners with an independent water supply.
• The technology used for these systems is fairly simple and low-maintenance
• Collecting rainwater helps reduce the burden on municipal drainage systems and water treatment plants.

Suitability

Collection tanks have traditionally been used in locations such as islands with salt-contaminated ground, remote areas far from water sources, and tropical regions where annual rainfall is plentiful, but there is also a dry season. Recently, many have discovered that even in areas without those criteria, rainwater can be a money-saving source of high-quality water.

Some considerations include:

• Monthly rainfall. (Use data from national weather agencies for at least the previous ten years.)
• Total catchment area—the area of the roof that is available for collecting rain.
• Loss factor—the water that does not go into the tank.
• How much water is needed to accommodate building uses.

Tank Size, Type of System, & Cost

Rainwater collection tanks can range in size from 50–30,000 gallons. The need for water and amount of rainfall, as well as period of drought, will help determine the appropriate tank size.

In areas of evenly distributed rainfall, the tank might be sized to hold a month’s worth. In regions with both rainy and dry seasons, a bigger tank might be desirable to store water for times of drought.

The intended purpose of the collected water is another key factor. A simple 50-gallon drum placed under the downspout might be adequate for watering plants, depending on the climate. (One-fourth inch of rainfall from an average home’s roof would fill the barrel.) Larger needs will require a bigger tank. The planned use of the water will also determine the need for filtration systems and other features.

The cost of rainwater receptacles varies widely, depending on materials and size. A small 75-gallon rain barrel made from recycled plastic can be purchased for about $150. A large, high-end, underground tank could cost roughly a dollar per gallon of capacity ($5,000 for a 5,000-gallon tank). Galvanized tanks are less expensive than polyethylene, but tend to rust and will need to be replaced at some point. Fiberglass tanks range in size from several hundred to 30,000 gallons. These cost roughly $2,000 for a 4,000-gallon tank.

This article is an excerpt from the book Green Building: Project Planning & Estimating which can be purchased through the RSMeans Bookstore.

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