Harvest the Rain
(Page 4 of 8)
August/September 2003
Adapted from Environmental Building News
To be most effective, the roof should be fully exposed and away from overhanging tree branches. This reduces the risk of contamination from rotting leaves or droppings from birds and insects in the trees. If possible, avoid using roofs of buildings that rely on wood heat, as the smoke particles and soot deposited on the roof may contain polynuclear aromatic hydrocarbons and other hazardous materials.
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WASH YOUR ROOF
Between rainstorms, various pollutants can settle out of the air and onto your roof. Many rainwater harvesting systems incorporate a roof washer to keep these contaminants from entering the cistern. Roof washers capture and discard the first several gallons of rainwater during a storm before conveying the rest to the cistern. A very simple roof-wash system can be made out of a 6- or 8-inch vertical PVC or polyethylene pipe installed beneath the gutter, with an inlet just above each downspout to the cistern. Commercial roof washers range in price from $100 for a water diverter (available by mail order only from SafeRain, an Australian company) to $600 for a separate roof washer.
CISTERN
The cistern is the single largest investment for most rainwater harvesting systems. A cistern can range from a recycled whiskey barrel under the eaves of a house (suitable for watering plants) to a large aboveground or buried tank that will hold 30,000 gallons or more. Cisterns are constructed out of a wide range of materials.
Most cisterns are cylindrical for optimal strength-to-weight ratios. A cistern with a 10,000 gallon capacity (a reasonable minimum for a family of four depending solely on rainwater) might be 12 feet in diameter and 12 feet deep, for example. No matter what the material, most experts recommend keeping the tank tightly closed to prevent evaporation and to keep out sunlight (which will support algae growth) and animals (insects, rodents and amphibians). Cisterns often are designed with settling compartments that keep sediment from mixing with the water. The cistern also needs an overflow pipe for additional rain that falls after the cistern is full. If the rainwater harvesting system is the only water source, it makes sense to locate the cistern so that it can be filled by a water tank truck if necessary.
In cold climates, protect the cistern from freezing by burying it underground or incorporating it into a basement. In northern climates where rainwater is the sole water source, it may be necessary to oversize the cistern to provide carry-over during a significant portion of the winter when snow falls instead of rain.
ROUTE YOUR RAINWATER
Rainwater falling on the roof can be captured and conveyed to the cistern via gutters and downspouts constructed of roll-formed aluminum, galvanized steel, PVC (vinyl) or copper. As with the roofing, make sure that lead-based solder was not used in gutter or downspout connections. Downspouts should be designed to handle 1 1/4 inches of rain in a 10-minute period. Depending on the cistern location, 4-inch PVC or polyethylene piping may be used to convey water around the building to the cistern. A continuous 1/4-inch mesh screening and basket strainers at the downspouts help catch leaves and other debris. Gretchen Rupp, director of the Montana Water Center at Montana State University, recommends providing a downspout for each 50 feet of gutter run.
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