Harvest the Rain
(Page 7 of 8)
August/September 2003
Adapted from Environmental Building News
Concrete tanks are generally site-built using forms, though smaller pre-cast tanks are available. High-strength concrete (7,000 psi) is recommended for watertightness. Such a tank can be integrated into the basement, with the tank sides serving as the foundation walls. Tanks inside a building may serve a secondary function as thermal storage.
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Ferrocement tanks are made by spraying or plastering a cement mortar over a wire mesh form. Wall thicknesses as thin as an inch can be produced, depending on the materials and the skill of the contractor. Because cracks can develop, some maintenance and repairs can be expected. Ferrocement is potentially one of the least expensive cistern materials. Mortared stone was traditionally used in some areas for cisterns. Construction cost is high, but resulting cisterns can be highly attractive. Fiberglass tanks are available with FDA approval for potable water. They are relatively inexpensive, lightweight and available in large sizes (up to about 10,000 gallons).
Polyethylene tanks are available with FDA approval for potable water in sizes up to several thousand gallons. They are lightweight, easily moved, and can be used above or below ground (tanks designed for burial may be more expensive). Durable wood, such as redwood or cypress, also can be used for tanks. If properly built, such tanks are highly durable — often lasting 50 years or longer. Salvaged wood such as old wine vats and whiskey-aging casks may be available.
A polyethylene liner over a non-watertight frame usually provides the lowest-cost cistern option, though it also is the least permanent. Liners for cisterns should be 20 or 30 mils thick and made of a UV-stabilized, FDA approved material.
Calculating Catchment Capability
If you're using a roof to harvest rain, the size of your catchment area is the square footage of the building's footprint. Each square foot of collection area should yield 0.6 gallons of water per inch of rainfall, although some water will be lost to evaporation and leakage, which is factored in by an efficiency factor of 0.8.
To determine your roof's water-harvesting potential:
1. Multiply the square feet of collection area by 0.6 gallons (per square foot of area).
2. Take this total times 0.8 (the efficiency factor).
3. Multiply the total from Step 2 by your area's annual average rainfall (in inches). You can find the average precipitation in your area by visiting the National Oceanic and Atmospheric Administration. This data chart gives monthly and yearly averages of data collected for specific cities during the past 30 years.
For our sample calculation, we used Austin, Texas, which receives an annual average rainfall of 32 inches. From 2,500 square feet of collection area, about 38,400 gallons of water per year, or 105 gallons per day, can be harvested.
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