Harvesting Rain: System Design for Strategic Rainwater Capture

Conserving water through rainwater harvesting saves natural resources, providing water for use in buildings and for site irrigation
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Sponsored by Watts Water Technologies, Inc.
Celeste Allen Novak, AIA, LEED AP

Storage Capacity

After determining the potential for rainfall capture, the design professional can begin to calculate storage capacity. As seen in the table from the Georgia Rain Water Harvesting Guidelines, the total storage in gallons will vary according to the amount of rain and size of the storage tank. The storage tank may need an initial deposit if installed during a dry season. Therefore, it is best that a cistern be installed as early as possible during the building process to take advantage of rain events. Demand and supply will vary month by month based on the amount of rainfall received into the system as well as the users of that system.

Architects can increase the catchment surface area with awnings or canopies as well as increasing the footprint of a building. Catchment surfaces can also include rooftop solar or hot water photovoltaic panels. Based on the amount of rain and the demand for water, rainwater systems are designed to meet performance targets as considering budget restraints.

Rainfall (in.) Area (Sq. Ft.) X Gallons/Sq. Ft. Total Gallons
1 2,200 0.62 1,364.00
5 2,200 0.62 6,820.00
10 2,200 0.62 13,640.00
40 2,200 0.62 54,560.00
50 2,200 0.62 68,200.00
1 3,500 0.62 2,170.00
5 3,500 0.62 10,850.00
10 3,500 0.62 21,700.00
40 3,500 0.62 86,800.00
50 3,500 0.62 108,500.00
1 5,000 0.62 3,100.00
5 5,000 0.62 15,500.00
10 5,000 0.62 31,000.00
40 5,000 0.62 124,000.00
50 5,000 0.62 155,000.00
Table for rainwater potential collection from roof surfaces as shown in the Georgia Rain Water Harvesting Guidelines 2009

Estimating Demand

Rainwater systems can provide water exclusively for outdoor use or for a variety of uses in buildings or for both. According to the American Water Works Association (AWWA) Research Foundation, North American households use approximately 146,000 gallons of water annually. Of this amount, 42 percent is used indoors and the remaining 58 percent is used outdoors. By far the largest percentage of indoor water use occurs in the bathroom for toilet flushing (18.5 gal/person/day) and showering (11.6 gal/person/day). Clothes washers were the second largest water users (15 gal/person/day).7

A study by the North Carolina Department of Environment and Natural Resources calculated that commercial, industrial and institutional uses accounted for as much as 40 percent of total municipal water use. Facilities were using as much as 20 percent of the potable water for cooling and heating. Other intensive water uses included irrigation and restrooms.8

Water conservation and efficiency are two parts of the same demand equation. Architects and engineers should combine their specifications of energy efficient plumbing and water saving equipment along with an efficient rainwater harvesting system. WaterSense is a new EPA partnership program with manufacturers that identifies products, programs and practices that conserve water resources. Many WaterSense products also provide rebate programs in many areas of the United States. Sizing demand for water using a rainwater harvesting system includes the incorporation of best practices for water conservation. Rainwater used inside a building is typically filtered and treated to a higher standard than rainwater used outdoors for irrigation.

The climate zone, type of plants and outdoor activities like equipment washing determine the demand for outdoor water use. To reduce the demand for irrigation, many design professionals are choosing plants that are native to the climate zone or designing xeriscapes that require very little water. Rainwater that is collected from pavement surfaces should be separated from rainwater collected from roofs if the resulting water is used indoors. In general, rainwater collected from roofs is lower in contaminants than that from surface water.

The demand for indoor water use includes using water for flushing toilets, as well as for cooling tower make-up water. In some states, rainwater can also be used for clothes washing and drinking water. Water expert Peter Coombes has studied both the benefits and analyzed the health risks for the use of rainwater. His studies have shown that the quality of rainwater meets most governmental water standards for clean water. Properly collected and stored rainwater is ideal for showering/bathing, laundry, toilet flushing and drinking. Although widespread use of rainwater has been hindered by uncertainty over quality and the perceptions of health risk, more research is demonstrating that rainwater can be used for many building uses.9

Graphic courtesy of BRAE

Diagram of a residential above-ground rainwater system


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Originally published in June 2013