Toward Wiser Water Strategies

To lessen the burden on increasingly constrained fresh-water supplies, designers and building owners turn to alternative sources
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Alex Wilson

Rainwater Harvesting

While rainwater harvesting systems provide the sole source of water for buildings in some locations-in Volcano, Hawaii, and the U.S. Virgin Islands, for example-rainwater is more commonly used as a source of nonpotable water for landscape irrigation, cooling tower makeup, and occasionally for toilet flushing.

At the Chesapeake Bay Foundation headquarters in Annapolis, Maryland, rainwater is harvested and used for fire suppres- sion, hand-washing, mop sinks, laundry, and other miscellaneous washing. The building has composting toilets that do not rely on water for flushing.

Photo courtesy Prakash Patel, SmithGroup

Rainwater cisterns at the Chesapeake Bay Foundation are made from salvaged pickle barrels.

Photo courtesy Alex Wilson

Very simple rainwater harvesting systems can consist of a rain barrel positioned beneath the eaves of a building with a downspout emptying into it, an overflow, and a spigot. More sophisticated systems, such as those typically installed on commercial buildings, sometimes include a first-flush system which diverts the first rain that falls during a storm, carries off accumulated particulate matter, and includes a coarse filter to keep out leaves and other detritus. Metal roofing is generally preferred because its smooth surface collects less organic matter and particulates than roofs with rougher textures, so less debris ends up in the stored water. Other components include a cistern large enough to serve expected water needs and, depending on intended uses, a treatment system for purifying the water.

At One Bryant Park, the LEED Platinum-registered Bank of America office building under construction in New York City, harvested rainwater will be stored high in the 55-story tower, enabling gravity-feed for its use in toilet flushing. Similarly, the Chesapeake Bay Foundation headquarters in Annapolis, Maryland, completed in 2000, has rainwater tanks-in this case made from salvaged pickle barrels-on its roof. Rainwater stored on upper levels of a building can also serve as supply for fire suppression, though doing so increases the required size of cisterns and limits the amount that can be drawn down for other uses.

It is also possible to harvest rainwater at ground level by channeling stormwater from parking lots or landscaped swales into retention ponds, from which water can be drawn. Heifer International's headquarters in Little Rock, Arkansas (GreenSource, January 2007, page 47), harvests rainwater for landscape irrigation and for use in its innovative cooling system.

Capturing Air-Conditioning Condensate

In much of the country, air-conditioning equipment in commercial buildings generates large volumes of condensate. Standard practice is to send this down the drain into wastewater lines, but increasingly designers and building owners are treating condensate as a valuable resource that can be harvested.

At Heifer International head- quarters building in Little Rock, Arkansas, captured stormwater is used for landscape irrigation and cooling.

Photo courtesy Tim Hursley

The quantity of condensate that can be recovered depends on the outdoor and indoor conditions-both temperature and relative humidity-and the quantity of outdoor air that is provided. BuildingGreen, McGraw-Hill Construction's partner in the publication ofGreenSource, has anonline calculator that can be used to estimate condensate production in buildings based on these variables. In a hot, humid climate like San Antonio, Texas, considerable condensate is produced. According to the San Antonio Water System, the San Antonio Public Library produces about one gallon per minute, or 1,400 gallons per day. Condensate recovery also makes sense in cities like Washington, D.C., Philadelphia, New York, and Chicago, where high humidity coincides with the hottest summer temperatures and greatest cooling loads.

Air-conditioning condensate can be piped directly into cooling towers as makeup water because the condensate will never exceed the amount of water that evaporates from the cooling tower. This water can also be stored in a cistern and drawn off for a variety of applications, including cooling-tower makeup water, landscape irrigation, and toilet flushing.

In some buildings, combining air-conditioning condensate with other nonpotable-water sources makes a lot of sense, especially in regions with erratic rainfall patterns. The combination of rainwater harvesting and air-conditioning condensate recovery is referred to as "rainwater plus" by San Antonio water utility officials. "The combination is great," says Karen Guz, director of the Conservation Department for the San Antonio Water System. "A stand-alone rainwater system would require an enormous, expensive tank in order to ensure supply during long periods without rain," she says. But because condensate production is fairly steady and increases as the weather gets hotter, smaller storage tanks are possible with combined systems.

Because air-conditioning condensate is essentially distilled water, it is very pure when first produced, but bacteria and other contaminants can collect in the piping and storage vessels, therefore some form of treatment is typically provided-even for nonpotable uses. Chemical treatment, typically with chlorine, is most common, but UV treatment is also used.

 

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Originally published in GreenSource
Originally published in July 2008

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