February 2013

Volume 22, Number 2

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Green Topics

Water Budgets: A Holistic Look at Efficiency

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Illustration: Peter Harris

Have we been focusing on the wrong water-efficiency measures all along? Even though low-flow and low-flush fixtures have become common over the years, fewer project teams have been aware of the water savings that could be realized by looking at cooling towers, commercial kitchen equipment, medical equipment, and even municipal steam systems. The water consumption from these can dwarf that of bathroom fixtures.

Fortunately, there are tools available to help look holistically at how to get the biggest—and most cost-effective—water savings in a building. A “water budget” is a framework that has long been used to look at the relationship of input and output of water flows in a region. By looking at the interplay of precipitation and evapotranspiration, planners and hydrologists can understand when an area has a water deficit or surplus.

Applying the water budget concept at the building level is similar but puts a focus on identifying and quantifying water uses in plumbing fixtures, irrigation, mechanical systems, and any processes in the facility—and in turn identifying possible water supply sources other than the conventional municipal source or onsite well that might be used to meet some of those needs. Those might include collected rainwater, mechanical system condensate, graywater, or treated wastewater.

If done at a time when a lot of options are on the table, such as early in design, the water budget exercise can illuminate water conservation opportunities that might otherwise go unnoticed. For example, the water use represented by a cooling tower can often come as a surprise; members of the project team could be debating specifying 1.0 versus 1.2 gallon-per-flush toilets while not being aware that the cooling tower planned by the mechanical engineer will consume tens of millions of gallons per year—a lot of toilet flushes.

By talking about these and other uses in the same conversation (likely as part of an integrative design process), a team might explore opportunities to reduce or reuse cooling tower blowdown water; use another mechanical system, paired with energy-efficiency measures, to reduce the size of the cooling tower (thus making energy efficiency gains as a water conservation effort—a dual “watergy” approach); or look at rainwater catchment to provide cooling tower water while reducing stormwater runoff.

The water budget can also be used to look at landscape irrigation, aiding landscape design based on an amount of water appropriate for the region and the site. There is even a tool that does most of this work for you: using a project’s zip code, the WaterSense Water Budget Tool automatically looks up rainfall and evapotranspiration data and then provides a menu of appropriate plantings and irrigation technologies for each landscape zone. Although developed for single-family homes, the WaterSense tool works for landscapes around other building types and is referenced in LEED v4’s proposed irrigation efficiency credit.

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January 28, 2013