Water Reuse Trends
By: Rod Yeoh, P.Eng., P.E., LEED AP BD & C, Principal, Dialog
The LEED Platinum-certified University of Calgary Energy Environmental Experiential Learning Project collects rainwater, reused river water and grey water and recycles it in several different ways. Grey water from restroom sinks is used to dilute acid wastes from laboratories; river water used to cool condensers is collected in a rainwater storage tank and reused by the building’s cooling tower. Photo courtesy of Dialog
Many commercial real estate professionals are starting to realize that water is a precious resource and that we have to treat it as such. Those of us who design and build buildings need to reconsider “the way we’ve always done things” and stop thinking of water as an endless resource that we can continue to use wastefully.
While rainwater (also known as stormwater) is currently the most common source of recycled water, many other sources of water also can be reused. These include condensate from cooling as well as grey and black water. (For more on stormwater recycling, see “Better Stormwater Management for a Better Bottom Line” in the spring 2014 issue of Development.) This article focuses on the less well-known recycling of condensate, grey water and black water.
Recovering and Reusing Condensate
Almost all modern commercial and institutional buildings are mechanically cooled in some way. During the process of cooling the air, a significant amount of moisture is condensed and typically is just discharged to a stormwater or sanitary drain. This water is usually quite clean and can be reused without much, if any, treatment.
Owners of buildings in hot, humid, climates such as Eastern Canada or the Southeastern U.S. should consider cooling condensate recovery. An article by Thomas Lawrence, Jason Perry and Tyler Alsen of the University of Georgia in the May 2012 ASHRAE Journal presents their study of the economics of cooling condensate recovery in 47 U.S. cities. They evaluated cooling condensate recovery potential and rates along with the cost of water in these cities, and found that simple payback periods ranged from one year in Miami to 1,113 years in Spokane, Washington.
Obviously, the more condensate that can be recovered and the higher the cost of water, the shorter the payback time will be. Despite this large range of payback periods, the study identified 18 “first tier” cities as obvious locations for condensate collection systems — including Dallas, Honolulu, New York, Oklahoma City, San Antonio, St. Louis and Washington — with simple payback periods of nine years or less. As the cost of water increases, cooling condensate recovery will become a more viable option.
Recycling Grey and Black Water
Grey water generally is defined as the discharge from domestic appliances such as sinks, bathtubs and showers, washing machines and other washing functions. In a commercial or industrial building, the definition of grey water can extend to discharge from commercial washing processes such as vehicle washing. Black water is the discharge from toilets and urinals, and typically is characterized by the presence of organic materials and bacteria; in most cases, it is basically raw sewage. (Wastewater from kitchens sometimes is classified as black water because of its organic loading.) Depending on the proposed use, grey water can be reused with minimal treatment, while black water almost always must be heavily treated to be reused.
The most common uses for recycled grey water include toilet flushing, washing, and irrigation. In fact, the nitrogen and phosphorus often contained in grey water can act as nutrients for irrigated plants.
Two types of black water treatment are currently available on a commercial scale: packaged active mechanical systems and passive systems that filter the black water through engineered wetlands. Passive systems require a large amount of land, whereas mechanical systems can be installed in much smaller areas but require more energy to run. Both types of systems can be quite costly to install.
The main challenge in any water reuse system is to ensure that the system does not adversely affect the health and well-being of building occupants. Recycled water must be treated appropriately and clearly identified as nonpotable. Many pipe manufacturers now offer a special pipe that is designed and marked for recycled water use. This pipe is typically a different material and color than pipe used for domestic water, and is clearly marked as “non-potable.”
Another barrier to reuse is the cost of recycled water systems. In order to collect and reuse water, no matter what the source, large storage tanks, treatment systems, and additional piping need to be installed, sometimes at very high costs. (A stormwater collection and reuse system could cost on the order of $50,000.) As water becomes scarcer and more costly to treat and pump, and as the cost of city-supplied potable water increases, the economics of recycled water will improve and more systems will be installed.