June 12, 2017 0 Comments
Water consumption is an area of building operations in which facility managers can make a big difference. According to the Building Owners and Managers Association (BOMA) International, commercial office buildings alone use one-sixth of the world’s fresh water supply. It is incumbent on those who manage water usage to employ methods to reuse it whenever possible.
One method is to use reclaimed water—that which is previously used and has gone through advanced treatment—in cooling tower systems. These systems are large consumers of water for a facility, with approximately three gallons of water per minute needed for each ton of refrigeration the system must provide.
There is also the waste factor to consider. “Generally speaking, about 1% of that water is going to evaporate,” says Mark Hodgson, director, environmental air quality at Clayton Services Group in Edison, NJ. “Bearing in mind that larger buildings use thousands of tons of refrigeration per day, there are thousands of gallons of water recirculating and hundreds of gallons evaporating every day.”
Gray water, defined by the U.S. Environmental Protection Agency (EPA) as “wastewater composed of wash water from kitchen, bathroom, and laundry sinks, tubs, and washers” can be used for a number of secondary purposes, including water for cooling systems. Gray water can also be defined as any that is not suitable for drinking. This can include other types of used water, such as that from toilet flushing.
Since the water used in cooling towers does not need to be of drinking quality, with the proper level of treatment, reclaimed water can meet the needs of those systems without constantly drawing on the fresh supply. Instead, a wastewater treatment plant takes used water in, treats it, and sends it out to a facility.
The Orange Water and Sewer Authority (OWASA) in Carrboro, NC will begin providing this service to the University of North Carolina (UNC) in 2007. The university will initially use reclaimed water in cooling towers on its main campus. The 2007-2008 average day demand is estimated at 515,000 gallons per day.
Prompting the project was a severe drought in the region during 2001 and 2002. In order to address future droughts, OWASA and UNC conducted a joint study, with the help of an engineering firm, to evaluate the feasibility of using reclaimed water. After technical and health issues were addressed, it was determined such a system would be beneficial.
Margaret Holton, P.E., water, wastewater, and stormwater manager with UNC Energy Services, says, “The cooling towers were chosen [for this project] because they use large amounts of water throughout the year. Also, the peak usage coincides with the summer peak demand community-wide.”
The $15 million project is partially funded through state and federal grants totalling $2.5 million. Because UNC will initially be the sole customer, it is paying the remainder of the project costs. About 14,200 feet of reclaimed water distribution pipes are currently being installed from an OWASA treatment plant to the UNC campus.
The American Society of Heating, Refrigerating and Air-Conditioning Engineers (ASHRAE) is currently exploring this type of water reuse for cooling towers. A forum entitled, “What Do You Need To Know About Gray Water Before It Can Be Used in HVAC Cooling Systems?” was held this past June at the ASHRAE annual meeting to focus on the potential of tertiary treated municipal effluent.
Hodgson, who chaired the forum, says, “Cooling towers were discussed as one aspect, since the towers are probably the largest single consumer of water in a building system.
“While cooling towers don’t require drinkable quality water,” he continues, “in most instances, such as office buildings, that is the only available supply.” This is largely because the infrastructure to transport the treated water to a facility is not in place in most municipal systems.
In the forum, issues on the use of this type of effluent in cooling towers included health safeguards, the current design of the towers, and the infrastructure requirements.
“The primary concern that most engineers face is health and safety,” notes Hodgson. “There must be a certain amount of care taken and a certain amount of additional safety precaution for water treatment. However, the water treatment industry has always expressed a great deal of confidence that it knows how to treat this [effluent].”
With regard to the design of cooling towers themselves, attendees of the ASHRAE forum zeroed in on two issues. “This water tends to have a higher level of suspended solids than city water,” says Hodgson. “Those suspended solids may precipitate out onto surfaces and reduce heat transfer. This would restrict flow within the system, which would present an energy penalty.” This means more energy would need to be expended to maintain the proper flow of water through the cooling system.
“There was also some concern as to whether the metallurgy of existing chiller systems is sufficiently robust to withstand what could be a somewhat more harsh environment [from the effluent],” says Hodgson. “It increases the risk of corrosion.”
In their study, OWASA and UNC addressed the issue of corrosion with pilot scale testing to confirm the suitability of the reclaimed water for use in the cooling towers. In addition, chemical treatment will be needed to minimize scale formation, corrosion, pitting, and biofouling.
Hodgson notes that infrastructure can be an obstacle for existing buildings. It is possible, however, to retrofit. For new facilities, infrastructure can be included in original construction, as with the UNC project.
With the proper amount of research, facility managers can determine if this water saving practice fits into their buildings.
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