Save Water While Quenching Cooling Towers
With proper infrastructure and treatment in place,
reusing water can be a viable option for a facility.
By Anne Vazquez
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.”
[For more on cooling tower corrosion, see “The
Material Revolution” September 2005.]
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.
Information for this article was provided, in part,
through interviews with Hodgson and Holton. To find
out more about water reuse, visit the U.S. EPA (www.epa.gov/water).
To learn more about the OWASA/UNC project, visit www.owasa.org/pages/WaterReuse/questionsandanswers.html.
Do you use gray water at your facility?
E-mail your thoughts to avazquez@groupc.com.
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