October 2012
8-9
Reject water is typically sent directly to the sanitary sewer, although it is often suit-
able for use in other onsite applications. As long as sanitary conditions are main-
tained for storage and transfer, reject water can be appropriate for end uses requiring
higher water quality, including: toilet and urinal flushing; cooling tower make-up
water; above-ground irrigation; make-up water for decorative ponds, fountains, and
waterfalls; or other processes or uses not requiring potable water.
20
If used for irriga-
tion water, it should only be applied to plants with high salinity tolerances, due to
elevated levels of TDS. In addition, if this water is to be used as cooling tower make-
up water, compare the TDS concentration in the source to the cooling tower TDS set
point, to make sure that it provides a benefit as make-up water to the system.
Cooling Equipment Blowdown
As water is evaporated from cooling equipment, the concentration of TDS builds up.
If left undiluted, the TDS can cause scaling on equipment surfaces. As a result, some
of the water remaining in the cooling equipment must be periodically blown down
and replaced with make-up water. Cooling equipment that requires blowdown can
include cooling towers, evaporative air condensers, evaporative coolers, and evapo-
rative cooled air conditioners.
21
Although cooling equipment blowdown is typically discharged to the sanitary sewer,
it is often of sufficient quality to be used in other onsite applications such as irriga-
tion. It should be noted that the TDS content is significantly higher than that of the
original source water, often by two to five times. In addition, the water could con-
tain algae, bacteria, or pathogens and water treatment chemicals, such as biocides
or corrosion inhibitors. For these reasons, this water should never be used where it
can come into contact with humans. In addition, if the cooling equipment is using
water very efficiently, the TDS content could be too high for use in irrigation, unless
it is diluted with water from another source.
22
Blowdown could be treated through
nanofiltration or RO to make it suitable for other uses, particularly for recycling as
make-up water for the cooling equipment. Facility managers should carefully assess
the possible impacts of using this water on equipment, fixtures, or plants.
Additional Resources
Alliance for Water Efficiency (AWE). Blow-Down Water Introduction.
.
AWE. Condensate Water Introduction.
.
AWE. Graywater Introduction.
.
AWE. RO Discharge Water Introduction.
.
20
Ibid
.
21
AWE. Blow-Down Water Introduction.
.
22
Ibid
.
8.
Onsite Alternative Water Sources
