ASHRAE Journal - October 2013 - 30

Technical FeaTure

Cooling Tower Blowdown

AC Condensate

or into the ground as irrigation.
Figure 1 Membrane bio-reactor sewage treatment plant flow diagram.
Dissolved solids entering the buildSolids
ing water stream, either in the
Aeration Zone
Membrane
Grid
Pre-Treatment
Removal
incoming potable water, or added
to the water as part of the building
Effluent
Wastewater
function leave the building only
through the sewage discharge and
Air
irrigation. As noted previously, flow
Sludge
reduction of potable water through
conservation results in higher levels
of TDS in sewage discharge.
Figure 2 Flow diagram for low TDS cooling tower makeup.
Local packaged sewage treatment plants have been
Cooling
Potable Water
Storm Water
Cooling Tower
Tower
installed to provide recycled water or non-potable uses.
Evaporation
Sewer Outflow
Pure Non-Potable Water
Makeup
Grey Water
Brackish Water
Typical space-conserving sewage treatment technologies
Roof
Cooling Tower
include membrane bio-reactors and multi-reactor-phytoremediation systems. Sewage is subjected to tertiary
treatment in the building and is then recycled for specific non-human contact end-uses, including irrigation
and cooling tower makeup.
Water
Typical sewage treatment plant technologies, while
Packaged
effective at removing organic contaminants and parSewage
Treatment
High TDS Low TDS
ticulates from wastewater, remove very little of the
Plant
Water Irrigation
Water
dissolved inorganic contaminants. Some “natural”
Storage
Storage
Sewer
wastewater treatment protocols use halophytic plant
Outflow
species to absorb and concentrate inorganic solutes,
TDS wastewater to the municipal sewage stream may
but their biomass must be regularly harvested to main5 Space
be unacceptable or unlawful because of the impact
tain the desired removal rate of dissolved solids.
requirements and maintenance costs for this technology of the treated sewage effluent on TDS in rivers and
make it unsuitable for most building applications. The
streams.
most common packaged sewage treatment technology,
The third option uses treated sewage effluent for coolbecause of its compact size, reliability and low mainteing tower makeup, but relies on a structured water
nance requirements, is the membrane bio-reactor, illus- treatment protocol to deal with issues of corrosion, scaltrated in Figure 1.
ing and biological growth.

Dealing with High TDS in Building Wastewater

Low TDS Cooling Tower Makeup

Three strategies were identified for using high TDS
wastewater to achieve extraordinary reductions in
potable water use. All strategies use local wastewater
treatment and harvest the treated sewage effluent
(TSE) for non-potable uses, but differ in how that effluent is used.
The first strategy recycles treated sewage effluent for
flushing and discharges the remainder as a mediumhigh TDS stream to the city sewer system. The second
option uses reverse osmosis-purified locally treated
sewage for both cooling tower makeup and irrigation,
requiring discharge of a very concentrated reverse
osmosis brine to a suitable sink. Discharge of high

The first strategy reserves HVAC condensate and harvested storm water for cooling tower makeup with the
blowdown rate controlled to maintain conventional
upper limits of basin TDS. A portion of the sewage water
generated by the building is treated and the effluent
used for flushing while the remainder is discharged to
the municipal sewer.
This strategy is appropriate to humid or high-rainfall
climates, where condensate and storm water recovery
can meet the preponderance of cooling tower makeup
and irrigation requirements. For these projects, storm
water recovered from the roof, typically, would be
reserved for cooling tower makeup because of its relative

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ASHRAE Journal - October 2013

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