Chemical Engineering August 2014 - 57

Solids Processing
Environmental Manager
Cooling Towers: Managing Tighter
Water-Discharge Regulations
Tightening regulations
for cooling tower waterdischarge
quality are
requiring plant engineers
to evaluate enhanced
treatment options,
sometimes including
zero-liquid-discharge
systems
Brad Buecker
Kiewit Power Engineers
H
istorically, many large industrial
facilities, including
power plants, have relied
on once-through cooling, in
which the entire cooling water volume
flows through the plant heat exchangers
and then is discharged to the original
source. However, the U.S. Environmental
Protection Agency (EPA; Washington,
D.C.) has, for over a decade, been developing
regulations to protect aquatic life
from impingement and entrapment at
once-through cooling intakes. This development
has essentially eliminated
once-through cooling as an option at new
plants. Rather, cooling towers now tend
to be the preferred choice, with air-cooled
condensers increasing in popularity.
Cooling tower blowdown
Subsequent to the passage of the
Clean Water Act in the late 1960s,
EPA began controlling industrial
plant wastewater discharges per
the National Pollutant Discharge
Elimination System (NPDES)
guidelines. In many cases, NPDES
guidelines focused on a small core
of primary impurities in wastewater
discharge streams. The two most
common for cooling water were pH
(typical pH control range of 6.0 to
9.0) and residual oxidizing biocide
at a common limit of 0.2 parts per
million (PPM). These guidelines, or
perhaps even more stringent limits,
are still in place at virtually all
facilities, but the EPA is currently
preparing new national guidelines
that will place limits on additional
constituents. These new effluentlimitation
guidelines, expected to
be finalized in 2015, will include the
heavy metals, chromium and zinc,
with projected limits of 0.2 and 1.0
ppm, respectively.
However, the story does not end
there. Individual states are allowed
to develop their own discharge
guidelines, as long as they are as
stringent as those issued by the
EPA. In many cases, states are promulgating
tighter regulations that
may place limits on some or all of
the following additional constituents
that are commonly found in
cooling tower discharge:
* Total dissolved solids (TDS)
* Sulfate
* Zinc
* Copper
* Chromium
* Phosphate
* Ammonia
* Quantity of discharge
Concurrently, regulators recognize
water as an increasingly scarce resource
and are requiring some facilities
to use alternatives to fresh
water as cooling-tower makeup.
Potential alternative sources include
reclaim water from municipal
wastewater-treatment plants
and low-purity (untreated) groundwater.
(This trend is particularly
evident in California, where water
rationing is becoming increasingly
important). In the case of reclaim
water ammonia, phosphorus, and
organic material are often problematic
constituents, and thus the cooling-tower
makeup might require
pre-treatment, such as ammonia
stripping and phosphate precipitation,
and organics removal by clarification.
As
an example of the impact of
ChemiCal engineering www.Che.Com august 2014 57
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Chemical Engineering August 2014

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