POWER March 2017 - 37

WATER & WASTEWATER
2. Unfiltered. Use of the stress-tolerant terpolymer kept ortho-phosphate (o-PO4) in the
cooling tower within safe levels even without softening. Source: GE
120
100
80
60
40
20
3/1
o-PO4 (UF)
PO4 (F)
4/10 5/20 6/29 8/8
Date
Table 3. Operating cost analysis. Source: GE
Variable
Makeup water usage
Sulfuric acid (pH control)
Bleach (biological control)
Cooling water treatment (specialty) chemicals
Dechlorination
Cold lime softening chemicals
Lime
Soda ash
Ferric
Coagulant
Hydrochloric acid
Sludge handling and disposal
Labor
Maintenance
Operating cost savings per year
plant showed that the designed program met
all requirements for treating the reclaimed
water without the side stream process.
Following these evaluations, a nine-month
full-scale trial was completed in one cooling
system using the STP while the other continued
to operate as normal. The goal of the
treatment program was to maintain equipment
cleanliness and optimal performance.
Bleach was added to control microbiological
growth, and non-oxidant biocide and biodispersant
were fed periodically to maintain
surface cleanliness of the tower fill and condensers.
Controlling pH within the designed
range of 6.4-7.0 was a critical operating parameter
to monitor, as high pH excursions
without the softening process would significantly
increase the chance of calcium phosphate
deposition.
Guidelines were prepared for managing
the plant's cooling process without side
March 2017 | POWER
9/17
10/27
12/6
Change
Slightly higher
Same
Same
Higher
Higher
Eliminated
Eliminated
Eliminated
Eliminated
Eliminated
Eliminated
Eliminated
Eliminated
$750,000-$1,000,000
stream softening, and key performance indicators-including
condenser
backpressure,
condenser cleanliness, and water flow
through plate and frame heat exchangers-
were monitored online on an hourly basis.
Basic wet chemistry tests on makeup and
tower water were routinely performed, and
full water analyses were conducted three
times per week. Microbiological activity was
monitored, and data related to water quality,
plant operation, and condenser performance
was uploaded to an online data management
system for further analysis and calculations.
Results of the full-scale trial demonstrated
that the STP treatment program maintained
the cooling system in excellent condition.
The o-PO4 in the makeup water cycled to
100% of what would be expected based on
hydraulic cycles, indicating that the STP
polymer was able to keep PO4 soluble even
when the tower's PO4 level reached 100 ppm.
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A delta PO4 target of less than 3 ppm was
maintained with the average delta PO4 at approximately
1.2 ppm (Figure 2). Excellent
deposition inhibition performance was also
maintained throughout the trial with no signs
of any scaling issues. Discharge pH levels
did not exceed permit requirements, and at
the end of the nine-month trial, a visual inspection
and data review concluded that no
condenser scaling had occurred.
Removing the side stream softening process
simplified the cooling system's operation
by eliminating chemicals and the need
for sludge disposal. Labor and maintenance
associated with softening were also eliminated.
The resulting annual overall net operational
savings were calculated to be $750,000
to $1,000,000 (Table 3).
Following the trial period, the plant continued
with the STP treatment, and the second
cooling system was converted to the new
treatment program six months later, shutting
down the other side stream softening process.
Savings and Reduced Complexity
By employing STP, such low-quality stressed
water (with variable and high phosphate concentrations)
may be treated without the added
capital expenditure for softening. In these
cases, the use of STP can lead to a capital cost
avoidance for applicable new construction facilities
as well as facilities that are considering
expanding but do not have access to additional
freshwater supplies. For example, for the power
plant discussed in this paper, it is estimated
that the associated installed capital cost for the
cold lime softening process-including clarifiers,
belt presses, and ancillary equipment-
was $5 million to $10 million.
Reclaimed water offers tremendous potential
as a viable and long-term supply option
for power plant cooling applications. But as
with any makeup water supply alternative, a
range of built-in risks and challenges exist
that must be addressed to achieve a successful
implementation.
Several treatment solutions are available
for managing the contaminant challenges associated
with reclaimed water. As shown, the
proper cooling tower treatment-including
dosage, delivery, and control-can lead to the
ability to treat harsh waters with elevated and
varying phosphate levels without the need to
invest in costly and more complicated processes.
Moreover, in more overarching terms,
a wider array of harsh reclaimed waters may
indeed prove to be viable for use as cooling
tower makeup water. ■
-Robert A. Hendel is a senior engineer-
cooling, Caroline Sui is a senior research
scientist, and Jeffrey Melzer is a water
services technology leader with GE Water
& Process Technologies.
37
o-PO4 (ppm)
http://www.powermag.com
POWER March 2017

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