POWER March 2017 - 36

WATER & WASTEWATER
1. Sideways. This process schematic shows how one plant simplified its reclaimed water
treatment process and eliminated side stream softening. Courtesy: GE
Treatment Approaches
A cooling program that utilizes reclaimed
water as a makeup water supply must be resilient
against the inherent risks and able to
cope with the extreme variability of the water
chemistry. Facilities also need to be prepared
to adapt to a stressed water condition and
implement a robust treatment approach that
includes close monitoring and an effective
control strategy that takes into consideration
the unique contaminant challenges of the reclaimed
water supply as well as the specific
materials of construction used in the cooling
water system.
To address water hardness, a softening
process can be utilized to precipitate species
such as phosphate, calcium, magnesium, and
silica in clarifiers. If phosphate levels need
to be further lowered in secondary-treated reclaimed
water, a biological phosphorous removal
process can be installed. When excess
nutrients-such as ammonia nitrogen-become
a concern, an advanced biological nitrification
process can be added to remove the
ammonia-nitrogen by oxidizing it to nitratenitrogen.
While these supplemental processes
can generate adequate water, they also add
significant capital and operating costs.
Likewise, side-stream processing of recirculating
cooling water is able to successfully
remove contaminants from cooling water and
can be accomplished through processes such as
membrane filtration (such as reverse osmosis)
or by using softening (such as cold lime softening
using clarifiers). However, these processes
are also very capital intensive and carry high
operating expenses because of requirements for
ancillary unit operations such as dewatering,
solids management, or brine disposal.
36
Chemical
treatment
programs
represent
another option for dealing with highly
stressed reclaimed water and protecting cooling
systems from corrosion and deposition.
Such an approach can allow power plants to
utilize low-quality waters-including those
with elevated phosphate levels-without the
need for costly operations, such as additional
makeup water softening and side stream
softening. Moreover, in addition to eliminating
the investment that would otherwise
be required for installing additional water
pretreatment/treatment processes, chemical
treatment programs also offer supplementary
benefits related to more streamlined and simplified
operations, reducing operating costs
further. An example of a successful chemical
treatment program for addressing high-phosphate,
secondary-treated reclaimed water is
provided next.
Using a Terpolymer to Treat High
Phosphate Levels
At a 1,800-MW combined cycle natural
gas power plant in the southern U.S., a
novel stress-tolerant terpolymer (STP) was
employed to prevent calcium phosphate
deposition in a cooling tower system that
uses highly stressed reclaimed water as a
makeup water source. The use of STP was
able to successfully eliminate a capital- and
operational-intensive side stream softening
process that was previously used to treat recirculating
cooling water.
Two mechanical draft-cooling towers-each
of which comprises 11 cells, a high-efficient
film fill, and a recirculation rate of approximately
212,000 gallons per minute-are utilized
at the plant to meet cooling needs. A local
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municipal wastewater treatment facility supplies
the plant with reclaimed water treated to a
secondary level, which accounts for the entirety
of the plant's makeup water supply.
The reclaimed water contains an average
13 ppm ortho-phosphate (o-PO4) with a historical
maximum of nearly 30 ppm o-PO4.
Side stream softening units were originally
installed in the cooling loop to reduce o-PO4,
calcium, and magnesium concentrations so
that the cooling tower could operate at seven
cycles with o-PO4 controlled at about 25
ppm, calcium hardness at about 500 ppm as
calcium carbonate, and pH at around 7.
However, operation of the side stream
softening process was expensive, as lime,
soda ash, ferric chloride, and polymer had to
be added in order to reduce hardness. Acid
was also required at the effluent to control
post precipitation. The softening units also
required ancillary unit operations including
filtration, which demanded constant attention,
and the solids that were generated required
hauling for disposal.
In 2014, it was proposed that the plant
could operate the cooling water systems by
using STP and removing the side stream
softening units (Figure 1). This approach
promised significant operating expense reductions,
greater operational simplicity, and
better safety. Additionally, by eliminating
the need for sludge disposal, costs could be
reduced while improving the plant's environmental
footprint.
Without the softening process, it was
clear the plant would face challenges related
to the high concentration of phosphate, calcium
hardness, and particulates present in
the secondary-treated reclaimed water. The
elevated phosphate concentration presented
the extremely high risk of calcium phosphate
deposition on condensers and tower fill, and
pH excursions at such high concentrations
could cause even more severe condenser deposition
issues.
To evaluate the feasibility of using the
STP, and enabling for cooling tower operation
without the side stream softening process,
comprehensive studies were performed
addressing the inherent operational challenges.
As part of these efforts, a mitigation plan
was developed for high pH excursions so that
deposition could be effectively prevented on
the condensers in the event of a loss of acid
feed.
A laboratory evaluation demonstrated superior
performance of the STP for treating
worst-case scenario phosphate concentration
water, thus proving the feasibility to operate
the cooling tower without a softening process
under such conditions. Next, a test conducted
in a pilot evaporative research tower employing
the actual reclaimed water used at the
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POWER March 2017

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