Power August 2023 - 30

WATER & WASTEWATER TREATMENT
A Model for ZLD Treatment
of Blowdown from Cooling
Towers-Even the Big Ones
Pilot testing under the U.S. Bureau of Reclamation's Pitch-to-Pilot desalination
research funding proved an electro-coagulation, clarification, micro-filtration, reverse
osmosis with vacuum-assisted electro-distillation concentrate treatment process
train can recover the saline cooling tower blowdown in a zero-liquid discharge (ZLD)
configuration, preventing high-salinity wastewater from entering the sewer-shed.
Eric Dole, PE
recious water supplies are easily
contaminated with salt water, but
removing the salt isn't. As shown
in Figure 1, salt comes from a variety of
sources. Understanding the source in a
watershed sets the stage for salinity management.
Often there are best management
practices that can reduce salt loading,
so treatment intensity can be reduced.
Power plant cooling towers (CTs) are
sometimes a major salt contributor to
watersheds from CT blowdown. Many industrial
and commercial cooling systems
utilize evaporative CTs, especially in arid
regions where evaporative cooling has
the highest cooling efficiency and water
supply is scarce. According to the Central
Arizona Salinity Study, surveys conducted
by the City of Phoenix Water Conservation
Department indicate that manufacturers
use between 25% and 50% of
their total water use for cooling.
Cooling towers are typically operated
between three and four cycles of concentration
(COC), which is how many times
the water can be recycled before causing
operational issues with scaling. Hence,
the total dissolved solids (TDS) concenP
tration
entering the sewer from the blowdown
is three to four times higher than
the source water salinity level.
Seeing how critical CT blowdown
management is to watershed health, especially
in the era of indirect and direct
potable reuse, Garver saw this as a perfect
research opportunity. In 2019, Garver's
Denver office proposed researching
an innovative process train to recover
the saline blowdown from commercial
and industrial CTs under the U.S. Bureau
of Reclamation's (BOR's) Pitch-to-Pilot
(P2P) desalination research funding.
Garver was one of seven teams awarded
part of the $1.5 million P2P grant in
November 2019 aimed at researching
disruptive desalination technologies that
consumed less power than what was offered
on the market at the time. Garver
teamed with several entities that played
a significant role in project success,
such as Red Rocks Community College
(RRCC), Dr. Mike Mickley, and other industry
partners. Collectively, those partners
donated more than $480,000 of
in-kind equipment, instrumentation, and
labor on this $200,000 research project.
Pilot testing concluded in late November
2021, and a draft report was issued
to the BOR in June 2022. Currently, the
report is still in review status; however,
the P2P trailer has been operating in failure
mode analysis (FMA) for more than
1.5 years now. FMA results will be presented
at the end of this article.
The Solution
Garver proposed researching a 1.5-gallonper-minute
electro-coagulation (EC), clarification
(CLAR), micro-filtration (MF), reverse
osmosis (RO) with vacuum-assisted electro-distillation
(VAED) concentrate treatment
process train to recover the saline CT
blowdown in a zero-liquid discharge (ZLD)
configuration, preventing the high-salinity
wastewater from entering the sewer-shed.
Figure 2 shows a process flow schematic
of the treatment train and how it was integrated
into the existing CT system.
Water quality of the treated permeate
and distillate was analyzed to determine
if it could be repurposed for CT makeup
dilution water to balance water chemistry.
The salt slurry from the VAED system
was evaluated to see if it was pure
enough to use as a feedstock for onsite
sodium hypochlorite generation to use in
the CT and promote a circular economy.
The goal of this project was to prove
that an EC/CLAR/MF/RO with VAED concentrate
treatment system could be effective
in treating high-TDS commercial
CT blowdown from the 600-ton evaporative
cooling system located at RRCC's
Main Campus Building in Lakewood, Colorado.
The RRCC CT operated at three to
four cycles of concentration.
1. Watershed salt loading. Courtesy: Garver USA
30
www.powermag.com
The Research
As shown in Figure 2, the purpose of EC
is to precipitate the constituents in the CT
blowdown that could foul the CT and the
RO membranes used to treat the blowPOWER
| August 2023
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Power August 2023

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