POWER May 2020 - 31

WASTEWATER TREATMENT
ash, and other chemicals to replace
the calcium and magnesium ions in the
wastewater with sodium ions so that a
crystalline solid can be produced and
ZLD can be achieved. To overcome this
situation, conventional thermal facilities
require coagulation/flocculation, clarification,
and/or filtration, which have considerable
operating costs because of
their intensive sludge dewatering and
disposal expenses.
When water vaporizes and salts
are further concentrated in conventional
thermal processes, the energy
to achieve ZLD increases because of
the higher boiling temperature to reach
the salts' solubility limit. As low scrubber
wastewater temperature drives a
reduction in the solubility and boiling
temperature of high-solubility salts, it
became apparent that an operation under
lower temperatures concentrates
the solution, while precipitating dissolved
solids to the point of crystal formation.
In addition, by operating at low
pressures under vacuum, the boiling
temperature of the solution is reduced
from that at atmospheric pressure and
a solid can be obtained at a relatively
low concentration.
Simple, low-temperature crystallization
processes such as Veolia's CoLD
(crystallization of high-solubility salts at
low temperature and deep vacuum) take
advantage of this (Figure 2). Through
the use of an energy-saving, closedcycle
heat pump, the system supplies
the heat required to boil the solution
and the cooling necessary to condense
the water vapor. The integration of a
solids-dewatering device such as a centrifuge
or a filter press maximizes crystal
recovery by optimizing the residual
moisture of the wet cake to achieve the
maximum waste volume reduction and
solids collection efficiency.
3. The CoLD process requires no pretreatment or drying, which saves owners money on
chemical and energy costs. Courtesy: Veolia Water Technologies
Less Risk, More Water
One of the largest coal plants west of
the Mississippi River has used unlined
surface ponds to store FGD wastewater
since the mid-1970s. The operator,
one of the largest privately held American
energy and power generation companies,
has decided to turn offline older
operating units, as utilities in the U.S.
Northwest seek to remove coal from
their power portfolios because of climate
change concerns.
To comply with environmental mandates
resulting from the partial retirement,
Veolia Water Technologies was
selected to design and supply a ZLD
system integrating a falling film evaporator
that will feed a concentrated brine
to a CoLD crystallizer to treat 500 million
gallons of FGD wastewater. The
falling film evaporator can recover up
to 90% of the water and keeps energy
consumption low thanks to the use
of electrical mechanical vapor compression
(MVC), which drives higher
pressure on the recovered vapors to
provide the heating source applied to
the circulating brine.
The system will reclaim FGD wastewater
and produce two outputs: a dry,
stable waste of crystalline solids for
landfill disposal and a high-purity distillate
with very low dissolved solids, sodium,
chloride, sulfate, hardness, and
silica. The clean water will be used for
various processes within the power
plant, minimizing withdrawals of surface
freshwater.
Proven and Reliable Systems
For many years, scrubbers have been
the best technology to keep the emissions
of coal plants under control.
Today, low-temperature evaporation
systems are the best technology to
treat the FGD wastewaters collected in
those scrubbers.
Technologies such as Veolia's CoLD
provide effective treatment at lower
capital and operational expenses than
those of conventional thermal ZLD options
(Figure 3). Lifecycle costs are reduced
because pretreatment steps are
not necessary, and chemical use and
sludge generation are minimized. Further,
the use of a heat pump eliminates
the need for steam and keeps low energy
requirements to evaporate water.
Despite their recent downward trajectory
and challenging economic viability,
coal plants have been the backbone
of the U.S. energy supply for more than
a century and still deliver reliable power
to many electricity markets. To maximize
competitiveness
against
other
2. The CoLD process is founded on HPD evaporation and crystallization technology-a
water treatment solution Veolia has been using for more than 85 years. Courtesy: Veolia
Water Technologies
May 2020 | POWER
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sources, steam-electric utilities must
invest in optimal FGD wastewater management.
It not only enhances operations
today but it avoids the potential
for costly environmental liabilities in
the long run.
Remedial measures to restore ecological
damage resulting from uncontrolled
leaching or toxic spills are very
expensive. Treating FGD wastewater
with reliability and flexibility should not
be. By managing the risks associated
with FGD wastewater on the utility's
property, forward-looking producers
can control operating costs and extend
more profitable operations through the
end of their plants' economic lives, with
a focus on protecting human health and
the environment. ■
-Rob Lawson is business development
manager with Veolia Water Technologies
and Matias Navarro is manager of Marketing
and Strategic Market Development
with Veolia Water Technologies.
31
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POWER May 2020

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