POWER March 2017 - 22

WATER & WASTEWATER
Lowering Cost and Waste in Flue
Gas Desulfurization Wastewater
Treatment
Courtesy: Saltworks Technologies
Many informative articles have been published about options for treating flue gas
desulfurization wastewater, but new technologies are rapidly changing the treatment
possibilities. This article reviews key economic considerations and introduces an innovative
treatment solution that lowers costs and the amount of waste generated.
Mitchell Frank, Ben Sparrow, Joshua Zoshi, and Megan Low
F
lue gas desulfurization (FGD) wastewater
varies widely depending on the
source facility, the type of coal burned,
and the FGD system employed. However, almost
all FGD wastewater is highly scaling.
To achieve compliance with treatment regulations,
operators must successfully manage
scaling and they need to understand the different
types of scaling. This is especially important
for zero liquid discharge (ZLD) applications,
where scaling is of great concern.
FGD wastewater contains the following
types of scalants:
■ Low-solubility metals, such as iron, nickel,
cobalt, and aluminum. These can create
scaling compounds, but raising pH via
caustic or lime addition can readily precipitate
them.
■ Low-solubility anions, such as fluoride,
phosphate, and sulfate. These can combine
with calcium to create scale. They
cannot be readily removed to reliable levels
through precipitation, therefore further
management is required.
■ Divalent cations responsible for high hardness,
primarily calcium and magnesium,
as well as barium and strontium. Elevating
pH will precipitate magnesium, but it has
little effect on calcium, barium, or strontium,
unless carbonate is present, which
is uncommon in FGD wastewater. That is
why conventional treatment processes add
carbonates in the form of expensive soda
ash (sodium bicarbonate).
22
Conventional Treatment Processes
Although FGD wastewater chemistry varies
between facilities, there are common treatment
systems employed.
Lime (Calcium Hydroxide) Softening.
This system removes magnesium hardness
and heavy metals. Some calcium is removed,
but only calcium that is " associated " with
carbonate.
Soda Ash (Sodium Carbonate) Softening.
This system removes calcium hardness.
Soda ash is one of the most expensive
processes for FGD wastewater treatment.
Costs are often about $4 per cubic meter (m3)
inlet, accounting for more than 20% of the
total cost of ownership (capital plus operating
cost) of the treatment. Some waters are
higher in magnesium and lower in calcium,
so costs vary.
Ion Exchange. This system employs
resins to absorb calcium, reducing calcium
levels to less than 50 milligrams per liter
(mg/L). However, the resins are not effective
in high salinity FGD water and must be
regenerated with acid, resulting in high operating
costs and acidic chemical waste that
requires disposal.
Reverse Osmosis (RO). This is the most
dominant and widely practiced desalination
technology for removal of total dissolved
solids (TDS). RO is low cost, but requires
notable chemical pretreatment to be reliable
on FGD wastewater. On scaling ion chemistry,
RO is susceptible to fouling and does not
concentrate well past 60,000 mg/L, that is,
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about 50% to 65% recovery on FGD water.
This results in low recoveries and high brine
volumes (or requires very frequent chemical
cleaning). Advancements in modified RO
units, such as New Logic Research Inc.'s vibratory
shear enhanced processing (VSEP)
fouling-resistant membrane filtration product
line, reduce the need for chemical pretreatment
and boost recovery, but operators need
to plan for chemical cleaning to maintain operations.
Any RO unit can be hybridized with
the innovative treatment system presented in
this article, but a VSEP unit will add a further
boost to recovery.
Thermal Brine Management (Evaporator,
Crystallizer, SaltMaker). Evaporators
concentrate the final brine waste and
crystallizers produce solids. These systems
are required for brine management and
ZLD-and are the most expensive process
equipment in the treatment train in terms of
both capital and energy cost. The required
capacity and energy used by these systems
can be reduced if the recovery of upstream
membrane systems is increased. Membrane
systems are often one-fifth to one-tenth the
cost per unit volume processed; therefore, it
makes sense to maximize their recovery.
Reducing Treatment Costs
In smaller power plants and lower flow FGD
systems, such as less than 50,000 gallons per
day (189 m3/day), it may not be economical
to include an upstream membrane system
to reduce volume prior to the evaporation
POWER | March 2017

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POWER March 2017

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