POWER March 2017 - 43

WATER & WASTEWATER
1. ELG treatment options. Existing coal plants may opt for either biological or evaporator
flue gas desulfurizer (FGD) wastewater treatment. Courtesy: Aquatech International
2. ZLD approaches. FGD wastewater can be directly concentrated without softening,
followed by mixing in a pug mill with fly ash (and lime) to generate a solid mixture that fixates
metals (options 1 and 2). Alternatively, pretreatment with lime soda can be followed by concentration
and/or crystallization (options 3 and 4). Courtesy: Aquatech International
1
2
3
4
The biological solution is a scheme that
most plant operators seem to have comfort
with, as it is composed of multiple conventional
water treatment operations followed
by a biological system, also comprising
equipment with which plant engineers and
operators are familiar. Further, it may be
perceived by some in the industry that ELG
compliance via biological treatment is simpler
to operate with higher reliability, less
pretreatment, and is less costly to install and
maintain. These perceptions are the subject
of discussion below.
Comparing the Complexity of
Evaporator and Biological Treatment
Systems
The operational success of a biological system
is dependent upon the operator's ability
to operate the physical/chemical pretreatment
system to remove total suspended
solids (TSS), mercury, and arsenic, and to
condition the FGD wastewater to the input
specifications of the biological treatment step
for selenium and nitrate removal. Careful attention
by the operator is required to ensure
that the ELG limits are achieved.
March 2017 | POWER
By contrast, evaporator systems are comparatively
simpler to operate and more forgiving
to flow and chemistry variations, as
well as more tolerant to cycling operation. It
is relatively easy to change the feed rate, and
start and stop an evaporator system, whereas
biological systems are less tolerant of these
changes, and careful operation is required to
ensure the health of the biomass.
Pretreatment by physical/chemical treatment
may or may not be required (see discussion
on pretreatment below). If necessary,
conventional lime-soda softening is employed
to partially soften the FGD wastewater.
A target range is given for softening, as
the process is tolerant to a reasonable variation.
The evaporator equipment is fully automatic
with little operator attention needed on
a regular basis other than shift-wise sampling
and chemical analysis using test kits.
Operators unfamiliar with evaporator system
operation may believe it is more difficult
to operate. However, factors such as the right
training and utilizing operations and maintenance
services offered by the evaporation plant
supplier have changed operators' perceptions.
Also to be considered is the availability rewww.powermag.com
quirements
of the system. The biomass in a biological
system can die off due to a variety of
factors. These microorganisms require significant
time to grow and acclimate before being
placed in service. This risk necessitates that
redundancy be provided in the treatment train
so that an off-line biological unit can be placed
in service without interrupting FGD wastewater
treatment. However, a redundant biological
unit does not provide additional availability if
the cause of biomass mortality is common to
the on- and off-line units, and this should be
considered in the design of the system.
Evaporator FGD treatment systems, when
properly designed and operated, are highly
tolerant of changes in the FGD wastewater
composition that may be caused by seasonal
shifts in plant make-up water quality, changes
in coal composition, and upsets in the
FGD scrubber. The availability of a properly
designed system can be anticipated to exceed
96%. In addition, the distillate produced
by the evaporator system is of high quality
and can be recycled back to the scrubber as
makeup, which eliminates the risk of ELG
noncompliance.
Pretreatment Options
Existing facilities may already have a physical/chemical
treatment system to meet their
current discharge permit requirements for
TSS and regulated pollutants.
Therefore,
compliance with the ELGs for many plants
requires modifying the existing physical/
chemical treatment system and integrating
either a biological treatment system or evaporator
system.
The biological option requires physical/
chemical pretreatment using lime, sulfide,
and iron treatment for mercury, arsenic, and
metals removal. Biological treatment is employed
following pretreatment to reduce the
concentration of nitrate and selenium to the
ELG limits. For treatment by evaporation,
there are several schemes that need to be
considered, not all of which require physical/
chemical treatment, as shown in Figure 2.
Evaporative approaches to ELG compliance
are typically partial cold lime-soda
softening of FGD wastewater followed by
concentration and/or crystallization. Alternatively,
FGD wastewater can be directly
concentrated without softening followed by
mixing in a pug mill with fly ash (and lime)
to generate a solid mixture that fixates metals
and can be disposed of in a landfill.
Comparing Costs of Evaporator
and Biological Treatment
It is very difficult to make absolute claims
regarding the total installed cost or life cycle
cost of either biological or evaporator-based
FGD wastewater treatment systems, as these
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POWER March 2017

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