POWER March 2017 - 44

WATER & WASTEWATER
will be site specific. When comparing costs,
it is important to consider the various evaporator
configurations as discussed above as
well as redundancy requirements.
To account for less than 100% availability,
systems should be supplied with either 100%
redundant trains (very costly and unnecessary)
or with some amount of storage of the
FGD wastewater to provide a buffer between
the FGD scrubber and the FGD wastewater
treatment system. The amount of storage necessary
to achieve the desired availability is
dependent upon a number of factors. Anticipated
downtime due to single-point failures
of equipment and system design margins are
primary considerations. Decisions need to be
made as to the optimum configuration, and
such decisions are again site specific.
As an example, consider an FGD wastewater
treatment system to be rated for 100
gallons per minute. Based on operation at
other FGD evaporator installations, the cleaning
frequency for the evaporator is once per
year of operation, and the downtime duration
is 10 days. If the plant maintenance schedule
is to be down for an equal period on an
annual basis, then the FGD wastewater tank
can be sized for a single point failure, typically
one day maximum duration (144,000
gallons). Providing such a tank will always
be much less costly than a redundant train.
Nevertheless, the incremental cost of supplying
additional storage should be considered
in determination of the storage tank volume.
It is also important to consider the rated
capacity of the FGD treatment system versus
the expected normal capacity, as there can be
a large difference between the two. Such differences
can result from nonstandard operating
scenarios. If these scenarios are of short
duration, then it may make sense to consider
storage based on the normal capacity. The reason
for this is that shutdowns can typically be
anticipated and scheduled to occur when the
FGD wastewater is generated at a normal rate
(as opposed to the rated capacity). In some
situations, such considerations may make the
difference between choosing a redundant train
or larger FGD wastewater storage.
It is also important to note that the ELG rule
defers compliance until November 2023 for installation
of evaporator-based systems, whereas
facilities electing to comply using biological
treatment systems must be operational as early
as November 2018, or as determined by their
permitting authority. The economic impact
of the deferred investment must be evaluated
when comparing biological and evaporatorbased
ELG compliance solutions.
Choosing the Best Fit
Biological treatment and evaporator treatment
schemes will both play roles in FGD
wastewater ELG compliance. There are no
" one size fits all " solutions, as the capital, installation,
and operating costs are site specific.
Both treatment schemes have advantages
and disadvantages that should be evaluated,
including alternative evaporator options
that may eliminate the need for pretreatment,
crystallizers, and salt cake dewatering
equipment.
Further,
redundancy requirements
should be carefully evaluated for both
schemes. Consideration for providing buffer
volume between the FGD scrubber and the
wastewater treatment system should be considered
in lieu of redundant equipment.
Finally, the risk of future tightening of
effluent limits should be evaluated relative
to the projected life span of the plant. The
evaporator treatment scheme eliminates
FGD wastewater from the plants effluent discharge,
whereas the biological scheme may
not be able to adapt. ■
-Dan Bjorklund (bjorklundd@aquatech.
com) is vice president ICD at Aquatech
International.
44
www.powermag.com
POWER | March 2017
http://www.powermag.com

POWER March 2017

Table of Contents for the Digital Edition of POWER March 2017

Contents
POWER March 2017 - Cover1
POWER March 2017 - Cover2
POWER March 2017 - Contents
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