POWER March 2017 - 26

WATER & WASTEWATER
4. Linear recovery. Raw wastewater with high salinity will have a lower recovery rate than
low-salinity wastewater. Courtesy: Saltworks Technologies
ash) usage. However, all projects are unique, so
economic results may vary.
The Salt Splitter-RO system required addition
of sodium chloride in this particular case
due to insufficient monovalent ions in the raw
solution. That added $14,000 per year in operating
cost but removed the cost of adding soda
ash, saving $1.2 million per year. Salt Splitter
also consumes electrical power to move ions in
the process, but less than an evaporator, which
achieves similar brine concentration. Total system
electrical power savings were calculated to
be 38%.
due to brine concentration gradients.
If scaling limits are removed, such as in
the Salt Splitter-RO process, the system's
brine concentration is limited by osmosis
of water from the treatment circuit into the
concentrated brines. Results from three trials
reveal that the upper brine concentration
limit of the Salt Splitter-RO process is
about 210,000 mg/L TDS. Previously, no
membrane system could achieve such high
brine concentrations, with the exception of
forward osmosis including a thermal regeneration
step and soda ash softening-the Salt
Splitter-RO process requires neither.
Due to the upper limit of brine concentration
being about 210,000 mg/L TDS, higher
salinity inputs will result in lower recoveries.
For instance, if the inlet water were already
at 210,000 mg/L TDS, the system would not
be able to recover any water. Likewise, for
waters with lower inlet TDS, system recoveries
would increase. The relationship between
system recovery and inlet TDS is depicted in
Figure 4. Readers can roughly estimate membrane
system recovery for their system by dividing
their wastewater's raw TDS in mg/L
by 210,000 and subtracting the value from 1.
Comparison and Economics
The first step in a typical conventional treatment
train for highly scaling wastewaters,
such as FGD wastewater, is chemical pretreatment
including lime and soda ash softening
to reduce the scaling potential of the water.
An RO unit is then used for initial desalination.
The RO brine reject is then directed to an
evaporator for volume reduction, followed by
a crystallizer in ZLD systems. Lime and soda
ash softening may also be required upstream
of the evaporator to manage scaling, depending
on the equipment being used.
A comparison of the costs associated
with the conventional treatment train and the
Salt Splitter-RO-crystallizer treatment train
was completed (see the online version of
this article to view a spreadsheet containing
the detailed analysis). The design basis for
comparison assumed 200-gallon-per-minute
26
(1,090 m3/day) inlet capacities, with the FGD
wastewater chemistry shown in Table 1. It also
assumed that true ZLD was required, meaning
only solids and treated water would be allowed
to leave the plant.
Capital costs were based on U.S. market
prices for chemical softeners, industrial reverse
osmosis, and evaporator-crystallizers.
Only process equipment costs were accounted
for in both options. Building and installation
costs were not accounted for and assumed to
be the same for both options. Capital costs
were amortized over 10 years at an 8% discount
rate. Plant availability was assumed to
be 95%, allowing 5% for maintenance and
downtime. Power costs were assumed to be
$0.065/kWh and thermal energy costs were
estimated to be $3/MMBtu, although lowpressure
steam (<5 psi) may be lower cost inside
coal-fired power plants.
The Salt Splitter option assumed that sodium
sulfate was produced and provided to
industry at no cost or revenue addition. The
calcium chloride brine was concentrated to
about 80% total solids by mass in an evaporative-crystallizer
and then solidified with the
addition of fly ash and cement. Similarly, for
the conventional treatment train, the crystallizer
discharge was concentrated to about
80% total solids by mass and then solidified.
Solidification agent and tipping fee costs for
both options were not included due to operators
having very different methods.
The Salt Splitter-RO system resulted in
capital cost savings of 25% compared to the
conventional treatment train, saving the operator
$2.2 million in capital cost on the process
equipment for a 200-gallon-per-minute plant.
In addition, by eliminating soda ash softening,
the Salt Splitter-RO option resulted in operating
cost savings of 37%, saving the operator
roughly $1 million per year. Ultimately, the
Salt Splitter-RO system resulted in a total cost
of ownership savings of 45%. In addition, the
Salt Splitter option resulted in a lower regulatory
risk profile due to reduced risk of hazardous
waste generation, and lower transportation
and safety risk due to reduced chemical (soda
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Final solids mass was reduced by 30% due
to eliminating soda ash addition and the opportunity
to beneficially reuse solid sodium sulfate
produced. Solidification agent costs were not
included, such as fly ash or cement for both
scenarios. As such, savings from the calcium
chloride brine benefiting solidification were not
accounted for, but will further benefit Salt Splitter-RO
economics. Future work will focus on
increasing the beneficial reuse of the produced
solids and lowering solidification costs.
A Viable Treatment Option Worth
Considering
Changing regulations are driving both innovation
and the uptake of FGD wastewater
treatment solutions. Treatment approaches
vary widely depending on the facility and
its particular water chemistry, flow rate, and
discharge regulations. However, to optimize
treatment economics, there are steps an operator
or plant designer can take that are consistent
across all facilities. They are:
■ Eliminate soda ash softening
■ Increase membrane system recovery to reduce
the size of thermal evaporation technologies
■
Minimize overall residual waste and minimize
the amount of hazardous waste
The case study and economic comparison
presented in this article demonstrates the Salt
Splitter-RO system's ability to achieve these
objectives. The system also provides operators
and plant designers with flexible options
for managing residual waste, including the
ability to beneficially turn waste into reusable
by-products. The Salt Splitter-RO system
has been utilized in other highly scaling
industry applications, such as mine discharge
water treatment, and it can be applied anywhere
reverse osmosis recovery is limited by
scaling ions. ■
-Mitchell Frank is science and knowledge
manager, Ben Sparrow is chief
engineer, Joshua Zoshi is a senior technical
leader, and Megan Low is process
engineering manager for Saltworks
Technologies Inc.
POWER | March 2017

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POWER March 2017

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