POWER March 2017 - 24

WATER & WASTEWATER
2. Stacked for success. The Salt Splitter stack arrangement produces calcium chloride
(CaCl2) and sodium sulfate (Na2SO4) from calcium sulfate (CaSO4) present in FGD wastewater.
Courtesy: Saltworks Technologies
changed low-solubility calcium sulfate into
highly soluble calcium chloride and sodium
sulfate. Solubility of these ion pairs is shown
in Figure 1 (low solubility means greater
scaling risk). Davis worked on a similar process
in 2008, when he developed an electrodialysis
metathesis (EDM) process for feed
waters comprised solely of calcium sulfate.
Saltworks further innovated on top of Davis'
excellent work and has an exclusive license
to Davis' patents.
Saltworks' system, trade named Salt SplitTable
1. Salt Splitter-RO system analytics. The parameters shown here are from
an actual field test. An optional upstream precipitation step was employed in this example.
Therefore, the reject brine has less silica and transition metal load, and excess calcium mass
relative to the raw water. Courtesy: Saltworks Technologies
RO blowdown
to
Parameter (mg/L)
Total dissolved solids
Aluminum
Arsenic
Barium
Bicarbonate (as CaCO3)
Boron
Cadmium
Calcium
Carbonate (as CaCO3)
Chloride
Chromium
Fluoride
Iron
Lead
Magnesium
Mercury
Nickel
Nitrate (as N)
Phosphorus
Potassium
Selenium
Silica (reactive)
Sodium
Strontium
Sulfate
Zinc
24
Raw FGD
wastewater
19,300
0.04
0.45
0.65
120
175
0.11
3,290
<1
11,050
0.05
9
0.11
<0.0002
1,850
0.1
1.32
61
26.8
22
0.28
40
663
15
1,945
1.52
CaCl2 brine
or other
28,900
0.01
4.22
1.19
<1
16,400
0.02
310
<1
4,150
0.01
3.4
0.02
<0.0002
173
0.02
0.25
1,145
402
412
5.25
150
3,500
56
2,190
0.28
CaCl2 brine Na2SO4 brine RO permeate
220,700
0.01
3.06
8.11
<1
209,700
<0.01
0.85
0.09
<1
1,190
0.02
73,230
<1
140,300
0.01
7.8
0.02
<0.0002
1,580
0.02
0.22
940
7.77
375
0.1
<1
2,400
266
352
0.26
1,240
<0.01
768
<1
93,100
<0.01
0.34
<0.01
<0.0002
17.4
0.0002
<0.01
115
375
4.14
4.9
<1
77,400
2.82
36,620
<0.01
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120
<0.0001
<0.0001
<0.0001
<1
77
0.0001
6
<1
23.2
<0.0001
<0.05
<0.01
<0.0002
2.2
<0.00001
<0.01
0.3
0.07
0.1
<0.0005
<1
9.3
0.0306
3.1
<0.0001
ter, expands on this work with several proprietary
innovations-monovalent selective
ion exchange membranes, process, and controls-that
enable an industrially applicable
treatment plant. Salt Splitter is built around
the two most common desalination technologies:
electrodialysis and RO.
The advanced salt splitting electrodialysis
unit acts as a turbocharger for the RO.
It removes scaling limits by permanently
changing water chemistry. Divalent scaling
low-solubility ion pairs, such as calcium
sulfate, are " split " into nonscaling high-solubility
ion pairs, such as sodium sulfate and
calcium chloride.
The RO unit operates on an electrochemically-softened
feed, at a greater reliability
level and lower pressure than conventionally
possible. The combined Salt Splitter-RO
hybrid produces two highly soluble output
brines with a combined average brine concentration
of about 210,000 mg/L TDS.
These can be sent to separate downstream
processing stages for volume reduction or solidification.
A simplified electrodialysis stack
arrangement diagram, showing how the ions
are selectively transferred into separate brine
compartments to produce nonscaling Na2SO4
and CaCl2, is shown in Figure 2.
The Salt Splitter-RO system provides the
following benefits:
■ Recovery of the membrane system is 85%
to 95% versus 50% to 65% for RO alone,
resulting in a two-thirds reduction in brine
volume relative to that of chemically softened
RO, while eliminating the need for
expensive soda ash softening
■ Improved RO reliability and reduced operating
costs from lower pressure requirements
■
The nonscaling, lower volume brines reduce
the need for chemical softening of
the evaporator inlet and allow downsizing
of brine management assets
■ Less residual waste mass is produced because
soda ash addition is eliminated and
because useful by-products, such as sodium
sulfate, can be recovered from the
waste.
POWER | March 2017

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POWER March 2017

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