1. Multi-State Salinity Coalition
Electrodialysis Metathesis to Improve Desalination Yield from Gypsum-Rich Groundwater
Thomas A. Davis
Director, Center for Inland Desalination Systems
University of Texas at El Paso, USA
Presented to
Multi-State Salinity Coalition
January 27, 2012

2. We live in the desert.

3. Tularosa Basin, NM Alamogordo, NM
Satellite Image: White Sands National Monument and Missile Range
Total Dissolved Solids (TDS) in groundwater ranges from 2 g/L to 10 g/L.

4. Each aquifer has unique composition, and there are variations within an aquifer.
Snake Tank Wells
BGNDRF 4

5. Desalination of inland groundwater
RO produces two output streams:
Drinking water (permeate).
Waste water (concentrate), which contains the removed salts.
Minimizing the volume of RO concentrate:
Avoids wasting water.
Avoids/Minimizes expensive and environmentally challenging disposal processes:
Surface discharge
Evaporation ponds
Deep-well injection
Off-site hauling
Zero liquid discharge
DARK ORANGE IS CHLORIDE TYPE WATER
BROWN IS SODIUM BICARBONATE
YELLOW IS SULFATE TYPE WATER
LIGHT GRAY IS CALIUM MAGNESIUM BICARBONATE TYPE WATER
DARK GRAY IS CONSOLIDATED BEDROCK

6. Limitations of Desalination Yield
High osmotic pressure (energy cost)
Solutes with limited solubility
Silica ~ 100 mg/L
CaCO3 > 15 mg/L, depends on pH
CaSO4 ~ 2 g/L, depends of salinity
BaSO4, SrSO4, CaF

7. Approaches to Improve Yield
Use antiscalant to disrupt crystallization.
Contact supersaturated RO concentrate with seed crystals to reduce supersaturation.
Remove troublesome solutes by ion exchange or chemical softening.
Remove troublesome ions by electrodialysis.
Separation of useful salts is possible.
Salts can be concentrated to high levels.

8. Standard electrodialysis (ED)
Na+
Cl- C A
Concentrated NaCl
Diluate
Na Cl Feed
NaCl Feed
Rinse + -
REPEATING CELL
The DC potential applied to the electrodes causes anions to move to the left toward the anode and cations to move to the right toward the cathode. The migration of ions carries electric current through the solutions and membranes. The membranes designated by A transport anions, and those designated by C transport cations. In the ED apparatus the two types of membranes are positioned in an array of alternating A and C membranes with solutions flowing in between. Na+ and Cl- ions in the center solution compartment, designated as Diluate, migrate through the first membrane they encounter, but they are blocked by the next membrane. Cl- ions moving left pass easily through the A membrane into the concentrated NaCl compartment, but they do not exit through the C membrane. Similarly Na+ ions in the Diluate solution compartment pass through the C membrane but cannot get through the A membrane that is allowing Cl- ions to enter from the next Diluate solution compartment. The repeating array of two membranes and two solution compartments is called a “cell pair”, and many cell pairs can be stacked between a single pair of electrodes to form what is called an ED “stack”.

9. Ca & SO4 in ED - + C A C A C Cl- Ca+ Na+ SO4= SO4= Ca+ Ca+ Na+ Cl- Na+
Concentrate
Diluate
Concentrate
Diluate C A C A C
Cl-
Na+
Ca+
SO4=
SO4=
Ca+ +
Ca+ -
Na+
Cl-
Na+
REPEATING CELL
Rinse
Brackish
Water Feed
Brackish
Water Feed
Rinse

10. Limitation of Conventional ED
When CaSO4 is the dominant salt, it becomes supersaturated in the concentrate stream.
EDR is not as effective in highly saline solutions, because longer times are required to return to steady state after current reversal.
Others have mitigated supersaturation by contacted ED concentrate with seed crystals to remove CaSO4.

11. EDM: Electrodialysis Metathesis
Na+
Ca2+
Mg2+
SO42-
Cl-
HCO3- C A
Mixed Na (conc.)
EDM
Diluate
Mixed Cl
(conc.)
Dilute
NaCl
EDM Feed
Rinse + -
REPEATING CELL (“QUAD”)
Figure 2 shows the ion transport in an EDM repeating cell, or “quad”, which is a set of four flow compartments: Mixed Na salts, EDM feed-diluate, Mixed Cl salts, and NaCl. When an electric potential is applied to the electrodes, the cations (Na+, Mg2+, and Ca2+) migrate to the right, and anions (Cl-, HCO3-, and SO42-) migrate to the left. The cations can penetrate the cation-permeable membranes (denoted by C) but are blocked by the anion-permeable membranes (denoted by A). Similarly the anions can penetrate the A membranes, but further migration is blocked by the C membranes. The result is that the anions and cations of the two salts change partners to form concentrated solutions primarily composed of CaCl2 (mixed Cl) and Na2SO4 (mixed Na), both of which are highly soluble.

12. Highly soluble salts are produced by EDM.
Solubility of Salts in water
Maximum 3.1M at 33°C
NaCl
CaCl2
CaSO4
Na2SO4

13. EDM treats RO concentrate to transfer Ca and SO4 into separate streams.
+ C A C A C - RO
Pretreated
Feed Water
Precipitation
CaSO4
(solid)
Drinking
Water
NaCl
Na2SO4
CaCl2
NaCl &
MgCl2
Solution

14. CaSO4 precipitated by mixing concentrated solutions from EDM

15. The ZDD* process (Zero Discharge Desalination)
Treat RO concentrate with electrodialysis metathesis (EDM).
In EDM the ions of troublesome salts in RO concentrate exchange partners with NaCl to form highly soluble Na salts and Cl salts.
Each of the two EDM concentrate streams contains about 1% of the water from the original feed.
The other 98% is in the RO permeate.
* T. A. Davis is a principal in ZDD Inc, licensee of the ZDD technology.

16. Dealing with Silica
After CaSO4 was removed from the RO concentrate, silica became the limiting solute.
RO and EDM membranes are impermeable to silica, so silica builds up in circulating loop.
Methods to avoid silica precipitation:
Purge some of the solution to reduce silica.
Add antiscalant to delay precipitation.
Add NaOH to precipitate Mg(HSiO3)2, filter with ceramic membrane, and recycle purge solution.
Replace RO with Nanoflitration (NF) membrane.

17. Latest ZDD Process Flow Diagram
The ZDD equipment used in year 1 is capable of producing gpm of permeate and is comprised of NF (housed in a 40-ft container) owned by UTEP and EDM (in a 20-ft container) owned by Veolia. Figure 5 shows the process flows for the equipment (the calcium sulfate reactor was not used this year, but is shown for completeness of the process). The NF system includes pre-treatment (cartridge filters, alkalinity removal, and anti-scalant addition. The NF itself is a 4 x 2 x 1 array with four Dow Filmtec NF270 4” x 40” membranes. The NF270 membranes were chosen for their lower silica rejection rate. The EDM system includes the EDM “stack”, pumps, chemical dosing systems, and DI tanks.
Focus on the NF Concentrate-EDM loop & silica
Talk about how NF mode differs from RO mode
EDM takes yield-limiting salts out of the NF concentrate and returns that depleted solution to the NF feed for more water recovery.

18. Compositions of ZDD process streams, 4/15/11

19. ZDD: Improvements in Recovery

20. Conclusions
ZDD process can provide substantial improvements in yield of water from groundwater containing CaSO4.
EDM is the heart of the ZDD process.
Separate concentrated streams containing Ca++ and SO4= are produced in EDM.
Concentrated streams are mixed to produce CaSO4 byproduct.
NaCl can be recovered from supernatant.
Use of NF versus RO eliminates silica problem in EDM.

21. Acknowledgements cids.utep.edu
Veolia Team: Brad Biagini, Larry Hart, Bud Krebs, Bernie Mack, Paul Choules, Mark Smock, Larry Jessup
UTEP/CIDS: Malynda Cappelle, Shane Walker, Lucy Camacho, Jesse Valles
Brackish Groundwater National Desalination Research Facility (BGNDRF) in Alamogordo, NM
cids.utep.edu