Electrodialysis Metathesis to Improve Desalination Yield from Gypsum-Rich Groundwater Thomas A. Davis Director, Center for Inland Desalination Systems.

Electrodialysis Metathesis to Improve Desalination Yield from Gypsum-Rich

GroundwaterThomas A. Davis

Director, Center for Inland Desalination Systems

University of Texas at El Paso, USA

Presented to Multi-State Salinity Coalition

January 27, 2012

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We live in the desert.

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Tularosa Basin, NM

Total Dissolved Solids (TDS) in groundwater ranges from 2 g/L to 10 g/L.

Satellite Image: White Sands National Monument and Missile Range

Alamogordo, NM

© The University of Texas at El Paso

Each aquifer has unique composition, and there are variations within an aquifer.

4

BGNDRF

Snake Tank Wells

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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

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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

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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.

ROED

RO CONC.

ED DILUATEFEED WATER

ED CONC.

DRINKING WATERPURGE

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Standard electrodialysis (ED)

Na+Cl-

C

Na+

Cl-

Na+

A C A C

Concentrated NaCl

Diluate Concentrated NaCl

Diluate

Na Cl Feed NaCl FeedRinse Rinse

+ -

REPEATING CELL

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Ca & SO4 in ED

Na+

Cl-

C

Na+Cl-

Na+

A C A C

Concentrate Diluate Concentrate Diluate

BrackishWater Feed

Brackish Water Feed

Rinse Rinse

+ -

REPEATING CELL

Ca+SO4

=SO4=

Ca+

Ca+

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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.

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EDM: Electrodialysis Metathesis

Na+

Ca2+

Mg2+

SO42-

Cl-

HCO3-

C

Na+

Cl-

Na+

A C A C

Mixed Na (conc.)

EDMDiluate

Mixed Cl(conc.)

DiluteNaCl

EDM Feed NaClRinse Rinse

+ -

REPEATING CELL (“QUAD”)

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Highly soluble salts are producedby EDM.

Solubility of Salts in water

Maximum 3.1M at 33°C

NaCl

CaCl2

CaSO4

Na2SO4

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EDM treats RO concentrate to transferCa and SO4 into separate streams.

+ C A C A C -

RO

PretreatedFeed Water

Precipitation CaSO4

(solid)

DrinkingWater

NaCl

Na2SO4 CaCl2 NaCl

NaCl &MgCl2

Solution

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CaSO4 precipitated by mixing concentrated solutions from EDM

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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.

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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.

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Latest ZDD Process Flow Diagram

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Compositions of ZDD processstreams, 4/15/11

CONSTITUENTSource Feed Perm Conc Feed Diluate

MixedCl

MixedNa

F 1.7 1.4 0.9 2.5 1.8 0.8 ND 61.3Cl 290 210 260 160 140 100 89,000 11,766SO4 1,300 1,300 9.50 3,600 2,600 1,300 1,600 90,764

Ca 250 220 19 NT 380 160 17,000 716Mg 94 94 7.50 NT 190 86 7,400 310Na 370 350 150 NT 580 350 21,000 57,078

SiO2 24.0 27.0 23.0 NT 33.0 33.0 5.7 11.4pH 7.4 7.0 7.0 7.0 6.9 6.6 1.8 7.9HCO3 205 110 70 135 128 43 NT 4,447Conductivity 2,857 2,724 872 5,021 4,271 2,404 124,800 109,000TDS (at 180 oC) 2,300 2,180 471 4,980 3,840 1,940 133,000 146,834

NF Streams EDM Streams

ANIONS

CATIONS & METALS

OTHER CONSTITUENTS

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ZDD: Improvements in Recovery

2%

70%

75%

80%

85%

90%

95%

100%

Conventional BWRO

BWRO+EDMw/purge

BWRO+EDMw/antiscalant

BWRO+EDMw/Silica removal

BWRO+EDMw/Silica removal,

projected dewatering

NF+EDMw/antiscalant

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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.

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Acknowledgements

• 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