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

We live in the desert.

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.

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

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

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

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.

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”.

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

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.

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.

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

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

CaSO4 precipitated by mixing concentrated solutions from EDM

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.

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.

Latest ZDD Process Flow Diagram The ZDD equipment used in year 1 is capable of producing 20-25 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.

Compositions of ZDD process streams, 4/15/11

ZDD: Improvements in Recovery

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.

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