Ion Exchange Membranes for Water Softening and High-Recovery
Desalination
Malynda A. Cappelle, Thomas A. Davis
The University of Texas at El Paso, Center for Inland Desalination Systems, El Paso, TX, USA
7.1 ION EXCHANGE MATERIALS AND WATER SOFTENING
Ion exchange materials are available in a variety of shapes, but the most common shapes are beads and membranes. A cation exchange polymer is illustrated inFigure 7.1. The polymer matrix, typically a copolymer of styrene and divinylbenzene, has sulphonate groups covalently bonded to the polymer matrix to provide a permanent negative charge.
(Cation exchange materials can be made with carboxylic or phosphonic acids as the per- manent negative charge, but those bind divalent ions so tightly that they are not useful for the applications considered in this chapter.) To maintain neutrality of ionic charge, there must be an equivalent quantity of cations to balance thefixed negative charge of the sulphonate groups. But the cations are not bound, and they are free to exchange with cations (counterions) in the solutions adjacent to cation exchange material.
Cation exchange beads are commonly used in water softeners where they are used to exchange calcium and magnesium ions for the sodium ions associated with the cation exchange beads. In a typical water softener, hard water containing calcium and magnesium ionsflows through a vesselfilled with cation exchange beads in the sodium ion form, and the calcium ions in the water displace the sodium ions in the beads. When
Figure 7.1 Structure of cation exchange material.
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the sodium ions are depleted, the beads are regenerated with a solution of concentrated sodium chloride. This cycle of exhaustion and regeneration typically occurs daily in in- dustrial processes and weekly in home water softeners, and it is often quite wasteful of salt and water.
7.2 DONNAN DIALYSIS
Water softening can also be accomplished with a cation exchange membrane. In dilute solutions, there will be very low concentrations of the co-ions (e.g. Cl) associated with counterions in hard water (e.g. Ca2þ) inside the cation exchange membrane as depicted inFigure 7.1. This principle is called Donnan exclusion[1,2]. To maintain electroneu- trality, an equivalent amount of calcium ions is exchanged for sodium ions in a cation exchange membrane. As illustrated inFigure 7.2, the hard water flows on one side of the membrane, and the salt solution, known as the stripping solution,flows across the other side.
The NaCl concentration in the stripping solution is maintained at a substantially higher level than the concentration in the feed stream. That concentration difference provides a driving force for the Naþ ions to diffuse into the feed solution. The cation exchange membrane is essentially impermeable to the Clions, because they are repelled by the negatively charged sulphonate ions attached to the polymer. The membrane pro- cess for water softening is just one example of a process called Donnan dialysis[1]. The Donnan equilibrium for a divalent Ca2þcation and a monovalent Naþcation is described usingEqn (7.1):
Ca2þ
strip
½Ca2þfeed
!12
¼ ½Naþstrip
½Naþfeed (7.1)
The brackets signify molar concentrations of the ions. This equation shows that Donnan dialysis is particularly effective for recovery or removal of multivalent ions[3].
The transport of one mole of Ca2þ through the membrane requires the transport of
C C
Na C NaCl
CaCl2
C C
Na Ca
NaCl Soft wate
Hard wat CaCl2
C C
Na Ca NaCl er
ter CaCl2
C
Figure 7.2 Donnan dialysis for water softening.
two moles of Naþ ions through the membrane in the opposite direction, in order to maintain electroneutrality.
In reality, the hard water will also contain Naþions that must be taken into account when the driving force is considered. An excess of Naþions is needed to ensure that there is still a driving force for Ca2þtransport when the NaCl solution reaches the end of the channel.
In contrast to the cyclic operation of water softeners, in which the ionsflow in and out of the beads in batch processes, Donnan dialysis can operate in a continuous, steady state where the ions flow countercurrently through the membrane, as illustrated in Figure 7.3. Rozanska and Wisniewski [4] described the use of Donnan dialysis to remove Ca2þ and Mg2þions from feedwater prior to electrodialysis (ED) in order to operate the ED at higher current densities. An effective device for Donnan dialysis was patented by Grot[5], the inventor of Nafion. Grot’s device utilised a bundle of small Nafion tubes.
The density of the solutions in Donnan dialysis is important in determining the direc- tion offlow in the solution compartments. Solution velocities are sufficiently slow in Donnan dialysis that buoyancy becomes a factor in the boundary layers. If the solution in the boundary layer adjacent to the membrane has lower density than the density of the solution entering the solution compartment, the solution should flow upwards in that compartment. The reader will notice that the NaCl strip solution isflowing down- wards inFigure 7.2and upwards inFigure 7.3, and this was intentional to emphasise the importance of solution density. Solutions of CaCl2are slightly more dense than solutions of NaCl of the same normality, so the boundary layer of the strip solution would be
Figure 7.3 Ion and water transport in Donnan dialysis with stream designations.
expected to become more dense as Naþions are replaced with Ca2þions. However, the hydration number is higher for Ca2þthan for Naþ[6], which could mitigate the density effect of Ca2þ if both Ca2þ and Naþ ions are fully hydrated as they are transferred through the membrane. Moreover,Figure 7.3 illustrates that water can flow through the membrane towards the strip solution and cause dilution that might increase the buoy- ancy of the boundary layer of the strip solution. In practice, it would be prudent to mea- sure the densities of the four process streams and select theflow directions that ensure that the less dense streams enter or leave the top of the Donnan dialysis stack. As depicted in Figure 7.3, the density of the extract is lower than that of the strip and the density of the feed is lower than that of the raffinate. For this reason, the strip solution should flow upwards. In contrast to that situation, uranyl ions imparted substantial density to the strip solution and required downwardflow of the strip solution[3].
7.3 ED FOR DESALINATION
Ions can diffuse through an ion exchange membrane in response to a concentration dif- ference as a driving force, and they can migrate through a membrane in response to an electrical potential. Because all of the cations in the membrane are free to migrate and almost all of the negative charges in the membrane are not free to move, the membrane is permeable to cations and impermeable to anions.Figure 7.1shows a few ion pairs. This intrusion of negative ions is extremely small when the concentration of the external solution is low, but the intrusion increases with the square of the concentration of the external solution [7]. The concentration of the co-ion in the membrane phase, Ccoion; can be calculated usingEqn (7.2):
Ccoion ¼
C2coion MR
g g
2
(7.2) where Ccoion is the co-ion concentration in the external solution, MR is the concentration offixed charges in the membrane phase,g is the activity coefficient in the external solution and g is the activity coefficient in the membrane phase [7].
Anion-permeable membranes typically are made with the same types of polymers, styrene cross linked with divinylbenzene, but the charged group attached to the polymer matrix is a quaternary amine, which imparts a permanent positive charge to the polymer.
ED is a desalination process that utilises an electrical potential to transfer ions through ion-permeable membranes out of a dilute solution into a concentrated solution. A description of how these ion-permeable membranes function follows, but first it is instructive to understand the context in which they will be used. ED utilises two types of membranese one membrane is permeable to cations, such as sodium and calcium ions, and impermeable to anions, and the other membrane is permeable to anions,
such as chloride and sulphate, but impermeable to cations. These two types of mem- branes are positioned in an alternating array as shown inFigure 7.4, with salt solutions flowing between them. When a direct electrical potential is applied to this array, ions in one solution migrate through the membranes into the solution on the other side of the membrane. The two solutions in the ED are called diluate and concentrate, and these solutionsflow through compartments formed by gaskets around the perimeter of the membrane. The solution compartments typically contain a plastic mesh material to hold the membranes apart and allow solution toflow through the compartment. The repeating unit, comprising a pair of membranes and their associated diluate and concen- trate compartment, is called a cell pair, and a commercial ED device can contain more than 100 cell pairs. The device is called an ED stack, because it is assembled by stacking membranes, gaskets and spacers and clamping them between end plates. The electrodes that provide the electric potential are housed in the end plates. An electrode rinse solu- tion, typically containing sodium sulphate, provides continuity between the electrodes and the membranes at the end of the stack. A commercial ED stack is illustrated in Figure 7.5.
When a salt, such as NaCl, is dissolved in water, the atoms in the salt go into solution as positively charged Naþcations and negatively charged Clanions. In an electricfield, these ions migrate towards the electrode of opposite charge. The Naþ cations migrate towards the cathode, and the Cl anions migrate towards the anode. As illustrated in Figure 7.4, an Naþ cation in the diluate compartment migrates to the right, and it can pass through thefirst membrane it encountersea cation-permeable membrane desig- nated by Cebut it is blocked by the next membrane, an anion-permeable membrane designated by A. Similarly, the Cl ion migrates to the left, passes through an A
Feed Solution Diluate
anode
+ Electrode Rin
anode
+ nse
+ + _
C A C
Cell Pair + _
+ _
A
E + _
A C
Concentra
Concentra
cathode
_ lectrode Rinse
ate
ate
Figure 7.4 Theflow of ions in electrodialysis (A¼anion exchange membrane, C¼cation exchange membrane).
membrane but is blocked by the C membrane. Hence, both ions can migrate out of the diluate compartments but cannot migrate out of the concentrate compartments.
How can a membrane selectively transport cations and block passage of anions? That charge selectivity is determined by the polymer in the membrane. A membrane like the one illustrated inFigure 7.1has the ability to transfer ions of only one charge type, but the ability of the membrane to discriminate between ions of different charge is limited. For example, because cation exchange material tends to prefer divalent cations, e.g. Ca2þ, over monovalent ions, there is a slight preferential transport of Ca2þover Naþ. The pref- erential transport of ions is quantified by the relative transport number (RTN) and calcu- lated usingEqn (7.3).
RTNCaNa ¼ Ca flux
Na flux ÷ Ca concentration
Na concentration (7.3)
InEqn (7.3), theflux is the rate of transport of the ion through the membrane, and the concentration refers to the diluate solution. The value of RTNCaNa is typically in the range of 1e2.5, which is inadequate for softening of hard water by ED. However, it is possible to modify cation-permeable membranes to reject divalent ions. Typically, monovalent-cation-selective membranes are made by attaching a thin coating of anion exchange polymer to the surface of the membrane. Values of RTNCaNa of about 0.2 can be obtained by this method. Membranes prepared by this method are used in ED to recover and purify NaCl from seawater.
Figure 7.5 Commercial electrodialysis stacks are often clamped in a hydraulic press.
The electrical resistance of an ion exchange membrane is roughly the same as a 0.1 M salt solution of the same thickness. Highly cross-linked membranes have higher electrical resistance. Electrical resistance is important, because it determines the amount of energy consumed in ED. Electrical resistance is inversely proportional to salt concentration in the solutions, but solution concentration has only a minor effect on membrane resistance.
When the solution being treated is highly concentrated, the resistance of the membranes can become the dominant resistance of the cell pair.
7.3.1 Electrodialysis Metathesis and the Zero Discharge Desalination Process
The zero discharge desalination (ZDD) process typically includes cartridgefiltration and antiscalant addition for pretreatment of reverse osmosis (RO) and/or nanofiltration (NF) feedwater, the primary desalination step. Electrodialysis metathesis (EDM) is then used to remove Ca2þ, SO42 and other ions from the RO/NF concentrate and return the diluate from the EDM to the RO/NF feed. Salt recovery systems can be added to the ZDD process to produce saleable salts such as NaCl (used in the ZDD process), Mg(OH)2, Na2SO4,and CaSO4. The ZDD system for a greenfield site (i.e. no existing desalination equipment) is shown inFigure 7.6. Brackish water is fed to an NF or RO to produce finished water in compliance with primary drinking water standards. The concentrate from the primary desalting process (RO or NF) is fed to an EDM system where undesirable dissolved constituents, i.e. scalants such as calcium sulphate, are
Figure 7.6 Simplified greenfield ZDDflow diagram.
removed, and the EDM diluate is returned to the RO/NF feed. EDM can be added to an existing plant to boost the plant’s recovery; however, it is possible that different mem- branes would need to be installed in the NF/RO. Special consideration is required when moderate to high levels of silica are present in the groundwater. Since EDM does not remove silicae silica is not ionically chargedeand because RO membranes have high silica rejection and are prone to fouling by silica at concentrations
>120 mg/L, a silica purge or silica treatment system would be needed. Alternatively, silica-permeable membranes, e.g. FILMTECÔ NF270, can be used to transfer silica to the permeate and avoid build-up of silica in the NF/EDM loop.
Similar to ED, an EDM stack is comprised of alternating cation and anion exchange membranes. However, the repeating cell in EDM includes two diluting streams and two concentrating streams. The salts in the two diluting streams, namely EDM feed and NaCl, change partners to form highly soluble sodium- and chloride-based concentrate streams. The repeating unit of four streams and the associated membranes and feed spacers is called a quad; commercial EDM stacks include 100 or more quads.Figure 7.7 shows a single EDM quad in detail and how it is incorporated into the EDM stack. The EDM feed, which can be brackish water or desalination concentrate, is fed to the EDM feed compartments. The applied voltage causes the cations to move towards the cathode and anions towards the anode. Two concentrating streams are produced in this process.
Cations from the EDM feed and chloride from the NaCl produce a stream with dissolved
Figure 7.7 Electrodialysis metathesis (EDM) membrane arrangement and ion transport (A¼anion exchange membrane, C¼cation exchange membrane).
chloride salts (called Mixed Cl). Anions from the EDM feed and sodium from the NaCl produce a stream with dissolved sodium salts (called Mixed Na). NaCl addition must match the removal rate of ions from the EDM feed. It is worth noting that, when the EDM is first started, the concentrate compartments are typically filled with deionised water or RO/NF permeate. In this case, the electric current builds up slowly; for a quick start, the concentrate compartments arefilled with Mixed Na and Mixed Cl solution from a previous run. In removing salts from the RO/NF concentrate, the EDM is similar to a kidney in the ZDD process. The desalinated product (called EDM diluate) can be returned to the RO/NF feed or blended directly with the RO/NF permeate.
EDM was first described in US Patent 2721171, which was issued in 1955 and assigned to DuPont. In this patent, H2SO4 and NaOH were produced from Na2SO4
[8]. A few years later, Winger described a process that employed EDM to generate NaOH from CaO and NaCl[9]. Another group described a process to convert a salt to an acid using an electrolytic cell fitted with a single set of membranes in an EDM arrangement [10]. EDM was also used to convert MgCl2 and Na2SO4to MgSO4and NaCl [11]. US Patent 6712946 describes how EDM can produce 2-keto-L-gulonic acid from its calcium salt [12]. HCl is the second diluting stream, and CaCl2 is a by- product. All of the aforementioned examples of EDM appear only to have been demon- strated at the laboratory scale. The ZDD process, described in US Patent 7459088[13], is thefirst commercial process using EDM.
7.3.2 ZDD Case StudyeAlamogordo, New Mexico, USA
ZDD has been tested at several scales and in multiple configurations in Alamogordo, New Mexico, at the Brackish Groundwater National Desalination Research Facility (BGNDRF). The demonstration activities were funded by the Desalination and Water Purification Research Program and the Texas Emerging Technology Fund. Activities in 2011e2013 focused on demonstrating systems capable of producing 109e218 m3/day (20e40 gpm) of permeate from a brackish feedwater with 2500e3000 mg/L TDS [14]. The greenfield ZDD configuration was tested during this period, and included a primary desalination system (NF) and EDM as described inFigure 7.6. The BGNDRF has four wells, with brackish water typically ranging from about 1200 to 6000 mg/L TDS. The feedwater to the ZDD equipment was a blend of Wells 1 and 4, intended to simulate the City of Alamogordo’s brackish water (available for desalination), called the Snake Tank Wells. The targeted blend was chosen to closely match the sulphate levels between the two sources (seeTable 7.1); since the BGNDRF wells have more Na, Cl and HCO3than do the Snake Tank Wells, the TDS is higher for the blend[14].
The ZDD equipment used at the BGNDRF is housed in two 12-m (40-ft) cargo containers. The primary desalter is an NF system with Dow FILMTECÔNF270 mem- branes. These membranes were chosen because of their low silica rejection. The NF sys- tem can be thought of as a single pressure vessel with eight NF270 membrane elements,
since it is a 4:4 array. Sulphuric acid was added to the brackish water feed to reduce bi- carbonate alkalinity, and Hydrex 4101 antiscalant was added at 2e3 mg/L to the NF feed to mitigate sulphate and carbonate scale formation. The NF was operated at 55e60%
recovery. The NF reject is transferred to the EDM system where 60e65% of the ions are removed before the solution is returned to the NF feed. The EDM system includes two MEGA EDM stacks with 4200 cm2 of each membrane exposed to the solutions.
Each MEGA EDM stack is designed to contain 100 quads comprising MEGA RALEXÒ AM-PES and CM-PES anion and cation exchange membranes, respectively. The EDM feed is comprised of the NF reject and a portion of the EDM desalinated product (EDM diluate). The recycle ratio, defined as the ratio of EDM diluate to NF reject, varied from 0.8 to 1.2. NaCl is the only chemical typically added to the EDM system. NF permeate is fed to a small RO system to produce water for preparing salt solutions and diluting the EDM concentrate streams. The only waste streams from the ZDD process are the EDM concentrate streams, namely the Mixed Cl and Mixed Na streams.
The ZDD system was operated for four runs during June and July 2013. Good water quality (TDS less than 700 mg/L) was produced consistently at 190e208 m3/day (35e40 gpm). The total system recovery averaged 97.5% for the same period and is related to the amount of dilution water used for the Mixed Na stream. Conservative set points were chosen to avoid crystallisation of NaHCO3 during the demonstration
Table 7.1 Comparison of compositions (all in mg/L unless otherwise noted) of brackish groundwater in ZDD demonstration (BGNDRF Well 1þ4) and City of Alamogordo (Snake tank wells average)
Well/Source name
Snake tank wells BGNDRF
Average Well 1D4 targeted mix
K 4.7 3.5
Na 164 382
Mg 88 143
Ca 413 334
Fe 0.35 0.24
Mn 0.16 0.012
Ba 0.02 0.02
Sr 7.3 5.3
HCO3 132 228
F 0.62 0.99
Cl 133 410
SO4 1433 1455
SiO2 28 21
TDS 2500 3019
Conductivity (mS/cm) 2967 3560
pH 7.58 7.9
Turbidity (NTU) 1.39 1.12
activities. Higher recovery is expected for a full-scale plant, because equipment would be installed indoors and because the brackish water available for the City of Alamogordo has lower alkalinity (less dilution would be required). Conductivity of the feed, product and waste streams is shown inFigure 7.8(a). Conductivity reduction by system and overall is shown inFigure 7.8(b). The ZDD system removed 63% of the ions, on average, from the blended source water.
The removal of various constituents is summarised in Table 7.2. The NF system rejects larger and multivalent ions better than smaller ions with a single or no charge.
Most of the silica passes to the permeate, as does the chloride and bicarbonate. The Cl concentration increases slightly from the NF feed to NF permeate, but the total mass is constant. Since the NF 270 membranes are able to reject 97% of the SO24 , on average, Cle passes through the membrane to maintain electroneutrality. The EDM
Figure 7.8 Zero discharge desalination (ZDD) operating data showing (a) Conductivity of ZDD inlet and outlet stream (b) Conductivity reduction.
Table 7.2 Reduction of various constituents by the ZDD system and its components
Reduction of: NF EDM ZDD
Conductivity 65% 52% 64%
TDS 77% 62% 75%
Ca 90% 73% 90%
Mg 92% 69% 92%
Na 52% 44% 48%
Cl 20% 73% 5%
SO4 97% 62% 96%
HCO3 19% 57% 58%
removed, on average, 62% of the TDS from the NF reject. The EDM removes more Ca2þ and Mg2þ as compared to Naþ and slightly more Cl as compared to HCO3 and SO42. The RTNCaNaand RTNMgNa averaged around 2e2.5, meaning more current is carried by the divalent cations. The RTNSOCl4 and RTNHCOCl 3 averaged around 0.3e0.5, meaning more current is carried by the chloride ions.
Zero liquid discharge (ZLD) processes generally require more power, more chemicals or both. The thermal ZLD approaches use brine concentrators and crystallisers to reduce the volume of industrial wastes, such as cooling tower blowdown, at power plants. Both can be quite expensive; one source indicates that the brine concentrators’
capital cost range is around $8000e26,000/m3/day ($30e100/gal/day) for RO brine with 6000e18,000 mg/L TDS [15]. Mickley has reported a specific energy demand of 20e24 kWh/m3 (75e95 kWh/kgal) for brine concentrators and evaporators [16]. The high-pressure pump in the NF/RO system and the applied voltage potential in the EDM are the primary energy uses in ZDD. At Alamogordo, the NF system is designed to operate at 55e60% recovery, which lowers the feed pressure required to desalinate wa- ter. A full-scale (15,000 m3/day or 4 MGD) ZDD system is estimated to require 2.28 kWh/m3[14]. The EDM system accounts for 76% of the total energy. A desalination plant could add solar salt recovery to make the ZDD process completely ZLD for 10% or less of the energy required by thermal ZLD.
7.3.3 ZDD with Salt Recovery
After energy costs, NaCl consumption is the next big operational cost driver for ZDD.
NaCl is added at a rate equivalent to the amount of ions removed from the NF/RO reject in ZDD. Daily NaCl consumption for a 15,000 m3/day (4 MGD) plant in Alamo- gordo is 22,000 kg/day, which represents 87% of the ZDD chemical cost and 33% of the total ZDD operating cost[14]. Feasibility studies using laboratory-scale equipment were performed as a part of the ZDD research. Incorporating salt recovery into the ZDD pro- cess is estimated to reduce the unit cost of ZDD by more than 40%. The incorporation of salt recovery in the ZDD process is shown schematically inFigure 7.9. Two approaches could be used to recover, purify and concentrate NaCl from the supernatant of the sec- ond precipitation tank: ED with monovalent-ion-selective membranes and evaporation ponds. The purified NaCl is blended with make-up NaCl and supplied to the EDM. In both cases, an additional small evaporation pond is required for excess Mixed Naflow, as shown inFigure 7.9.
The Mixed Na and Mixed Cl streams contain all of the salts removed by the NF/RO and EDM systems and many useful, saleable salts can be recovered from the streams, including Na2SO4, NaCl, Mg(OH)2and CaSO4. These streams can be combined to pre- cipitate CaSO4(actually CaSO4∙2H2O, but CaSO4is used for brevity). The remaining supernatant will be mostly NaCl, which can be recovered using several methods.
The brackish groundwater in Alamogordo has more SO42 than Ca2þon an equivalent basis (seeTable 7.3). However, the sum of Mg2þand Ca2þis nearly equal to the SO42. The Mixed Cl stream can be treated with lime to produce Mg(OH)2, which has a retail value of $800/tonne. The calcium ions from the lime replace the magnesium ions that are precipitated. The treated Mixed Cl stream and Mixed Na stream can be combined stoichiometrically to precipitate CaSO4 according to the following reaction (Eq. (7.4)):
CaCl2þNa2SO4þ2H2O/CaSO4$2H2Oþ2NaCl ð7:4Þ
Figure 7.9 Flow diagram for ZDD with salt recovery.
Table 7.3 Alamogordo brackish water (snake tank) water quality
Ion mg/L meq/L
Kþ 5 0.1
Naþ 164 7.1
Mg2þ 88 7.2
Ca2þ 413 20.6
HCO3-
132 2.2
Cl- 133 3.7
SO24 1433 29.8
TDS 2500
The supernatant from the CaSO4 precipitation would contain mostly NaCl and would be saturated with CaSO4. This solution can be treated with an ED fitted with monovalent-selective anion and cation exchange membranes to recover a very pure NaCl solution that could be used directly by the EDM.Figure 7.10shows the membrane configuration for the ED used in the experimental programme at The University of Texas at El Paso (UTEP). The monovalent-selective anion exchange membranes are able to block almost all of the sulphate ions present in the supernatant. The monovalent-selective cation membranes allow some leakage of calcium, but are able to block most of the calcium and produce a good quality NaCl. One way of evaluating NaCl purity is the ratio of Clto SO42 and Naþ to Ca2þ. Experiments suggest that both of these ion ratios should be greater than 12 to prevent precipitation in the EDM stack. A batch of NaCl produced using ED with monovalent-selective membranes had a Cl/SO4ratio of 200 and a Na/Ca ratio of 300. This level of NaCl purity corre- sponds to a purity of 99.5% or higher and is similar to the softener salt that is available for purchase worldwide [14,17]. The cost of power to produce NaCl with ED is estimated to be 5.3 US cents per kg (2.4 US cents/lb) of recovered NaCl.
A lower cost method for recovering NaCl employs evaporation ponds to concentrate the NaCl in the supernatant. Experimental results suggest that the recovered NaCl would be of suitable quality for direct use in the EDM. Two sets of experiments were performed to demonstrate ways to recover and use simple, lower cost, methods to recover salts from EDM concentrates. In thefirst, 19 L samples were gathered from the ZDD demonstra- tion system while in use at the BGNDRF. The Mixed Cl was treated with lime and then combined with the Mixed Na solution at an approximated stoichiometric mix based on
Figure 7.10 Membrane arrangement and ion transport in electrodialysis stack with monovalent- selective membranes (SA¼monovalent-selective anion exchange membrane, SC¼monovalent- selective cation exchange membrane).
earlier water quality analyses. The supernatant was then used to dissolve NaCl crystals and produce a saturated NaCl solution. A laboratory-scale EDM system fitted with MEGA RALEXÒmembranes was operated at a similar cell velocity and current density as in the demonstration equipment. No evidence of scale formation was evident after 6 days of operation. Table 7.4 summarises the analytical results for the experiments and compares the quality of the recovered NaCl in the bench scale to the demonstration-scale NaCl streams.
The second approach evaluated at a laboratory scale utilised samples of Mixed Cl and Mixed Na from the ZDD demonstration system. Similar to the previously described experiments, the Mixed Cl stream was combined with the Mixed Na stream. The super- natant was then boiled. This method and its results are summarised inFigure 7.11. The ion ratios in the highly concentrated NaCl solution indicated sufficient purity for direct use in the EDM.
Table 7.4 Summary of prepared NaCl solutions and circulating NaCl streams (all concentrations in meq/L)
Ca Mg Na Cl SO4 HCO3 Na/Ca Cl/SO4
Supernatant (average of 3) 121 7.4 1565 1602 136 8.7 14.7 12.6 SupernatantþNaCl
(average of 2)
121 14.6 4880 4696 205 8.0 40.8 26.8 NaCl to EDM
(benchebeginning)
3.4 1.0 238 230 15.0 2.3 70.8 15.4 NaCl to EDM
(bencheend)
7.3 3.6 474 468 24.4 3.2 65.2 19.2 NaCl to EDM
(demoelow Cl/SO4)
2.6 2.0 540 534 15.7 1.8 209.3 34.1 NaCl to EDM
(demoehigh Cl/SO4)
8.6 6.7 597 494 74.6 3.6 69.7 6.6
Figure 7.11 NaCl recovery using lime treatment and evaporation.
7.4 CONCLUSIONS
Ion exchange membranes can provide many benefits over conventional softening or desalination technologies such as water softeners (or other devices with ion exchange resins) and RO/NF processes, particularly when considered in the general context of sus- tainability. Donnan dialysis could substantially reduce the amount of salt used to soften water and greatly reduce the amount of salt that is released to the environment. Scale for- mation can be mitigated by the use of Donnan dialysis for pretreatment of hard water before desalination by ED or RO.
Demonstrations of ZDD with commercially available NF and EDM equipment have shown potential to boost desalination recovery to 96e98%, while providing good quality drinking water or blend water at multiple sites. Equally sized ZDD and brackish water RO (BWRO) systems are expected to have similar total capital cost (desalination system and concentrate management). However, the relative amount of capital cost associated with concentrate disposal is quite different for ZDD and BWRO. Additionally, since ZDD can recover more drinking water from the brackish water feed, less groundwater pumping is required (alternatively, for the same amount of groundwater pumping, more drinking water is produced with ZDD than with BWRO). ZDD with salt recovery (NaCl, Mg(OH)2) closes the material loop and would allow a utility to achieve ZLD. If Mg(OH)2 could be sold for half the retail price, the unit cost of ZDD decreases by as much as 50%. ZDD, like other high-recovery desalination approaches, is technically more complex and involves higher water production, but when compared against the much less sustainable alternatives (no water or water transfers from other watersheds) it can appear quite attractive.
LIST OF ACRONYMS AND ABBREVIATIONS
BWRO brackish water reverse osmosis ED electrodialysis
EDM electrodialysis metathesis gpm gallons per minute kgal 1000 gallons lb pound
MGD million gallons per day NF nanofiltration
RO reverse osmosis
RTN relative transport number TDS total dissolved solids
UTEP The University of Texas at El Paso ZDD Zero Discharge Desalination
ACKNOWLEDGEMENTS
This project was made possible through the Desalination and Water Purification Program, U.S. Bureau of Reclamation under Agreement No. R10AP81212 and with funding from the Texas Emerging Technology Fund. The views and conclusions contained in this document are those of the authors and should not be
interpreted as representing the opinions or policies of the U.S. Government. Mention of trade names or commercial products does not constitute their endorsement by the U.S. Government.
The authors would like to thank all of the organisations and people that have contributed to this work.
Collaborative work takes many talents and this project had plentiful support.
REFERENCES
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[2] H. Strathmann, Ion-Exchange Membrane Separation Processes, in: Membrane Science and Technol- ogy Series, 9, Elsevier, 2004.
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