By Avijit Dey

Abstract
Electrodeionization (EDI) is a separation process combining electrodialysis and conventional ion exchange; the resulting hybrid process entails zero regenerant chemicals. One benefit of this technology includes the elimination of secondary hazardous waste associated with the chemical regeneration of ion exchange tanks. For fundamentally the same reason that cation and anion units are cost-effectively replaced by reverse osmosis (RO) units, EDI units in many cases can cost-effectively replace mixed bed deionizers. RO and EDI technologies enjoy a symbiotic relationship with each contributing complimentary characteristics that enhance the performance of overall treatment process. EDI technology in combination with RO offers a chemical-free option for the removal of ionic impurities from raw water. Industrial scale EDI devices are available in two major configurations: plate-and-frame or spiral wound. Omexell developed, patented, and commercialized the most advanced Spiral Wound Electrodeionization (SWEDI) system over five years ago. The proven reliability of Omexell spiral wound EDI modules in the high purity water generation application is substantiated by more than 40,000 gallons per minute (gpm) capacity of successful installations. These systems have now been in operation at several micro-electronics and power plants for over three years. SWEDI is a robust and forgiving technology. SWEDI represents a breakthrough in desalination of RO permeate water as compared to plate and frame configuration. Associated benefits include leak-free operation up to 100 psig feed pressure because of the ease of sealing cylindrical pressure vessels, higher feed water hardness tolerance due to unique cross flow operation, ion exchange resins and/ or membranes replacement, excellent electrical insulation due to the use of FRP housing, light module weight, and simple system integration. Cross-flow is the term given to the mode of EDI operation where the direction of flow in the concentrate chambers is perpendicular to that in the diluting chambers. Conventional plate and frame devices are essentially operating in co-flow configuration. Co-flow is the term given to the mode of EDI operation where the direction of flow in the concentrate chambers is the same as in the diluting chambers. The unique cross-flow design of the Omexell SWEDI modules results in higher feed water hardness tolerance compared to the conventional plate and frame devices. Vendors of plate and frame EDI devices suggest that the maximum total hardness in the feed to the EDI be limited to 0.5 to 1 ppm as CaCO3 whereas Omexell SWEDI modules can accept a maximum total hardness of 2 ppm as CaCO3 in the feed to the EDI. It sometimes translates to a reduction in capital expenditures for water treatment systems by eliminating the need for softeners and/ or double pass RO ahead of the EDI systems.

Introduction
The purification of aqueous streams by reducing the concentration of ionic impurities has been an area of considerable technical interest. Many techniques have been developed for this purpose. The most well known processes include electrodialysis, electrodeionization (EDI), liquid chromatography, membrane filtration, and ion exchange. EDI is a separation process combining electrodialysis and conventional ion exchange; the resulting hybrid process does not entail regenerant chemicals. EDI technology has been reported in the literature since 1950s1,2. The first patent for EDI was granted to a Dutch company in 19573. A patent for the purification of acetone with EDI was also granted to Kollsman in 19574. For fundamentally the same reason that cation and anion units are cost-effectively replaced by RO units, EDI units in many cases can cost-effectively supplant mixed bed deionizers5. EDI process is preferred in many systems primarily because of the environmental benefit of not requiring hazardous regenerant chemicals and inherent superiority of a continuous process over a batch process6. Although it may be possible to regenerate mixed bed resins using regenerants of high purity to achieve quality of water required even in electronics industry, this is inept due to the cost of producing and maintaining ultra-pure regenerant. Additionally, it is critically important for mixed bed deionizer operation that the resins settle properly and the division between them occurs at the middle collector. Furthermore, the particle size range is limited by the separation requirement. The separation occurs because of the difference in the density of the two types of resin. The cation resin, being heavier, settles on the bottom, while the anion resin, being lighter, settles on top of the cation resin. The degree of separation can be determined using the terminal settling velocities (TSV) of the cation and anion resins respectively. The TSV is calculated using Stokes’ Law7:

Eq. 1

Where,

V = Resin terminal settling velocity, cm/ s
g = Gravitational constant (= 981 cm/ s2)
a = Radius of resin bead, cm
d1 = Density of resin, g/ cm3
d2 = Density of water, g/ cm3
η = Viscosity of water, Poise

In the case of regenerable mixed beds, it is essential that distinct separation of cation and anion resins occurs upon backwashing to assure complete regeneration of the resins and avoid contamination by the other regenerant. However, the very nature of a regenerable mixed bed requires compromise: the resins must be able to separate for regeneration and to remain intimately mixed during exhaustion8. These are obviously contradictory requirements. If cation resin remains in the anion zone, it will absorb sodium from the caustic regenerant. Conversely, anion resin remaining in the cation zone will attract either sulphate or chloride depending on whether sulphuric or hydrochloric acid regenerant is used for the regeneration of cation resins. Consequently, EDI systems provide a technically superior alternative in a cost-effective manner.

One key factor stimulating the EDI market growth is the desire to move to a water treatment approach that consumes fewer chemicals. A main benefit of the EDI system is the elimination of the need for on-site bulk storage of concentrated acid and caustic regenerants and associated operator exposure. Moreover, no hazardous waste stream is generated by this technology. In combination with RO, EDI provides a continuous, chemical-free system. On the other hand, purifying water through regenerated resin beds may lead to organic release onto the downstream treatment steps. When newly regenerated resin beds are installed, high TOC leakage may occur. It is interesting to note that EDI systems can readily remove charged organics9. Moreover, EDI systems use a continuous process, which does not require any redundancy like the conventional mixed bed deionizers. The main consumable in EDI is electricity. The electrical cost of running an EDI system is usually between 0.5 to 3.0 kW-h/ 1000 gallons of product water, depending on feedwater quality and product water specifications10. Recent developments in EDI technology have resulted in significant cost reductions arising from an expandable, modular system approach built upon a base module. EDI systems require minimal civil works for installation.

The patented Omexell™ EDI modules utilizes a spiral-wound membrane and ion exchange resins, sealed in a high strength FRP pressure vessel11. The EDI system is composed of an arrangement of flow-directing spacers separated by anion and cation semi-permeable membranes. Spacers are provided between alternating cation and anion exchange membranes to maintain separation of associated membranes. Spacers support even distribution of liquid through the chamber. Spacers are usually manufactured from thermoplastic materials. Permselective ion exchange membranes are arranged to form parallel flow chambers. In EDI systems, ions are removed from water by a combination of ion exchange resin, ion exchange membranes, and a DC electric current. The feedwater entering the Electrodeionization apparatus is divided into at least three parts. A small percentage flows over the electrodes, a majority of the feed passes through the diluting chambers and the remainder passes along the concentrating chambers. Diluting chambers are filled with specially graded ion exchange resins. A conductive path is developed through the resin beads that are much lower in electrical resistance compared to the path through the bulk solution. The chambers bounded by the anion membrane facing the anode and the cation membrane facing the cathode become exhausted of ions and are hence called diluting chambers. The chambers bounded by the cathode-facing anion membrane and anode-facing cation membrane are called concentrating chambers. The concentrating chambers will then “trap” ions that have electrically migrated in from the diluting chambers. Consequently, the ionic concentration of the water will decrease in the diluting chamber with a corresponding increase in the ionic concentration of the water in the concentrating chamber. These flow chambers are hydraulically in parallel but electrically in series.

In the present SWEDI systems, feed water enters the modules from below and is diverted into vertically spiraled dilute chambers. Concentrate enters the module through the central pipe from below and is diverted into spirally flowing concentrate chambers (See Figure 1). The patented cross-flow design of the concentrate stream in the present SWEDI modules is unlike that of the co-flow design used in conventional plate and frame devices. This imparts a higher hardness tolerance for the present SWEDI devices. The present SWEDI module offers the only serviceable EDI system in the market as the replacement of resins and/ or membranes is possible at anytime while reusing the housings, end caps and titanium anode. This unique feature dramatically reduces the operating cost by extending the useful life of the present SWEDI modules. Therefore, operating costs are lowered by reducing the high replacement cost for the conventional EDI modules. The influence of EDI module life is far greater on the overall operating cost than the influence of power consumption by the EDI module and associated electrical equipment in the EDI system. As these SWEDI modules are serviceable on-site, it is not necessary to send the modules back to the manufacturing facility.

Higher conductivity in the concentrate stream accomplished by concentrate recirculation and occasional brine injection in the concentrate stream facilitates the transfer of current while back-diffusion can limit effluent quality. Consequently, a certain percentage of the concentrate flow must go to the drain. The increase in the electric current flow due to higher electrical conductivity of the concentrate water lessens the power consumption of the device because the EDI module resistance and applied voltage decreases as a result. Further improvement in the power economy was accomplished by passing the current through a series of cell pairs with electrodes only at the terminal points of the series. A cell pair consists of a cation membrane, spacing material through which the feed and the treated water flows in the diluting chamber, an anion membrane, and the spacing material through which the concentrate water flows. Consequently, the loss of energy at the electrodes per equivalent of salt removal from the RO permeate water will reduce to a minimum.

The current also splits water molecules into hydrogen and hydroxyl ions. The EDI systems operate in two different modes: Electrodeionization and Electro-regeneration12. The system is working in the first mode when the feedwater salinity is high13. On the contrary, the system is working in the second mode when the feedwater salinity is very low due to the passage of strongly ionized species to the concentrating chamber in the upstream flow path. This allows a portion of the resins in the EDI to always be in the fully regenerated state. This will form a highly conductive path through the resin beads. Moreover, higher degree of regeneration of resin beads will reduce the leakage of all the ionic species including boron and silica.

EDI Module Construction
Industrial scale EDI devices are available in two major configurations: plate-and-frame or spiral wound. While the majority of these systems employ first generation plate and frame designs, the spiral wound version has turn out to be the fastest growing technology in the EDI field. The reliability of the present Omexell SWEDI modules in the high purity water generation application is substantiated by more than 40,000 gpm capacity of successful installations in the last 3 years. The plate and frame type EDI devices are similar in construction to a plate heat exchanger, with multiple fluid chambers sandwiched between a set of endplates (and electrodes) that are held in compression by bolts or threaded rods. Plate and frame devices are commercially available in two main configurations. These are thin cell and thick cell, designated as such based on the thickness of the diluting chambers. Plate and frame device are large in size and characteristically suffer from leaks because of the difficulty of sealing large vessels. Moreover, the units often are oversized because of inflexibility in designing for capacity, necessitating undesirably high capital and operating costs14. The spiral wound membrane design incorporates a non-metallic pressure vessel more similar to RO membranes15. Spiral-wound EDI devices are commercially available in two main configurations: cathode at the center or anode at the center. The present SWEDI modules employ central cathode.

The present SWEDI membranes are similar to that of RO membranes in that the membranes and spacers are rolled to form a cylindrical element (See Figure 2). The EDI element is manufactured by placing a stainless steel concentrate pipe on a rolling machine and winding the membrane and spacers around the pipe. The element is then placed into a fiberglass pressure vessel and dilute chamber spacers are filled with resin. The unit is sealed inside the pressure vessel. The central stainless steel pipe acts as the concentrate distributor/ collector and the cathode. A titanium anode lines the inside of the fiberglass pressure vessel and become the anode15.

Omexell SWEDI System
When multiple EDI modules are present in one system, the feed flow enters the system through a header and is distributed to all modules. The treated water exits the diluting chambers while the water from the concentrating chambers is recirculated to promote mixing. The product flow is collected on the outlet of the modules and exits the system through another header. It is commonly known that the high flow rate of the concentrate stream achieved by employing recirculation of concentrate stream inhibits scales from forming. To prevent ion concentration from reaching the point of precipitation, a small stream is bled from the concentrated stream loop. By employing the recirculation of concentrate stream, the ion concentration within the concentrate water increases and this in turn leads to higher electrical conductivity of the concentrate water. If necessary, Omexell recommends injecting food grade brine that is low in metallic impurities in the concentrate loop of their SWEDI modules to maintain the conductivity of the concentrate stream between 250 and 600 mS/ cm at 25 ºC. A small portion of concentrated stream is also used to flush the electrodes to eliminate any build-up of gases. Electrolyte chambers are provided so that reaction products from the electrodes can be separated from the other flow chambers. The present SWEDI modules incorporate two terminal electrode chambers containing an anode and a cathode respectively, which are utilized to pass DC current transversely through the module. A small make-up stream from the feed water balances the flow lost through concentrate bleed and electrode flush. Figure 3 illustrates a typical process flow diagram. It is imperative to note that EDI systems are typically available with plastic piping due to the danger of accelerated corrosion from stray currents16. Stainless steel piping can be used if proper grounding techniques are adopted. Omexell offers skid mounted SWEDI systems that considerably reduce the footprint.

Omexell adopt a modular system approach for the demineralization of RO permeate water comprising a skid mounted plurality of EDI modules, patent-pending DC rectifier, concentrate recirculation pump, brine dosing system, instrumentations, PLC based control system, and interconnecting UPVC piping and fittings.

Industrial Applications
The requirements for high purity water in various industrial uses are immense. The purification of aqueous streams using EDI has become of increasing interest in many industries. EDI is the answer in demanding applications that require reliable and economical removal of ionic contaminants from aqueous streams without the application of hazardous chemicals. EDI has been extensively used in electronics, pharmaceutical, and power industries for high purity water production. Production of high-purity water in a cost-effective manner is becoming more and more important in these industries. To be competitive in the deregulated power market, authorities are looking for ways to lessen operating costs while maintaining an unfailing supply16. Effective application of EDI systems can in many cases provide a critical competitive edge.

Water Recovery and scaling
Scaling in an EDI device takes place when sparingly soluble impurities are concentrated in the concentrating chambers beyond their solubility limit. Scaling occurs in the concentrating chambers under conditions of high water recovery. For example, if an EDI system is operated at 90 percent recovery, the concentrations of various impurities in the concentrate chambers will be ten times the concentrations in the EDI feed stream. Scaling potential increases with the increase in the water recovery.

The concentrate stream accepts ions removed from the dilute stream and provides necessary pressure balance with the dilute stream to control salt transfer. Unfortunately, scaling predominantly occurs because the concentrate stream becomes rich in hardness or heavy metals- the ions which scale. Consequently, recovery rate is decided by EDI feedwater hardness. The following guidelines for the water recovery based on the feedwater total hardness are established for the present SWEDI modules (Table 1).

Scaling has been found to take place in localized regions of electrodeionization devices, and mainly those where high pH is characteristically present17. The Langelier Solubility Index (LSI) in the concentrate stream is commonly negative during the operation of the commercial electrodeionization devices that accepts feed water according to the recommendation of the EDI manufacturers. Thus, on the basis of consideration of LSI alone, one wound not expects the precipitation of calcium carbonate scale that occurs within concentrating chambers. This phenomenon is instead explained by the existence of high pH regions on the surface of the concentrate chamber side of anion exchange membranes18. It is believed that the pH at the concentrate chamber side of anion exchange membrane boundary layer augments with applied current. Therefore, the current needs to be optimized based on the desired product water quality and the incidence of scaling.

In the same way, scaling occurs in the alkaline environment of the cathode chamber. Due to high pH, scaling problem is expected at the cathode chamber. The following reaction takes place at the cathode:

2 H2O + 2 e- ® 2 OH- + H2 Eq. 2

CO32-, HCO3-, and OH- migrates through the anion exchange membrane from the diluting chambers are concentrated near the concentrate chamber side of anion exchange membranes. In addition, hardness contributing polyvalent cations in water in the concentrating chambers is drawn or driven to the anion exchange membrane, so that CO32-, HCO3-, and OH- react with Ca2+ to form scales of calcium carbonate on the surface of the concentrate chamber side of anion exchange membranes19. Such scale formation results in higher electrical resistance and less electric current flow at that section20. At the extreme condition, enough current to achieve the desired product water specification cannot be applied within the maximum voltage of the device, and the quality of the treated water declines.

Cross-flow designs in concentrate chambers in Omexell SWEDI Modules
Co-flow is the term given to the mode of EDI operation where the direction of flow in the concentrate chambers is the same as in the diluting chambers. Hardness-contributing polyvalent cations are readily removed from the diluting chambers flow path to the concentrate water flowing in the concentrate chambers. Such polyvalent cations continue to move further towards the cathode and reach the concentrate chamber side of anion exchange membranes near the concentrate chamber outlet. A high average rate of water splitting is normally achieved by applying a high voltage drop across the diluting chambers. Substantial amount of OH- ions migrate from diluting chambers to the concentrate water flowing in the concentrate chambers in the downstream section of the modules. The pH of the solution on the concentrate chamber side of the anion exchange membranes near the concentrate chamber outlet becomes very high and triggers the hardness scaling phenomenon.

Counter-flow is the term given to the mode of EDI operation where the direction of flow in the concentrate chambers is opposite to that in the diluting chambers. Assuming the flow in the diluting chambers to be upward, the flow in the concentrate chambers is introduced at the top of the module in a counter-flow operation. Counter-flow operation is intrinsically more efficient compared to the co-flow operation to avoid scaling. In this flow configuration, once the polyvalent cations migrate from the diluting chambers to the concentrate water flowing in the concentrate chambers, they exit the module immediately before reaching the surface of the anion exchange membranes. Consequently, the scaling potential reduces significantly in this mode of operation. However, it is imperative to operate the system with the concentrate inlet pressure be 5 – 10 psig less than the dilute outlet pressure in this flow configuration. To satisfy this criterion, this flow configuration necessitates a very high feed pressure for the operation of the EDI system. For instance, the minimum feed pressure should be at least 90 psig to provide 10 psig for the ΔP (Dout – Cin) when the concentrate inlet pressure and feed to product pressure drop are respectively 45 and 35 psig (See Figure 5). Such a high feed pressure augments the power consumption drastically and interferes with the sealing efficiency of the modules. Additionally, variation in ΔP across the membranes in counter-flow operation also undesirably stretches them. At this point, it is important to recognize that feed to product pressure drop increases significantly at higher flow rates and lower operating temperature. Pressure drop also increases with fouling of the ion exchange beads in the diluting chamber and agglomeration of organic contaminants in the concentrate spacers. Consequently, the feed pressure requirement may exceed the maximum allowable pressure of 100 psig in the counter-flow mode when the modules are operated on water streams with high organic impurity levels at high flow rates during the winter months. On the contrary, the minimum feed pressure is only 55 psig in conventional co-flow mode to provide 10 psig for the ΔP (Din – Cin) when the concentrate inlet pressure and feed to product pressure drop are respectively 45 and 35 psig (See Figure 5).

Cross-flow is the term given to the mode of EDI operation where the direction of flow in the concentrate chambers is perpendicular to that in the diluting chambers. Omexell SWEDI modules operate on a cross-flow mode (See Figure 6). This patented concentrate flow design of the present SWEDI modules is unlike that of the co-flow design employed in conventional plate and frame EDI devices11. The concentrate chamber is separated into two compartments: lower and upper. Spiral flow in the lower compartment is away from the central cathode while the flow direction in the upper compartment is towards the central cathode. Migration of strongly ionized polyvalent cations always occurs in the lower compartment of such concentrate chambers. Reversal of flow direction in the concentrate chambers in these SWEDI modules can be described as a two pass design. Such a two pass flow in the concentrate chambers promotes turbulence and lowers the scaling potential as the thickness of the localized alkaline boundary layers on the concentrate chamber side of the anion exchange membranes is greatly reduced. The use of multiple flow paths connected in series provides a tortuous path and good fluid distribution within the concentrate chamber. However, the hardness contributing polyvalent cations continue to move further towards the cathode at the center of the EDI modules and may reach the concentrate chamber side of the anion exchange membranes. In the cross-flow configuration, concentrate water flows spirally in a direction perpendicular to the upward flow path in the diluting chambers and the transverse DC electric field drives the various cations in a direction perpendicular to that of the spiraled concentrate flow. Resultant movement of the polyvalent cations under the presence of the boundary layer of the anion exchange membrane in a plane perpendicular to the plane in which dilute stream is flowing ensures a low concentration of such cations in the concentrate stream near the outlet of the dilute flow where the alkaline environment mainly exists in the concentrate side. This phenomenon greatly reduces the scaling potential in such cross-flow EDI operation. The unique cross-flow design of the present SWEDI modules results in higher feed water hardness tolerance compared to the conventional plate and frame devices. Vendors of plate and frame EDI devices suggest that the maximum total hardness in the feed to the EDI be limited to 0.5 to 1 ppm as CaCO3 whereas Omexell SWEDI modules can accept a maximum total hardness of 2 ppm as CaCO3 in the feed to the EDI. It sometimes translates to a reduction in capital expenditures for water treatment systems by eliminating the need for softeners and/ or double pass RO ahead of the EDI systems. Furthermore, even higher feed hardness levels are possible with increased frequency of cleaning.

Cross-flow mode enjoys fundamentally a similar pressure profile with co-flow mode. The minimum feed pressure is only 55 psig in cross-flow mode SWEDI to provide 10 psig for the ΔP (Din – Cin) when the concentrate inlet pressure and feed to product pressure drop are respectively 45 and 35 psig. Consequently, this unique cross-flow mode allows higher feedwater hardness without augmenting the power consumption compared to conventional plate and frame design operating in co-flow mode. It is worthy to note that the maximum allowable inlet pressure is 100 psig, which allows product water pressure up to 70 psig.

Ion Exchange Resin
Ion exchange resin acts as a path for ion transfer and also serves as an increased conductivity bridge between the membranes for the movement of ions. Customized intimate mixture of cation and anion resin is used in the present SWEDI modules to maximize its performance for a given feed water quality. Specially graded resins are used in the present SWEDI modules. The resin beads are flushed out in less than 10 minutes with water under a voltage gradient between the cathode and the anode in order to remove extractable ions from the resin beads.

As stated earlier the present SWEDI modules, the only serviceable EDI technique in the market, allows the replacement of resin at any time. Replacement of mixed resins in the present SWEDI modules is accomplished by various proprietary methods.

Feedwater requirements
Appropriate pretreatment of the water is a basic prerequisite for optimum performance of the EDI process to remove hardness, particulate matter, and chlorine. A major limitation of this process is the tendency of the membranes to become fouled by hard water constituents. Because of their requirements for softened pretreated water, EDI devices are always coupled with upstream RO units21. Chlorine will attack ion exchange resins and cause de-crosslinking, which results in reduced capacity. Particulate matter, organics, and colloids can cause the plugging and fouling of membrane and resin beds. Since carbon dioxide is typically present at significant concentrations in the RO permeate water and is removed in a combination of anionic bicarbonate and carbonate forms, the total exchangeable anion (TEA) load is usually greater than the cation load (total exchangeable cation, TEC) and determines the required EDI operating conditions22. If free carbon dioxide is present in the EDI feed, as it generally is, it will also be present in the concentrate, and since it is not ionic it will diffuse without restraint through the cation membrane back to the diluting chamber. It cannot diffuse through the anion membrane because most of the anion membrane is alkaline and the carbon dioxide would be converted into bicarbonate in the membrane and forced back into the concentrating chamber by the voltage gradient. Bicarbonate or carbonate ions are forced by the voltage gradient within the concentrating chamber towards the cation membrane. The boundary layer next to this membrane is acidic, as is the membrane itself. This converts both bicarbonate and carbonate into carbon dioxide, which can diffuse without restraint into the diluting chamber through the cation membrane23. Consequently, occurrence of carbon dioxide and alkalinity at more than the recommended values are mostly accountable for the degradation of EDI product water quality. The feed water requirements for the present SWEDI modules are as follows:

TEA ≤ 25 ppm as CaCO3
pH 5.0 – 9.0
Hardness ≤ 2 ppm as CaCO3
Dissolved Silica ≤ 1 ppm as SiO2
TOC ≤ 500 ppb
Free Chlorine ≤ 0.05 ppm
Fe, Mn ≤ 0.01 ppm
Carbon dioxide ≤ 10 ppm
Alkalinity ≤ 20 ppm as CaCO3
Temperature 10 – 38 ºC

At this point it is important to realize that feed water conductivity does not show a complete picture of the total ionic load in a water system. Conductivity measurement devices do not detect the full amount of weakly ionized species like CO2, silica, and boron24. Consequently, total ionic load is more accurately described by TEA (Total Exchangeable Anion) and FCE (Feed Water Conductivity Equivalent).

Eq. 3
FCE (µS/ cm) = Feed Water Conductivity (µS/ cm) + ppm CO2 X 2.66 + ppm SiO2 X 1.94 Eq. 4

All the EDI units have an upper operating temperature limit of approximately 38 °C. This limit is established by the spacer material, the membrane material, and the anion exchange resin present in the diluting chamber. As the feed water temperature decreases, reaction kinetics and diffusion rates decelerate and EDI module electrical resistance increases, causing a decline in performance.

Power Consumption
The main consumable in EDI is electricity. The applied DC current, not the voltage, is the main parameter for the design of EDI units. The amount of current used by an EDI module is a function of the flow rate and the amount of salt being removed. Faraday’s Law states that 96,500 amperes of electric current is required for one second to move one mole of ionic charge between electrodes. This is same as 26.8 amperes for one hour. One can also write one Faraday of charge is required to transfer one-gram equivalent of salt.

1 Faraday = 96,500 ampere-seconds = 26.8 ampere-hours

Eq. 5
Where,
I = Applied Direct current, amperes
F = Faraday’s constant, ampere-hours/ g-equivalent
Qp = Flow rate through dilute chambers, m3/ hour
DN = Change in normality of dilute stream between inlet and outlet, g-equivalent/ liter
E = Current efficiency in fraction, dimensionless
Ncp = Number of cell pairs, dimensionless

The above Equation 5 indicates that higher impurity concentrations and higher flow rate will entail a higher applied DC current in order to achieve the desired product water quality. Current efficiency is defined as the ratio of the current that transfer salt to the total amount of current applied. At this stage it is imperative to realize that applied current level primarily determines the resistivity, silica, and boron concentration in the product stream.

The current is supplied by a power source capable of automatically increasing or decreasing voltage in response to a change in the electrical impedance of the EDI module to maintain constant current. Over time, the electrical impedance of all the modules increase resulting in a gradual decrease in current at a given voltage level. All EDI manufacturers limit the maximum DC rectifier voltage to 350 – 600 VDC, in order to elude the need for the more expensive wiring construction that is necessary for higher voltages. Omexell rectifiers require 350 VDC for 40 gpm systems and larger, even though the modules are limited to 160 VDC due to the patent pending design of wiring multiple modules in series (See Figure 7). Although the rectifier is sized for 160 VDC/ module but the actual voltage requirement to achieve a desired current is normally in the vicinity of 75 – 110 VDC/ module (See Table 2). Actual power consumption can be computed by the following simple equation (6).

Power Consumption Eq. 6

Where,
V = Applied voltage, amperes
ηR = Rectifier Efficiency (fraction)

As the applied DC voltage to obtain a given current through the SWEDI modules is significantly lower compared to its plate and frame counterpart, SWEDI configuration may reduce power consumption by 20 – 25 percent.

Ohm’s law indicates that the current flowing through an EDI module is directly proportional to the voltage applied, and inversely proportional to the overall resistance of the module.

Eq. 7

Where, R represents the overall resistance of the EDI module. The above Equation 7 indicates that lower overall EDI module resistance will entail a lower voltage in order to pass a fixed amount of current through the module. The overall resistance is equal to the sum of the individual resistances, just as is the case in the flow of electric current through a series of resistances. The overall resistance of an EDI module is equal to the sum of that offered by individual membranes, resins, concentrate stream, anolyte, and catholyte at a given feed water temperature and ionic composition. For a given feedwater composition, electrical power necessary to attain a desired product water quality increases with the decrease in the water temperature for both plate and frame and SWEDI configurations.

The structural design of the present SWEDI modules reduces the distance between anode and cathode resulting in less overall module resistance when compared to conventional plate and frame modules. It is interesting to note that the distance between anode and cathode in the present SWEDI modules is smaller than its radius while anode and cathode are separated by the overall length of the module in the conventional plate and frame configuration. Lower overall module resistance in SWEDI configuration leads to reduction of necessary voltage when compared to conventional plate and frame counterpart to obtain a given current through the module. Consequently, reduction in power consumption by 20 – 25 percent is expected in SWEDI configurations when compared to conventional plate and frame modules. The steep escalation of energy costs in recent years has forced many to minimize the power consumption of various unit operations in high purity water systems. Rational utilization of energy in high purity water treatment facilities has received close attention of top management in the recent years. Power consumption has become critical plant design and operation consideration. The use of EDI devices in the spiral wound configuration can provide a significant contribution towards the needed conservation effort.

Performance Data
A full-size Omexell SWEDI system was operated on a varying feed water. Module desalination performance was evaluated by constructing a desalination performance curve (See Figure 8). Desalination performance of the present SWEDI modules was evaluated at various applied DC currents and feed water conductivities. The dilute flow rate was kept constant at 2.0 m3/ h per EDI module. During such experiments water temperature was maintained at 25 ± 1ºC. The curves all have the same characteristics shape, in the beginning showing a high resistivity plateau as the feed conductivity increased, which eventually gives way to deterioration in product resistivity at higher feed conductivity values. The rudimentary requirement is to provide sufficient current to attain the desired product water quality for a given feed water conductivity.

The removal of silica by EDI modules depends on the flow rate, feed silica levels, current, feed water conductivity, and temperature. Several experiments were carried out on the removal of silica by the present SWEDI modules at a constant flow rate of 2.0 m3/ h and 25 ºC and typical values are presented in Table 2. The silica rejection was studied under relatively high background feedwater conductivity. The silica rejection data are shown in three groups in Table 2 based on the applied DC current. The effect of variation in feed water conductivity and feed silica on the rejection of reactive silica at various current levels was investigated. It is apparent from Table 2 that the key parameter is the selection of the appropriate operating current for a desired silica rejection. Further enhancement in the rejection of silica can be obtained by reducing the flow rate through the EDI modules.

Similar experimental studies were conducted on the rejection of boron by the present SWEDI modules at a constant flow rate of 2.0 m3/ h and 25 ºC. The boron feed level was restricted to less than 300 ppb during the experimental study. Experimental results imply that the key parameter is the selection of the appropriate operating current for a desired boron rejection. Boron rejection exceeds 96% at an applied current level of 8 ampere and feed boron concentration of 100 ppb.

Case Study of an Omexell SWEDI System at the Aquila Lake Road Power Plant, Missouri
In Missouri, Aquila serves 282,000 electric and 48,000 natural gas customers. Based in Kansas City, the company provides electricity and natural gas service to 1.3 million customers in Kansas, Colorado, Missouri, Nebraska, Michigan, Minnesota, and Iowa25. Aquila utilizes a number of generating sources including coal, natural gas, and renewable energy such as wind power. Lake Road plant in Buchanan County is 253.8 net MW coal, natural gas, and oil-fired power plant. Consolidated Equipment Company, Omaha, Nebraska has won a contract to build a 125 US gallons per minute (gpm) high purity water treatment plant to provide condensate make-up for their existing boiler operation. The SWEDI system was first put into service in April 1, 2004. Feed water to this water purification plant comes from a well water source of average 983 ppm of total dissolved solids (TDS). This plant includes the following train of water treatment unit operations: 1 micron bag filter, UV disinfection unit, cartridge filter, double pass RO, and SWEDI. Consolidated installed the inlet filter and UV units according to the specification provided by Aquila. Consolidated provided PMD150 UV unit with an initial UV dosage level of 68 mJ/ cm2 and 30 mJ/ cm2 at end of lamp life (EOL) from Aquaionics for the purpose of water disinfection. Omexell supplied the remaining unit operations that include cartridge filter, double pass RO, and SWEDI. Cartridge filters are provided directly upstream of the RO high–pressure pump to protect the pump and lead end RO membrane elements. The double pass reverse osmosis system was designed to produce adequate SWEDI feed water using BW30-400 FilmTec membranes. The overall recovery rate of the double pass RO system is 71.8 percent. Consolidated preferred to stay with two pass RO, as opposed to single pass, due to fluctuation in the raw water TDS and high iron content of the well water used to generate the boiler feed water. The SWEDI capacity is set at 125 gpm. The primary motivation of the customer for selecting EDI was to eliminate the chemicals needed for regeneration conventional mixed bed deionizers. This skid mounted SWEDI system operates at 95 percent recovery. Conductivity of the concentrate stream is maintained at 215 – 275 µS/ cm at 25 ºC. SWEDI performance, measured as product resistivity, has remained consistently high at 14 – 16 MΩ-cm at 25 ºC. Silica removal has also remained very high. Silica feed to the SWEDI unit from our double pass RO is typically in the range of 4 – 18 ppb with silica in the SWEDI product water averaging a consistent below the instrument detection limit of 1 ppb.

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About the author
Dr. Avijit Dey received his Ph.D. in chemical engineering from Jadavpur University, Calcutta, India. He is currently technical manager at Omexell Inc., a research and development, design, engineering, manufacturer specializing in Integrated Membrane Technology (IMT) equipment used in the process of water treatment. Omexell has been developing membrane technologies since 1998 and has now emerged as a leader in IMT with its patented spiral wound EDI and unique hollow fiber UF technologies. Dey can be reached at (713) 973-9731 or via email at info@omexell.com.

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