WO2001020060A1

WO2001020060A1 - Apparatus and methods for electrodeionization of water

Info

Publication number: WO2001020060A1
Application number: PCT/US2000/024858
Authority: WO
Inventors: Leon Mir
Original assignee: Leon Mir
Priority date: 1999-09-13
Filing date: 2000-09-11
Publication date: 2001-03-22
Also published as: WO2001020060A9; AU7367800A

Abstract

Electrodeionization apparatus (10) for the purification of water including a cathode (12), an anode (14), a plurality of alternating cation and anion permeable membranes (16, 18) that define concentrating and diluting flow channels (22, 20) between adjacent membrane pairs. The diluting channels include cation and anion exchange materials (24) fixed in close contact position to provide conductive paths for ions of the adjacent membranes and flow passages for water between the material. The packing material can include one or more macrostructural elements made of smaller microstructural elements. First and second stages can be used to purify water including calcium, carbon dioxide, and its hydrates, wherein the ion exchange material differs in the diluting flow channel of each stage, and the concentrating flow channels can include a brine channel between two guard channels.

Description

APPARATUS AND METHODS FOR ELECTRODEIONIZATION OF WATER

TECHNICAL FIELD

The invention relates to apparatus and methods for carrying out electrodeionization to purify water.

BACKGROUND

Electrodeionization is a process for removing ions from liquids by sorption of these ions into a solid material capable of exchanging these ions for either hydrogen ions (for cations) or hydroxide ions (for anions) and simultaneous or later removal of the sorbed ions into adjacent compartments by the application of an electric field. (See Glueckauf, E., "Electro-Deionization Through a Packed Bed", Dec. 1959, pp. 646-651, British Chemical Engineering for a background discussion.) The hydrogen and hydroxide ions needed to drive the ion exchange process are created by splitting of water molecules at the interface of anion and cation exchanging solids that contact each other in the orientation that depletes the contact zone of ions, when in the presence of an electric field. This orientation requires that the anion exchanging material face the anode and the cation exchanging material face the cathode. The created hydroxide ions enter the anion exchanging material, and the created hydrogen ions enter the cation exchanging material.

The electrodeionization process is commonly carried out in an apparatus consisting of alternating diluting compartments and concentrating compartments separated by anion permeable and cation permeable membranes. The diluting compartments are filled with porous ion exchanging solid materials through which the water to be deionized flows. The ion exchanging materials are commonly mixtures of cation exchanging resins and anion exchanging resins (e.g., U.S. Patent 4,632,745), but alternating layers of these resins have also been described (e.g., U.S. Patents Nos. 5,858,191 and 5,308,467). Ion exchanging materials consisting of woven and non-woven fibers have also been described. (E.g., U.S. Patent No. 5,308,467 and 5,512,173). The compartments adjoining the diluting compartment into which the ions are moved by the applied electric field, called concentrating compartments, may be filled with ion exchanging materials or with inert liquid permeable materials. An assembly of one or more pairs of diluting and concentrating compartments, referred to as a "cell pair", is bounded on either side by an anode and a cathode which apply an electric field perpendicular to the general direction of liquid flow. Flow of water is provided past the anode and cathode.

The diluting compartments are each bounded on the anode side by an anion permeable membrane and on the cathode side by a cation permeable membrane. The adjacent concentrating compartments are each correspondingly bounded by a cation permeable membrane on the anode side and an anion permeable membrane on the cathode side. The applied electric field causes anions to move from the diluting compartment across the anion permeable membrane into the concentrating compartment nearer the anode and cations to move from the diluting compartment across the cation permeable membrane into the concentrating compartment nearer the cathode. The anions and cations become trapped in the concentrating compartments because the movement of anions toward the anode is blocked by a cation permeable membrane, and the movement of cations toward the cathode is blocked by an anion permeable membrane. A flow of water is set up to remove the ions from the concentrating compartments. The net result of the process is the removal of ions from the water stream flowing through the diluting compartments and their concentration in the water flowing through the concentrating compartments.

The removal of the ions from the diluting compartment is a multi-step process involving diffusive steps as well as electrically driven steps. First, it is clear that the movement of ions directly from the diluting solution across the bounding membranes, under the influence of the applied electric field, contributes insignificantly to the overall removal of these ions, because the concentration of ions in the diluting solution is typically 1 ,000 to 100,000 times smaller than the concentration of ions in the solid ion exchanging materials. While the mobility of ions in the solid material may be on the order of 20 times smaller than their mobility in the solution, the electric field acting on the ions in the two phases is the same, so the product of mobility times concentration times electric field strength, which determines the rate of ion removal, is 50 to 5,000 times as large in the solid ion exchanging material.

Glueckauf showed that the mechanism of ion removal from the diluting compartment solution includes two steps. The first step is the diffusion of cations to the cation exchanging solids and the diffusion of anions to the anion exchanging solids. The second step is electrical conduction within the solids phases to the bounding membranes of the diluting compartment. Because the concentration of ions in ion exchanging solids is so high, the process that controls the overall removal of ions is their rate of diffusion from the solution to the surface of the ion exchanging solids. This diffusion rate is a function of three factors; the diffusion rate is proportional to surface area between the ion exchanging solids and the flowing solution, inversely proportional to the thickness of the liquid layer through which the ions must diffuse, and proportional to the difference in concentration of the ions in the bulk of the diluting solution and their concentration next to the ion exchanging solid. In order to achieve high rates of ion removal, the product of the above three factors should thus be as high as possible. The ratio of the surface area to the diffusion distance is inversely proportional to the characteristic dimension of the ion exchanging solid material; the characteristic dimension is particle radius for ion exchange resins and is fiber radius for ion exchange fibers. In designing electrodeionization apparatus, this characteristic dimension can be made as small as possible, commensurate with avoidance of excessive pressure drops or plugging by particles in the water to be treated. Particle diameters on the order of 500 to 600 micrometers are typical, and fiber diameters can be on the order of several tens of microns.

As noted above, the third factor controlling the rate of ion removal is the difference in concentration of the ion being removed between the bulk of the solution and its concentration in the liquid adjacent to the surface of the ion exchanging solid where it is being exchanged for either a hydrogen or a hydroxide ion. The concentration of the ion in question at the surface of the ion exchanging solid is in equilibrium with the concentration of that ion in the solid. For cations, the equilibrium concentration is approximately equal to the ratio of the cation concentration to the hydrogen ion concentration in the cation exchanging solid times the concentration of the cation in solution. For anions, the equilibrium concentration is approximately equal to the ratio of the anion concentration to the hydroxide ion concentration in the anion exchanging solid times the concentration of the anion in solution. In order for this equilibrium concentration to be low, and the rate controlling concentration difference to be large, the cation exchanging solid should be predominantly in the hydrogen form, and the anion exchanging solid should be predominantly in the hydroxide form. In fact, if the two solids are completely in the ionic form rather than in the hydrogen or hydroxide form, there is no concentration difference, and ions will not be removed by this diffusive mechanism. In order for the ion exchanging solids to be predominantly in the hydrogen and hydroxide forms, the so-called "regenerated forms," the rate of hydrogen ion and hydroxide ion creation (water splitting) must be both high and spatially uniform. A high average rate of water splitting can be achieved by applying a high voltage drop across the diluting compartment. With equinormal mixtures of ion exchange particles, voltages of between 1 and 5 volts are adequate for the purpose. The achievement of a uniform distribution of water splitting is a more difficult problem and much effort has gone into designing structures that achieve this (e.g., U.S. Patents Nos. 5,858,191, 5,868,915 and 5,308,467). The random nature of mixtures of cation and anion exchanging particles tends to cause some portion of the particles to be regenerated to a needlessly high degree and others inadequately regenerated. Water flowing through the regions of inadequately regenerated material will be inadequately purified. The essence of the difficulty that existing approaches have had in dealing with the problem is that the number of contacts between cation exchanging and anion exchanging material where water splitting can take place is limited by the relatively large characteristic dimension of the ion exchanging material. This results in regions of inadequately regenerated resin between the water splitting sites.

Electrodeionization (EDI) stacks frequently suffer from precipitation of calcium carbonate in the concentrating compartments as well as in the cathode compartment. (See AEDI and Membranes: Practical Ways to Reduce Chemical Usage when Producing High Purity Water, William E. Katz in Ultrapure Water, Vol. 16, No. 6 July/August 1999, pp 52 - 57). The consequence of this "scaling" is an increase in stack resistance, a drop in current density and eventually a sharp decrease in the purity of the product water.

Vendors of EDI equipment suggest that the concentration of calcium in the feed to the EDI be limited to very low levels; e.g., less than 0.5 ppm. (U.S. Filter Literature No. US2006). While this concentration can be achieved when the electrodeionization apparatus is fed with reverse osmosis (RO) permeate from an RO system with new membranes, and the RO system is operating properly, the suggested values can be exceeded when these conditions do not hold.

In order for calcium carbonate to precipitate in solution the Langelier Saturation Index (LSI) has to be positive. In the cathode compartment the pH can be high enough for the LSI to be positive; precipitation of calcium carbonate is therefore to be expected under some circumstances. The LSI of RO permeates is always negative. Even in the EDI brine the concentrations of calcium and bicarbonate are so low that the LSI is still negative, at the prevailing pH. Thus, on the basis of consideration of LSI alone, one would not expect the precipitation of calcium carbonate that occurs within EDI concentrating compartments. This phenomenon is instead explainable based upon local conditions.

When a concentrating compartment from a "scaled" EDI stack is examined, the scale is observed on the anion membrane, predominantly halfway between the inlet and the outlet of the stack. This pattern can be explained on the basis of the mechanism by which an EDI stack removes weak acids like carbon dioxide and silica. At the pH of RO permeate, only a tiny fraction of silica is ionized, and a large fraction of the total inorganic carbon (TIC) is in the form of carbon dioxide. In order for silica and carbon dioxide to be removed by EDI, the feed solution needs to contact anion exchange resin in the diluting compartment, which is partly in the OH- form, regenerated. Carbon dioxide and silica diffuse from solution into the partly regenerated anion resin and react with the OH- to form the HC03-, CO 3 •= and HSi03- anions which are moved, along with substantial amounts of OH-, by the applied voltage gradient, into the concentrating compartment. In order for the above mechanism to operate, the voltage drop in the diluting compartment has to be high enough, typically 2 to 3 volts, to regenerate some portion of the anion resin by the splitting of water into OH- and H+.

At the inlet portion of an EDI stack the extent of resin regeneration in the diluting compartment is low. Carbon dioxide and silica are therefore not removed in the front part of the stack. Toward the middle of the stack the concentrations of the ions in the feed water have dropped sharply and water splitting takes place. The resins are partly regenerated and the carbon dioxide and silica are removed. The pH of the solution on the concentrating side surface of the anion membrane is therefore very high; the concentration of C03= is also high, and the LSI can be positive at the concentration of calcium prevailing in the concentrate. (See U.S. Patent No. 5,593,563). Calcium carbonate can therefore precipitate, as shown in Fig. 5. Note that the LSI within the bulk of the concentrate is still negative because the pH of the concentrate is virtually the same as that of the feed. The high pH at the surface of the anion membrane and the corresponding low pH at the surface of the cation membrane are boundary layer phenomena.

Toward the outlet of the stack virtually all of the anions have been removed. Although the concentrating side of the anion membrane is still at a very high pH, the concentration of C03= is so low that the LSI index is negative, and calcium carbonate does not precipitate. If it were not for the need to remove the weak acids by operating the EDI stack in a partly regenerated form, there would not be any problem with calcium carbonate precipitation. In order for EDI stacks to replace ion exchange beds, which remove these weak acids, EDI stacks must be operated in a partly regenerated form and consequently calcium carbonate precipitation is always a threat.

The problem of calcium carbonate precipitation has been broadly recognized, and various suggestions have been made to deal with it. One approach is the periodic reversal of the role of the diluting and concentrating compartments with a simultaneous reversal of the polarity of the electrodes. (E.g., U.S. Patent Nos. 4,956,071 and 5,558,753). Drawbacks of this approach include the production of low quality water during some parts of the operating cycle and the complexity and expense of the valving system needed to implement the process.

The special problem of calcium carbonate precipitation in the cathode compartment, exacerbated by the formation of hydroxide ions, has been dealt with by filling the cathode compartment with an electrically conductive medium. (E.g., U.S. Patent No. 5,593,563). This is said to reduce the concentration of hydroxide ions at the surface of the electrode by distributing the current over a larger area and thus decreasing the degree of calcium carbonate supersaturation.

Calcium carbonate scaling can be prevented by reducing or eliminating any of the three prerequisites of scaling: calcium, carbon dioxide and bicarbonate or alkaline pH. The brute force chemical approaches—softening the RO feed or adding acid to the cathode compartment or to the concentrate~re-institute the very problems of chemical supply and waste disposal that EDI is designed to eliminate and are therefore fundamentally unacceptable. More acceptable approaches are the softening of the EDI feed, which has a much lower concentration of calcium than the RO feed, or the removal of carbon dioxide by air stripping. These approaches entail additional capital costs and operating expenses. It has also been suggested that the concentration of salts in the concentrating compartment be reduced by reducing the fraction of the feed water that is recovered as the pure water stream. This approach is fundamentally unacceptable because of its expense.

Although at times product water from an electrodeionization (EDI) stack may exceed 18 mega ohms, in general vendors of EDI equipment will only guarantee a much lower water purity. (US Filter Literature No.: US2006). Application of EDI for the preparation of water for high-pressure boilers and semiconductor manufacture therefore requires that mixed ion exchange polishers be used to treat the EDI product water. It would be desirable to have an EDI system that could reliably produce water equal to that of mixed ion exchange beds. If an EDI system were developed that reliably produced a water with conductivity in the range of 18 mega ohms it would offer very significant advantages in the design of high purity water systems.

In an EDI stack operated at a sufficiently high current density, the ion exchange resin will be so highly regenerated that it should be capable of producing water equivalent to that produced by mixed ion exchange beds. There are two mechanisms responsible for the degradation of the potential performance of EDI stacks. The one with the largest impact is the diffusion of carbon dioxide from the concentrating compartment back into the diluting compartment. 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 freely through the cation membrane back to the diluting compartment. 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 compartment by the voltage gradient. Even when no free carbon dioxide is present in the feed water, but bicarbonate ion is present, the problem persists. Bicarbonate or carbonate ions are forced by the voltage gradient within the concentrating compartment toward 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 freely into the diluting compartment through the cation membrane. The involvement of carbon dioxide in degrading the purity of EDI product water is described in the patent literature, (e.g., U.S. Patent No. 5,868,915). Efforts to remedy this problem have centered on increasing the degree and uniformity of regeneration of the anion exchanging resin in the diluting compartment. These efforts have not been successful because the origin of the problem lies, as pointed out above, in the concentrating compartment.

The second, more fundamental mechanism that limits the ultimate purity of the water that can be produced by EDI is the imperfect permselectivity of ion exchange membranes; i.e., some cations from the concentrating compartment penetrate the anion membrane and some anions penetrate the cation membrane. In both cases the voltage gradient will then force them into the diluting compartment. The Donnan equation (see ADemineralization by Electrodialysis@ by J.R. Wilson Butterworth Scientific Publications, 1960, p. 56) predicts that the penetration of co-ions into the membranes decreases with a decrease in the concentration of these ions in the concentrating compartment. The parasitic processes are illustrated in Fig. 33, which shows a single concentrating channel 510, having cation permeable membrane 512, anion permeable membrane 514 oriented as indicated between anode 516 and cathode 518.

It is possible to reduce the impact of these parasitic transfer processes by, e.g., reducing the concentration of ions in the concentrating stream by reducing the fraction of feed water that is recovered. This approach has a relatively small effect at any reasonable water recovery. Another possibility is to divide the EDI system into two sequential parts. The second, polishing part, would operate with a very low concentration of ions in the permeate and thus produce a higher quality water.

SUMMARY

In one aspect, the invention features, in general, electrodeionization apparatus for purifying water. The apparatus includes a cathode, an anode, and a plurality of alternating anion permeable membranes and cation permeable membranes between the cathode and anode that define concentrating and diluting flow channels between adjacent pairs of membranes. The diluting channels include cation exchange materials and anion exchange materials that are fixed in close contacting position with respect to each other and provide conductive paths for cations and anions to the adjacent membranes and provide flow passages for water between the materials. The anion exchange materials and cation exchange materials each have a characteristic dimension that is smaller than the characteristic dimensions of the flow passages. The use of exchange materials with small dimensions and the fixed intimate contact of cation and anion exchange materials provides increased, uniform water splitting and resin regeneration, and a high rate of ion removal from the water flowing through the diluting channels compartments.

Particular embodiments of the invention may include one or more of the following features. Individual particles of cation exchange material and anion exchange material can be fixed together with a binder in sufficient particle concentration to provide conductive paths for cations and anions. The particles of cation exchange material, anion exchange material and binder can form larger combined particles packed into the diluting flow channel in contacting relation between adjacent membranes. The combined particles are sufficiently large so as to cause an acceptably low pressure drop in the diluting flow channel.

Alternatively, the particles of the cation exchange material and the anion exchange material and binder can form filaments provided as a matrix between the adjacent membranes. The openings in the matrix for water flow are larger than the diameter of the filaments. A further alternative is to have particles of cation exchange material, anion exchange material and binder form an open cell foam between adjacent membranes, with the openings in the foam being sufficiently large to provide flow passages through the foam with an acceptably low pressure drop.

The fixed ion exchange material could also be provided as cation exchange filaments and anion exchange filaments that are intimately commingled or joined together.

When the fixed ion exchange materials are in the form of filaments, they can be provided in multiple filament bunches or as multiple filament braids. The strands, made of bunches, braids, or individual filaments, can be fixed with respect to other strands by providing them as a woven fabric, nonwoven (randomly oriented) fabric or extruded netting. The fabric could also be provided by extrusion.

Preferably the majority of combined particles (also referred to as "macrostructural elements" herein) have dimensions greater than 0.1 mm, and the majority of individual particles of the cation and anion exchange material have dimensions less than 0.1 mm. (When the combined particles do not have pores, the particles can be smaller.) The combined particles preferably are sufficiently large so as to cause an acceptably low pressure drop (e.g., less than 100 psig) in the diluting flow channel. The filaments (as macrostructural elements) can have diameters between 0.1 mm and 3.0 mm. The fabric includes groups of generally parallel filaments, with filaments spaced center-to-center by a distance equal to or greater than the diameter of filaments. The binder used to fix the individual cation and anion particles is preferably a thermoplastic polymer or thermosetting polymer, but can be any water insoluble bonding material. The cation and anion exchange materials are made of styrenic ion exchange resin, acrylic ion exchange resin, phenolic ion exchange resin, or carbohydrate ion exchange resin.

In another aspect, the invention features, in general, obtaining an increased velocity in the diluting channels of electrodeionization apparatus by reintroducing a portion of the water from the diluting channel outlet to the diluting channel inlet or using flow diverters in the diluting channel to provide a tortuous path for the flowing water, while keeping the volume of the diluting channel substantially unchanged. The diffusion distance is decreased by increasing the velocity of the water flowing past the ion exchanged particles.

Embodiments of the invention may include one or more of the following advantages. A substantially spatially uniform rate of water splitting is achieved in the diluting channels; the uniform rate is conducive to a high and uniform degree of resin regeneration and consequently a high rate of ion removal from the water flowing through the diluting compartments. The uniform regeneration of the anion resin additionally facilitates removal of silica and carbon dioxide. The small size of the ion exchanging particles or filaments insures that numerous and uniformly distributed sites for water splitting are created without creating excessive pressure drops, because the dimension of the passages for water flow can be made larger, without affecting adversely the water splitting properties of the material.

In another aspect, the invention features, in general, a packing for an electrodeionization compartment that includes one or more macrostructural elements (e.g., beads, strands, brands or foam) made up of smaller, microstructural elements (e.g., particles or fibers). The microstructural elements are in fixed, close contacting position with respect to each other in the macrostructural elements so as to provide porosity in the macrostructural elements. A majority of the microstructural elements have a characteristic dimension between 5 and 50 micrometers, and the macrostructural elements have a void fraction interior to the macrostructural elements between about 25% and 50%.

In another aspect, the invention features electrodeionization apparatus including a cathode, an anode, a plurality of alternating anion permeable membranes and cation permeable membranes between the cathode and anode defining concentrating and diluting flow channels, and ion exchange packing in the diluting flow channels including macrostructural elements and microstructural elements as already described.

Preferred embodiments of the invention may include one or more of the following features. The microstructural elements can be cation exchange material, anion exchange material or a mixture of the two. The characteristic dimension of the microelements preferably is between 7 and 40 micrometers, and the void fraction is less than 45%. The macrostructural elements, when other than a foam, can each have a characteristic dimension between about 0.3 and 3 mm. Microstructural elements in the form of particles can be held together by binder. Macrostructural elements in the form of beads can include particles as microstructural elements held together by binder. Each bead includes micropassages between the particles in a bead and macropassages in the packing between the beads.

Macrostructural elements in the form of strands can be single-fiber strands that are porous (e.g., including particles as microelements held together by binder) or strands made up of a plurality of fibers. In both of these cases there will be micropassages between the microelements and macropassages between the strands. The strands can be provided in braids, in which case the braids preferably are oriented so that the longitudinal axes of the strands make an angle of between 30 and 60 degrees with the direction of water flow through the diluting channel.

Macrostructural elements in the form of an integral foam element can include particles as microelements that are held together in a porous polymer foam binder.

Embodiments of the invention may have one or more of the following advantages. A packing having macroelements made up of a plurality of microelements of specified size and void fraction permits large effective active surface area for the ion exchange material at the same time that pressure drop is maintained at acceptable levels. The microelement size and void fraction provide pores (also referred to as micropassages) between the microelement particles or fibers that are sufficiently large so that adequate quantities of water can flow through these pores under the influence of the pressure gradient established in the diluting compartment by the flow of water through the large channels between the beads, strands or braids. This flowing water supplies the interior of the beads or braids with a much larger quantity of ions than can be supplied by simple diffusion.

In another aspect, the invention features, in general, using first and second stages in electrodeionization to purify water including calcium and carbon dioxide and its hydrates. The diluting flow channels of the first stage include only anion exchange material or cation exchange material, and thus remove either carbon dioxide and its hydrates (and other anions) or calcium (and other cations) but not the other. The diluting flow channels of the second stage receive the diluting channel effluent from the first stage and include the other type of exchange resin (or a mixed resin) and remove the oppositely charged ions. The brine effluent from the concentrating flow channels in the first stage is isolated from the second stage, and calcium and carbon dioxide and its hydrates tend to be removed in different stages so as to deter calcium carbonate precipitation in any of the concentrating flow channels. In another aspect, the invention features, in general, using electrodeionization to purify water including calcium and inorganic carbon. The concentrating channels include cation exchange material nearer to the anion membrane than the cation membrane, and pH is lowered at the surface of the anion so as to limit calcium carbonate precipitation in the concentrating flow channel.

Preferred embodiments of the invention may include one or more of the following features. Each concentrating flow channel can include anion exchange material between the cation membrane and the cation exchange material so that water splitting occurs between the anion exchange material and the cation exchange material. The anion exchange material in the concentrating channel is in a first fixed structure, and the cation exchange material in the concentrating channel is in a second fixed structure. The anion exchange material and cation exchange material in the concentrating channel can directly contact each other, or they can be separated by an a cation membrane, cation/anion membrane pair, or a bipolar membrane. Water splitting occurs at the interface of an anion material or membrane with a cation material or membrane. The anion exchange material or cation exchange material in the concentrating channel can be provided as two layers with a membrane that inhibits mixing of the brine in the anion exchange layer with the brine in the cation exchange layer, such as a dialysis membrane, located between the two layers. This membrane is chosen so as not to increase the electrical resistance of the concentrating compartment unduly. One can use the various membranes and layers in the concentrating channels in single stage electrodeionization or in two-stage electrodeionization to provide even further improvements in reduced scaling.

In another aspect, the invention features, in general, using electrodeionization to purify water including calcium and inorganic carbon. There is countercurrent flow in the diluting flow channels and the concentrating flow channels, such that calcium is avoided at the concentrating side of the anion membranes in the region where scaling is likely to occur so as to limit calcium carbonate precipitation in a concentrating flow channel. In particular embodiments, the diluting flow channels include a mixture of cation and anion resins. Alternatively, the diluting flow channels can include cation resin only at the diluting inlets.

In another aspect, the invention features, in general, using electrodeionization to purify water including calcium and inorganic carbon by flowing the feed to the concentrating compartment first through a region that renders it substantially acidic before it enters a region that contains calcium.

In particular embodiments the concentrating flow channel includes first and second flow channel portions in overlying relation, with the outlet of the first flow channel being connected to the inlet of the second flow channel. The first flow channel includes an anion resin, and the second flow channel includes a cation resin. The first flow channel and the second flow channel can be separated by a cation membrane or by a bipolar membrane. The first channel inlets and the second channel outlets can be adjacent to the diluting outlets, and the diluting inlets can be the adjacent to the first channel outlets and the second channel inlets. Alternatively, the first channel inlets and the second channel inlets can be adjacent to the diluting outlets, and the diluting inlets can be adjacent to the first channel outlets and the second channel outlets. The second channel outlet can be connected to divert a portion of its effluent to the first channel inlet in order to maintain a high flow rate without the use of excessive amounts of fresh feed.

Embodiments of the invention may include one or more of the following advantages. The tendency of scaling is reduced by modifying the design of the EDI stack without additional unit operations.

In one aspect, the invention features, in general, electrodeionization apparatus for purifying water. The apparatus includes a cathode, an anode, and a plurality of alternating anion permeable membranes and cation permeable membranes between the cathode and anode that define concentrating and diluting flow channels between adjacent pairs of membranes. Each concentrating flow channel includes a first guard channel adjacent to the anion permeable membrane, a second guard channel adjacent to the cation permeable membrane, and a brine channel between the first and second guard channels. The first and second guard channels have water with lower concentration of dissolved ions than water in the brine channel so as limit parasitic transfer from a concentrating flow channel to a diluting flow channel.

Preferred embodiments of the invention may include one or more of the following features. In preferred embodiments a further anion permeable membrane separates the first guard channel from the brine channel, and a further cation permeable membrane separates the second guard channel from the brine channel. The first guard channel includes anion exchange material, and the second guard channel includes cation exchange material. Embodiments of the invention may include one or more of the following advantages. The anion exchange resin in the first guard channel and the additional anion membrane act as a transfer layer for the anions moving to the middle brine channel, while the cation exchange material in the second guard channel and the additional cation membrane act as a transfer layer for the cations moving to the middle brine channel. The water (e.g., feed or purified) flowing through both guard channels keeps the concentration of ions at a very low level and thus virtually eliminates both back-diffusion processes.

The details of one or more embodiments of the invention are set forth in the accompanying drawings and the description below. Other features, objects, and advantages of the invention will be apparent from the description and drawings, and from the claims.

DESCRIPTION OF DRAWINGS

Fig. 1 is a diagram of electrodeionizing apparatus.

Fig. 2 is a diagrammatic plan view of a woven fabric of deionizing material used in a diluting channel of the Fig. 1 apparatus.

Fig. 3 is a diagrammatic plan view of a fabric of extruded netting of deionizing material used in a diluting channel of the Fig. 1 apparatus.

Fig. 4 is a diagrammatic plan view of a nonwoven fabric of (randomly oriented) strands of deionizing material used in a diluting channel of the Fig. 1 apparatus.

Fig. 5 is a diagrammatic elevation of a multifilament strand of deionizing material useful in the Fig. 2, 3 or 4 fabric.

Fig. 6 is a diagrammatic elevation of a braided strand of multifilaments of deionizing material useful in the Fig. 2, 3 or 4 fabric.

Fig. 7 is a diagrammatic perspective view of a filament that contains cation exchange and anion exchange fibers in a binder and can be used in the Fig. 2, 3 or 4 fabric.

Fig. 8 is a diagrammatic vertical sectional view showing the woven fabric of Fig. 2 in a diluting channel of the Fig. 1 device.

Fig. 9 is a diagrammatic perspective view of an open cell foam that contains cation exchange and anion exchange materials therein and can be used in a dilutmg channel of the Fig. 1 apparatus.

Fig. 10 is a diagrammatic elevation of combined ion exchange particles useful in the diluting channel of the Fig. 1 apparatus. Fig. 11 is diagram showing a partial recirculation loop that can be used with the Fig. 1 apparatus.

Fig. 12 is a diagrammatic plan view showing the use of flow diverters in diluting channels of the Fig. 1 apparatus.

Fig. 13 is a diagram illustrating a braid structure that permits loose packing of filaments to achieve a desired void fraction.

Fig. 14 is a diagram illustrating filaments perpendicular and parallel to direction of flow.

Fig. 15 is a diagram illustrating orientation of filaments at an angle to direction of flow to promote flow through micropassages.

Fig. 16 is a flow diagram of a single-stage electrodeionizing apparatus.

Fig. 17 is a flow diagram of a two-stage electrodeionizing apparatus.

Fig. 18 is flow diagram of an alternative two-stage electrodeionizing apparatus.

Fig. 19 is a diagram showing the conditions involved in scaling in a concentrating cell of electrodeionization apparatus.

Figs. 20-25 are diagrams of alternative embodiments for concenfrating cells useful in the Fig. 1, 2 or 3 apparatus.

Figs. 26-30 are diagrams of alternative embodiments for the concentrating and diluting channels of an electrodeionizing apparatus.

Fig. 31 is a diagram of pH versus position in concentrating and diluting channels for the embodiment of Fig. 27.

Fig. 32 is a diagram of a serpentine flow path in a concentrating channel.

Fig. 33 is a diagram illustrating parasitic processes in a single concentrating channel of electrodeionizing apparatus.

Fig. 34 is a diagram of a concentrating channel of the Fig. 1 apparatus.

Like reference symbols in the various drawings indicate like elements.

DETAILED DESCRIPTION

Referring to Fig. 1 electrodeionization apparatus 10 includes cathode 12, anode 14 spaced from cathode 12, and a plurality of alternating anion permeable membranes 16, and cation permeable membranes 18. Diluting channels 20 ("D") are provided between each pair of an anion permeable membrane 16 that faces anode 14 and a cation permeable membrane 18 that faces cathode 12. Concentrating channels 22 ("C") are provided between each pair of an anion permeable membrane 16 that faces cathode 12 and a cation permeable membrane 18 that faces anode 14. Diluting channels 20 and concentrating channels 22 can be about 3.0 mm thick. Fixed ion exchange materials 24 are located in diluting channels 20, and ion exchange materials or other spacers 25 are located in concentrating channels 22. As discussed in detail below, fixed ion exchange materials 24 can take a variety of forms. Cathode 12, anode 14, membranes 16, 18 and spacer materials 25 can be made of components and materials typically used in electrodeionization apparatus, as described, e.g., in the above-referenced patents, which are hereby incorporated by reference. Water flows are provided past cathode 12 and anode 14. As is well known in the art, the components shown on Fig. 1 are assembled together as a stack between the pressure plates held together by bolts or a hydraulic ram or in a housing that contains the components and provides manifolds to direct the incoming liquid to and the outgoing liquid from diluting channels 20 and concentrating channels 22. Diluting channels 20 and concentrating channels 22 are typically between 1.0 mm and 5.0 mm thick, and there typically are 10 to 300 diluting channels. The surface area of each membrane is typically between 0.5 and 5.0 square feet.

Fixed ion exchange materials 24 include cation exchange materials and anion exchange materials that are fixed in close contacting position with respect to each other. Fixed ion exchange materials 24 can be provided in strands 26 of combined anion and cation exchange materials in woven fabric 28 (Fig. 2), extruded netting fabric 30 (Fig. 3) and nonwoven fabric 32 of randomly oriented strands 26 (Fig. 4). Fixed ion exchange materials could also be provided by open cell foam 50 (Fig. 9) and by combined exchange particles 60 (Fig. 10).

Strands 26 (Figs. 2-4) can also take a variety of forms. Strand 26 can be made in the form of bundle 34 of multiple filaments 36, as shown in Fig. 5. Strand 26 can also be in the form of braided strand 38, as shown in Fig. 6; braid 38 is made on a standard braiding machine. Strand 26 can also be in the form of combined exchange particle filament 40, which is made of cation exchange particles 42 (shown white on Fig. 7) and anion exchange particles 44 (shown dark on Fig. 7) that are held together by binder 46. Filaments 36, used in bundle 34 (Fig. 5), could be made of roughly equal, commingled amounts of individual filaments of cation exchange material and individual filaments of anion exchange material. Alternatively, combined exchange particle filaments 40 (Fig. 7), each having cation exchange particles and anion exchange particles, could be used as filaments 36 in bundle 34. Combined exchange particle filaments 40 could similarly be used in making braid 38, using either a single filament 40 or a plurality of filaments 40 in each braided together portion 48 of braid 38. Each portion 48 of braid 38 could also be made of a plurality of commingled filaments of cation exchange material and filaments of anion exchange material.

Referring to Fig. 9, fixed ion exchange materials 24 could also be provided as open cell foam 50, which (like filaments 40) includes cation exchange particles 52, anion exchange particles 54 and binder 56. Open cell foam 50 has an interconnected network of flow passages 58 therethrough.

Referring to Fig. 10, fixed ion exchange materials 24 could also be provided as combined particles 60, made up of cation exchange particles 62 (shown white), anion exchange particles 64 (shown dark) and binder 66. Combined particles 60 are sufficiently large so as to cause an acceptably low pressure drop in diluting flow channels 20 in the space between combined particles 60.

Individual cation exchange particles 42, 52 and 62 and anion exchange particles 44, 54 and 64 in filament 40 (Fig. 7), foam 50 (Fig. 9), and combined particle 60 (Fig. 10), respectively, have dimensions (roughly a diameter) of less than 0.1 mm, preferably less than 0.05 mm. Individual filaments 36 in bundle 34 (Fig. 5) and in braided strand 38 are between 0.01 mm and 1.0 mm in diameter. Strands 26, bundles 34, braid 38, combined exchange particle filament 40 and combined particle 60 have diameters between 0.1 mm and 3.0 mm. Particles 42, 44, 62, and 64 are preferably less than 1/3 the diameter of combined exchange particle filament 40 or combined particle 60, respectively.

Particles 42, 44, 52, 54, 62, 64 are provided in sufficient particle concentration to provide conductive paths for cations and anions through the bulk filament, foam, or combined particle structure, respectively. The volumetric concentration of the anion plus cation particle should exceed 60% and preferably is about 70% as a fraction of solid material.

In all described examples of fixed ion exchange materials 24, there is an intimate fixed, mixture of cation exchanging material and anion exchanging material, and the individual particles or filaments of the exchange materials have a small size. The small size of the ion exchanging particles or filaments and the intimate relationship of the two types of exchange resin insures numerous and uniformly distributed sites for water splitting. In all examples (Figs. 2-10) there also are relatively large passages for the flow of water (referred to as "macropassages") when compared to the particle size, thus providing good water splitting without excessive pressure drop. In particular, the respective ion exchanging materials have a characteristic dimension that is smaller than the characteristic dimensions of the macropassages through which the purified water flows. For the individual filaments of ion exchanging material, the characteristic dimension is the radius of the filament. For the individual particles of ion exchanging material 42, 44, 52, 54, 62, 64, the characteristic dimension is the radius of the individual particles. The macropassages for flow in all examples are not determined by spaces between individual particles or filaments, but instead are determined by the larger dimensions of the overall strands (for Figs. 2-7 and 9) or the combined particles (Fig. 10). As appears from the figures, the macropassages around the larger structures are substantially larger than the dimensions of the individual particles or filaments. As is described in more detail below in reference to Figs. 13 and 14, there also are micropassages within braids, strands and beads of combined particles; these micropassages are important in terms of promoting effective use of the large surface area of the small ion exchange particles or fibrils. Passages 58 in open cell foam 50 preferably include macropassages that are also substantially larger than the individual cation and anion exchange particles 52, 54 in foam 50 and micropassages to provide increased exposure of the ion exchange surface.

In manufacture, the majority of apparatus 10 is made and assembled the same as for known deionization apparatus. Anion and cation exchange filaments 36 and the individual anion and cation exchange filaments used in braid 38 can be made from any of the well-known ion exchange materials, e.g., styrenic ion exchange resin, acrylic ion exchange resin, phenolic ion exchange resin, and carbohydrate ion exchange resin. Individual ion exchange particles 42, 44, 52, 54, 62 and 64 can be made from the same well-known materials. Binder 44 and binder 64 can be any of the well-known thermoplastic polymers or thermosetting polymers used in the manufacture of ion permeable membranes. Binder 54 can be any suitable foam material such as polyurethane.

The individual filaments made of either cation exchange resin or anion exchange resin can be made by known techniques, e.g., as described in the above-referenced patents. The combined exchange particle filament 40 and combined particle 60 are made by obtaining (e.g., from a commercially available source) individual anion exchange particles and anion exchange particles in the desired size and approximately equinormal amounts, and then applying the binder 46 and 66 using the same techniques as used for making ion selective membranes, as described in the above-referenced patents. Open cell foam 50 is similarly made by known techniques for open cell material, adding the individual anion and cation exchange particles prior to initiating the reaction that creates the foam.

When using a fabric for fixed ion exchange materials 24, one or more layers of fabric 28, 30 or 32 is simply placed between membranes 16 and 18. When using open cell foam 50, it also is simply placed between membranes 16 and 18. In both cases, there is no need to pack individual ion exchange material particles, and there is no need for efforts to obtain uniform packing of particles in the diluting channel. WTien using combined particles 60 for fixed ion exchange materials 24, one packs the diluting channel with particles using the same techniques as presently used to pack ion exchange particles except that there is no need to obtain uniformity in the relative amounts of cation exchange particles and anion exchange particles, because only one type of particle is being added.

In operation of deionization apparatus 10, feed and brine are supplied to diluting channels 20 and concentrating channels 22, respectively, at typical flow rates (e.g., 1 to 3 cm sec) and pressure (e.g., 5 to 50 psig), and electric power is supplied to cathode 12 and anode 14 to provide an appropriate current density of 2 to 15 mA square cm and voltage of 1 to 5 volts per cell pair. The feed supplied to the inlets of diluting channels 20 is typically the permeate from a reverse osmosis process. The brine supplied to the inlets of concentrating channels 22 is typically a mixture of the reverse osmosis permeate and brine recirculated from the outlet of the electrodeionization apparatus.

The removal of ions from diluting channels 20 includes two steps. The first step is the diffusion of cations to the cation exchanging solids and the diffusion of anions to the anion exchanging solids. The second step is electrical conduction within the solid phases to the bounding membranes of the diluting compartment.

The concentration of the ion in question at the surface of the ion exchanging solid is in equilibrium with the concentration of that ion in the solid. It is desired to increase the exchanging sites having hydrogen ions and hydroxide ions to increase the transfer of ions from the liquid to the solid. The regeneration of exchanging sites with hydrogen ions and hydroxide ions is promoted by water splitting. Thus, by providing numerous, uniformly distributed sites for water splitting (the interfaces between individual ion exchanging particles or filaments), the transfer of ions to the solid is promoted. Because transfer to the solid is the limiting step in the removal of ions, the efficiency of apparatus 10 in removing ions is improved.

The applied electric field then causes anions on the exchanging material to travel along the anion exchanging material in a conductive path to and through the anion permeable membrane into the concentrating compartment nearer the anode. The applied electric field similarly causes cations to travel along the cation exchanging materials in a conductive path to and through the cation permeable membrane into the concentrating compartment nearer the cathode. The anions and cations become trapped in the concentrating compartments because the movement of anions toward the anode is blocked by a cation permeable membrane, and the movement of cations toward the cathode is blocked by an anion permeable membrane.

By providing numerous, uniformly distributed sites for water splitting in the diluting channels, the removal of ions is improved, and the ion concentration in the deionized output is advantageously decreased without excessive pressure drops.

The above-described approach of increasing the interfacial area by reducing the size of the particles or fibrils that constitute the ion exchange packing is most effective if certain criteria are observed. These criteria relate to the size of the particles and fibrils and the void fraction within the large beads and braids.

Because the dimension of the ion exchange beads or braids (also referred to as "macrostructural elements") need to be quite large in order to avoid high pressure drops, the diffusion of ions from the outer surface of these elements to the interior surface of the constitutive particles or fibrils (also referred to as "microstructural elements") can be slow. This can lead to the virtually complete capture of the inwardly diffusing ions by the ion exchange resin particles or fibrils nearer this outer surface of the bead or braid and the absence of ions in the interior. If this occurs, the ion exchange surface in the interior is almost completely ineffective in capturing ions, there being no ions to capture. This phenomenon becomes more pronounced as the size of the particles or fibrils becomes smaller.

The ions can be effectively delivered to the interior surfaces of the ion exchange materials by providing sufficient pores or micropassages between the particles of ion exchange resin or ion exchange fibrils. The pores need to be large enough so that adequate quantities of water can flow through these pores under the influence of the pressure gradient established in the diluting compartment by the flow of water through the large channels between the beads or braids. This flowing water supplies the interior of the beads or braids with a much larger quantity of ions than can be supplied by simple diffusion. As a result, the ion exchange surface in the interior of the beads or braids is in contact with a solution that has an ion concentration almost as high as the ion concentration at the exterior surface of the structure. The entire ion exchange surface is therefore effective in capturing ions, and a major improvement in the efficiency of the EDI stack is obtained.

Sufficient pores can be provided in a practical EDI device by controlling the size of the particles and the void fraction within the macrostructural elements. Assuming circular sectioned fibrils or spherical particles, the size of the pores can be estimated in terms of the size of the particles or fibrils and the void fraction within the bead or braid by the following formula:

Radius of pore = void fraction (l- void fraction)* Radius of (fibril or particle)

The flow rate though these interior pores is very sensitive to the pore radius, being proportional to its fourth power. The area within the beads or braids is inversely proportional to the radius of the constitutive particles or fibrils. Thus the quantity of ions that need to be supplied into the interior of the bead or braid increases with decreasing particle or fibril radius, while quantity of ions brought by the flow rate of water into the interior of the bead or braid drops sharply with decreasing particle or fibril radius. In order to utilize the interior surface area effectively, the void fraction must be increased in order to maintain the pore radius at a reasonable size and the supply of water at a level that matches the potential adsorptive capacity of the ion exchange surface.

For ion exchange packings in the form of beads that are composed of small particles, the void fraction in the interior of the bead, in the absence of a binder, would generally be between about 35% and 50%. The bonding agent needed to adhere the small particles to each other, in order to form a bead, occupies some of the available void volume. This can reduce the void fraction to levels that seriously reduce the flow of water through the interior of the bead. In constructing beads out of small ion exchange particles and binder, it has been discovered that it is useful to maintain the void fraction between 25% and 45% in order to maintain an adequate supply of ions to the interior surface of the beads. Such void fractions can be obtained by controlling the amount of binder applied to the particles in making the beads.

For ion exchange packing in the form of strands or braids, the prefened range of void fractions internal to the braids has been found to be 25% and 50%. Void fractions in excess of 40% can be obtained, without weakening the structure of the braid, by braiding or ananging the fibrils loosely and bonding the fibers together at regular intervals, as shown in Fig. 13.

The functionality of ion exchange structures composed of braids or similar filamentous ion exchange materials arranged as two approximately perpendicular sets can be further enhanced by employing a preferred orientation with respect to the direction of the pressure gradient, i.e., the direction of water flow. If the axis of one set of the braids is parallel to the pressure gradient, there is effectively no flow through the pores, because there is no component of the pressure gradient along the pores. The interior area of this set of braids is effectively wasted. There is adequate flow through the pores of the other set of braids, oriented in a direction perpendicular to the direction of water flow, because the pressure gradient is oriented along these pores and the effective internal area approaches the geometrical area, as shown in Fig. 14. The average effective internal area for the packing is about 50% of the total geometrical internal area of both sets of braids. If, however, the two sets of braids are oriented so that the axes of the two sets are at an angle of about 45 degrees to the direction of the pressure gradient, the effective area of the packing is about 71% of the geometrical area, as shown in Fig. 15. An angle of 45 degrees is an optimum orientation of the packing, though a range of angles between 30 and 60 degrees is still effective.

It has also been found that there is a prefened practical range for bead or particle size. As particle or fibril radius go above 40 micrometers, the decrease in internal mass transfer coefficient, being inversely proportional to the size of the pores, and effective surface area (which decreases with increasing radius) become reduced to such an extent such that it is preferable to use particles or fibrils with radii smaller than about 40 micrometers. Radii up to about 50 micrometers can still have some benefit in some situations.

It has also been found that the use of very small particles or fibrils is constrained by the ability of the resin material to conduct ions under the influence of the applied voltage. While the diffusion rate of ions from the solution to the surface of the particles or fibrils increases with decreasing particle or fibrils radius, the voltage drop required to conduct these ions, within the ion exchange material, to the bounding membranes of the diluting compartment, becomes inordinately large. This voltage drop is proportional to the square of the ratio of the channel thickness to the particle or fibril radius, and therefore increases very rapidly with decreasing particle or fibril radius. The voltage required to conduct the captured ions to the membranes that bound the flow channel can become impracticably large when this radius goes much below about 5 micrometers. Preferably the radius is greater than 7 micrometers.

Based on these considerations, it is prefened to use particles or fibrils whose radii range from about 5 micrometers to 50 micrometers and have internal void fractions between 25% and 45%.

Likewise the prefened braid should be composed of particles with radii between 5 and 50 micrometers and have a void fraction between 25% and 50%, the braids being oriented at an angle of between 30 and 60 degrees to the direction of the water flow.

The ion exchange material can be cation exchange only or anion exchange only in some embodiments; in these cases, the volumetric concentration of the cation or anion exchange resin should exceed 60% and preferably be about 70% as a fraction of solid material.

Other embodiments of the invention are within the scope of the appended claims. For example, fixed ion exchange material could be usefully used in the concentrating channels 22 as well. Also, a fabric of fixed ion exchange materials 24 could also be made by extrusion.

It is also possible to increase the rate of diffusion of ions from the fluid to the surface of the ion exchanging materials 24, by increasing the velocity of the fluid relative to ion exchanging materials 24. This goal can be accomplished, without reducing the residence time in the electrodeionization apparatus, which would counteract the benefits of the higher velocity, by two techniques shown in Figs. 11 and 12. The first (Fig. 11) involves mixing a portion of the water leaving the diluting channel 20 with the feed water entering channel 20 using recirculating loop 100. This increases the velocity of the water and also results in a dilution of the feed water and thus an improved water purity. The second technique involves providing a serpentine path whose cross section, normal to the flow direction, is smaller than the cross section of the electrodeionization compartments. This is achieved by placing non- permeable obstructions 102 in the flow path so as to create a tortuous path for the flowing water, while keeping the volume of the diluting compartment substantially unchanged.

Referring to Figs. 16-18, there are shown three different flow configurations for electrodeionization apparatus. Single-stage electrodeionization apparatus 210 is shown in Fig. 16. Apparatus 210 has a plurality of concenfrating chambers 212 (shown as a single box in Fig. 16) and a plurality of diluting chambers 214 (again shown as a single box) which have both anion exchange resin and cation exchange resin. Feed water 216 (typically the output of RO apparatus) enters inlet 218 of the diluting chambers 214, and is therein converted to deionized water 220 provided at outlet 224. Brine 226 enters inlet 228 of concentrating chambers 212, picks up ions removed from the diluting channels, and leaves outlet 230. By employing one of the anangements described in Figs. 20-25 for concentrating channel 212, single-stage electrodeionization apparatus 210 can be provided with resistance to scaling formation.

Two-stage electrodeionization apparatus 232 is shown in Fig. 17. Apparatus 232 has first stage 234 and second stage 236. Both stages 234, 236 have a plurality of concentrating chambers 212 and a plurality of diluting chambers, though diluting chambers 238 of first stage 234 have only cation exchange resin, while diluting chambers 240 of second stage 236 have both anion exchange resin and cation exchange resin. Feed water 216 enters inlet 242 of the diluting chambers 238, and the effluent 244 at the outlet 246 of diluting chambers 238 is connected as the inflow to the inlet 248 of diluting chamber 240 of second stage 236. Brine 226 enters inlet 250 of concentrating chambers 212 of first stage 234, picks up cations removed from the diluting channels, and leaves outlet 230 as brine 252, including Ca++. Water splitting at the interface of the anion membrane with the cation exchange material in the dilutmg channels 238 regenerates the cation resin and replaces the cations that are removed with H+, which converts HC03- in the feed water into C02. This suppresses the transfer of TIC into the concentrating compartment and thus reduces the scaling potential, even though the pH of the concentrating compartment is alkaline. Fresh brine 254 enters inlet 256 of concenfrating chambers 212 of second stage 236, picks up anions (in particular HC03-), weak acids and cations not removed in the first stage, and leaves outlet 28 as brine 60, including HC03-. Deionized water 222 leaves dilutmg chamber 240. Because Ca++ is removed in the first stage without HC03-, and HC03- is removed in the second stage without Ca++, Ca++ and HC03- do not coexist at the same location in the apparatus, and CaCO3 precipitation is avoided. Two-stage apparatus 232 thus inherently provides scaling resistance regardless of the type of design for concentrating channels 212. One can additionally use one of the anangements shown in Figs. 20-25 for concentrating channels 212, to reduce any scaling potential due to the presence of small concentrations of TIC that were transfened from the diluting channel via the solution phase. This provides even further resistance to scaling.

Two-stage electrodeionization apparatus 262, shown in Fig. 18, is similar to apparatus 232 (Fig. 17) except that the diluting chambers 264 of the first stage have anion exchange resins only, instead of cation exchange resin only as in diluting chambers 238 in Fig. 17. Brine 226 enters inlet 250 of concentrating chambers 212 of first stage 234, picks up anions removed from the diluting channels, and leaves outlet 230 as brine 266, including HCO3-. Water splitting at the interface between the anion exchange resin in channels 264 and the cation membrane renders the diluting channel alkaline. This converts carbon dioxide to bicarbonate and thus results in very complete removal of TIC into the concentrating compartment. The concentrating compartment 212 of first stage 234 is acidic, and hence no scaling can take place within it. It is possible that the LSI in the diluting compartment could become positive, for some feed compositions, and scaling could then take place. Minimization of this possibility requires careful control of the cunent density so as to avoid excessive regeneration of the resin in the diluting compartment. Fresh brine 254 enters inlet 256 of concenfrating chambers 212 of second stage 236, picks up cations (in particular Ca++) and cations not removed in the first stage, and leaves outlet 258 as brine 268, including Ca++. Because HCO3- is removed in the first stage without Ca++, and Ca++ is removed in the second stage without HC03-, Ca++ and HC03- do not coexist at the same location in the apparatus, and CaC03 precipitation is avoided. Two-stage apparatus 262 thus inherently provides scaling resistance regardless of the type of design for concentrating channels 212. When one of the anangements shown in Figs. 20-25 is used for concentrating channels 212, even further resistance to scaling can be provided.

Fig.l, discussed in detail above, shows the details of the electrodeionization stacks used in single-stage electrodeionization apparatus 210 and in first stage 234 and second stage 236 of two-stage deodorization apparatus 232 and two-stage electrodeionization apparatus 262. Ion exchange materials 24 (Fig. 1) are located in diluting channels 20, and spacers 25 are located in concentrating channels 22. In the embodiments of Figs. 16-18, ion exchange materials 124 can be fixed ion exchange materials. Spacers 25 can be ion exchange resin or ion inactive, permeable material; examples of different spacer anangements are described in Figs. 20-25. Cathode 12, anode 14, membranes 16 and membranes 18 can be made of components and materials typically used in deodorization apparatus, as described, e.g., in the above-referenced patents, which are hereby incorporated by reference.

Fig. 9 shows anion member 316 and the ions involved in scaling in the concenfration channel in the absence of measures to reduce or avoid such scaling as described herein.

Figs. 20-25 show six different spacer anangements for the concentrating channels 212 that are designed to provide a reduced pH at the surface of the anion membrane so as to avoid CaCO3 precipitation. The six concentrating channel anangements can be used with any of the three different system configurations of Figs. 16-18.

Referring to Fig. 20, concentrating channel 330 is filled with a layer of anion exchanging material 332 (also refened to as anionic spacer 332) next to the cation permeable membrane 318 and a layer of cation exchanging material 334 (also refened to as cationic spacer 334) next to anion permeable membrane 316. Water splitting takes place at interface 336 of layers 332 and 334. The effect of this water splitting is to render acidic the cation exchange layer 334 and its bounding anion exchange membrane 316, which prevents scale formation at anion exchange membrane 316. The cation exchange material is sufficiently close to the anion membrane so as to provide hydrogen ions to the surface of the anion membrane facing the concentrating flow channel.

Referring to Fig. 21, concentrating channel 340 is similar to concentrating channel 330 (Fig. 20), except that cation permeable membrane 342 is placed between anionic spacer 332 and cationic spacer 334. Membrane 342 creates two separate brine streams, and water splitting now occurs at interface 344 of anionic spacer 332 and cation permeable membrane 342. This creates two separate concentrate streams. The concenfrate stream flowing through the anion exchanging layer contains calcium, is slightly alkaline and contains no TIC and hence cannot scale. The concentrate stream flowing through the cation exchanging layer contains TIC and is slightly acidic, but it contains no calcium and hence cannot scale. This approach provides a very high degree of protection against scaling.

Referring to Fig. 22, concentrating channel 350 is similar to concenfrating channel 340 (Fig. 21) except that anion permeable membrane 352 is placed between anionic spacer 332 and cation permeable membrane 342. Water splitting now takes place between membranes 332 and 342, and cation spacer 334 is again rendered acidic, preventing scale formation at anion membrane 316. These membranes furthermore prevent the fransfer of cations to the acidic brine compartment in spacer 334 and of anions to the basic brine compartment in spacer 332.

Referring to Fig. 23, concentrating channel 360 is similar to concentrating channel 350 (Fig. 22) except that bipolar membrane 362 replaces anion permeable membrane 352 and cation permeable membrane 342. Water splitting now takes place within bipolar membrane 362 at the interface of the cation and anion parts, and cation spacer 334 is again rendered acidic, preventing scale formation at anion membrane 316. Bipolar membrane 362 furthermore prevents the transfer of cations to the acidic brine compartment in spacer 334 and of anions to the basic brine compartment in spacer 332.

In concentrating compartment 370, shown in Fig. 25, there is direct contact between cationic spacer 334 and anionic spacer 332, and there is dialysis membrane 372 and additional anionic spacer 374 between spacer 332 and cation membrane 318.

In concentrating compartment 380, shown in Fig. 25, there is direct contact between cationic spacer 334 and anionic spacer 332, and there is dialysis membrane 382 and additional cationic spacer 384 between spacer 332 and cation membrane 318. The dialysis membrane serves to inhibit the mixing of the alkaline portion of the brine, which contains Ca++, with the acidic portion of the brine and thus serves to reduce scaling potential.

In operation of deionization apparatus 210, 232 and 262 (Figs. 16-18), feed and brine are supplied to diluting channels 214 and concentrating channels 212, respectively, at typical flow rates (e.g., 1-3 cm/sec) and pressure (e.g., 5 to 50 psig), and electric power is supplied to cathode 12 and anode 14 (Fig. 1) to provide an appropriate cunent density of 2 to 15 mA/square cm and voltage of 1 to 5 volts per cell pair. The feed 216 supplied to the inlets 218, 242 (Figs. 16-18) of diluting channels 214 is typically the permeate of reverse osmosis processing. The brine 226 supplied to the inlets of concentrating channels 212 is typically a mixture of the reverse osmosis permeate and brine recirculated from the outlet of the electrodeionization apparatus.

The removal of ions from diluting channels 214 includes two steps. The first step is the diffusion of cations to the cation exchanging solids and the diffusion of anions to the anion exchanging solids in the diluting channels. The applied electric field then causes anions on the exchanging material to travel along the anion exchanging material in the diluting channels in a conductive path to and through the anion permeable membrane into the concentrating compartment nearer the anode. The applied electric field similarly causes cations to travel along the cation exchanging materials in the diluting channels in a conductive path to and through the cation permeable membrane into the concentrating compartment nearer the cathode. The anions and cations become frapped in the concentrating compartments because the movement of anions toward the anode is blocked by a cation permeable membrane, and the movement of cations toward the cathode is blocked by an anion permeable membrane.

Figs. 26-30 show alternative embodiments for concentrating channels and diluting channels to provide for reduced scaling. In general, in these embodiments, there are provisions for maintaining all parts of the concentrating compartments at a pH lower than a pH that can lead to "scaling" even in the presence of both calcium and TIC species. This improvement is achieved in two ways. The first approach, shown in Fig. 26, is by ananging the flow of solution in the concentrating stream in a counter-cunent direction to the flow of water through the diluting compartments so that little or no calcium is present at the concentrating side of the anion membrane in the region where scaling has a tendency to occur. The second, shown in different embodiments in Figs. 27-30, is by flowing the water feed to the concenfrating compartment first through a region that renders it substantially acidic before it enters a region that contains calcium. These embodiments will now be discussed in detail with reference to Figs. 26-30, which show a pair of a concentrating channel and a diluting channel between two anion membranes and a cathode membrane, which are all shown positioned between an anode and a cathode. In the actual stacks, there are a larger number of alternating diluting channels and concentrating channels defined between alternating anion membranes and cation membranes.

Fig.12 shows stack 400 with diluting channel 402 and concentrating channel 404 (also refened to as "compartments") between anion membranes 406 and cation membrane 408, which are all positioned between anode 410 and cathode 412. The diluting channels and concentrating channels include spacer material, as described above. Water enters the concentrating compartment 404 at concentrating inlet 414 at the same end of the stack that product water leaves the diluting compartment 402 at diluting outlet 416. Concentrate leaves the concenfrating compartment 404 at concentrating outlet 418 at the same end of the stack that feed water enters the diluting compartment 402 at diluting inlet 420. As has been explained above, the TIC enters the concenfrating compartment in the middle portion of the diluting flow path at TIC transfer zone 422. It is in this general area that scaling would take place on the concentrating side of the anion membrane 406, because calcium ions in the concentrating channel can diffuse into the high pH region adjacent to the anion membrane 406 where the LSI index is positive. It is known that calcium leaves the diluting compartment 402 and enters the concenfrating compartment 404 primarily at calcium transfer zone 424 at the feed water inlet side near inlet 420, because calcium is a doubly valent ion and is therefore adsorbed preferentially by the cation resin in the diluting compartment. In the flow configuration shown in Fig. 26 the brine stream (in concentrating channel 404) does not pick up calcium until it has passed the critical central portion of the flow path, where scaling could otherwise occur. In the region where calcium enters the flowing brine stream, the pH values are closer to neutrality, because, as has been explained above, little water splitting takes place at the feed water entrance end of the stack. By this means it is possible to avoid the introduction of calcium ions into the region of the concentrating compartment 404 where TIC is present at high pH, and calcium carbonate scaling is thus avoided.

The approach shown in Fig. 26 reduces scaling tendencies in a conventional electrodeionization diluting compartment, which contains a mixture of cation and anion resins. The approach is even more effective when used in conjunction with a diluting channel that has the feed end filled with just cation resin. By this means the removal of calcium is more completely confined to the front end of the electrodeionization stack and intrusion of calcium into the cenfral part of the stack, which as explained above, is prone to scaling, is more completely avoided.

Fig. 27 shows stack 430, which embodies the second approach mentioned above. Stack 430 has concentrating channels with two compartments 432, 434 therein, similar to the configuration shown above in Figs. 20-25. Water enters the lower channel 432 of the concentrating compartment 404 at the end of the stack where the product water leaves at diluting outlet 416. After flowing through this channel 432 it enters the upper channel 434 of the concentrating compartment 404, as indicated in Fig. 27, and leaves at the same end of the stack as it entered through concentrate outlet 418. The lower channel 432 of the concentrating compartment 404 is acidic because the entry of cations from the upper concentrating compartment 434 is very small as that channel is filled with an anion resin 436, and the conductivity of cations is therefore low. Electrical neutrality in the lower concenfrating compartment 432 is maintained by hydrogen ions, produced by water splitting at the interface of the cation membrane 438 splitting the concentrating compartment 204 and the anion resin 236 filling the upper part 434 of the concentrating compartment 204. Thus the ionic solute in the lower brine compartment 432 consists almost exclusively of the acids of the anion in the feed water, H₂C0₃, HC1 and H₂S0₄. The stream flowing through the lower brine compartment 432 picks up more and more acid, and at the outlet of this compartment its pH is between 2 and 3. On entering the upper channel 434 of the concenfrating compartment 404 it becomes gradually neutralized by the OH^" ions that enter this channel 434 in an amount conesponding exactly to the H⁺ ions that were present in the lower concentrating channel 432. Consequently the pH of the solution in the upper concentrating channel 434 is always acidic and only approaches the pH of the feed water to the electrodeionization apparatus at the concentrating outlet 418. Since the pH of feed water at the water inlet 420, being always a reverse osmosis permeate, to the electrodeionization apparatus is always slightly acidic, no scaling by calcium carbonate precipitation can take place anywhere in the stack. Fig. 34 illusfrates in semi-quantitative form the expected pattern of pH variation. The two lines are in fact close to coincident and are shown as slightly separated for the sake of clarity.

Other variants of the design in Fig. 27 are depicted in Figs. 28-30. Fig. 28 shows a stack 440 in which the brine stream enters the upper concentrating compartment 434 at the same end of the stack as it enters the lower concentrating compartment 432 by an concentrating inlet 414. In fact any combination of entry points is suitable as long as the brine stream enters the upper brine compartment 432 after having passed through enough channels of the lower brine compartment 434 so as to render it acidic.

Fig. 29 shows a stack 450 in which a portion of the brine leaving the upper concenfrating channel 434 is recycled to the lower concentrating channel 432. This design may be beneficial in maintaining a high flow rate in the concentrating channels 432, 434, without the use of excessive amounts of water. It does have the drawback of introducing some calcium ions into the scaling prone area of the lower concentrating channel 432 and should therefore be used only when the concentration of calcium in the feed water to the electrodeionization apparatus is low, preferably less than 2 ppm.

Fig. 30 shows a stack 460 in which a bipolar membrane 462 is used to generate the H⁺ ions required to render the lower channel 432 acidic. This design can be combined with any of the flow directions previously described in Figs. 27-29. Although, in Fig. 30, the stack is shown with anion resin in the upper concentrating channel 434 and cation resin the lower concentrating channel 432, it can be used with inert filling material in the channels since the water splitting function that it provides is within the membrane itself and contact with anion exchange resin is not required.

Although all of flow directions in the Figures are shown as being straight from inlet to outlet of each concenfrating channel, it is to be understood that serpentine paths as shown in Fig. 32 in concentrating channel 270 can be used, if it is desired to raise the average velocity of the concentrating stream.

The apparatus of Fig. 1 can be subject to parasitic processes as illustrated in Fig. 33, which shows a single concenfrating channel 510, having cation permeable membrane 512, anion permeable membrane 514 oriented as indicated between anode 516 and cathode 518.

Fig. 34 shows an anangement that can be used to avoid parasitic process by employing spacers/membrane anangements located in concentrating channels 22 (Fig. 1). Referring to Fig. 34, the spacer/membrane anangement 525 in each concentrating channel 522 includes cation resin 530, cation permeable membrane 532, spacer 534, anion membrane 536, and anion exchange resin 538. Brine flows through permeable spacer 534 in the central channel 540, while feed or product water, with a concenfration of dissolved ions that is much less than that in the brine, flows at a low flow rate through guard channels 542, 544.

During operation of the Fig. 1 device (discussed in detail above), cation exchange resin 530 in guard channel 542 acts as a transfer layer for the cations moving to the middle concentrating compartment 540. Anion exchange resin 538 in guard channel 544 acts as a fransfer layer for the anions moving to the middle concentrating compartment 540. A low flow rate of purified water or of feed water through both guard channels 542, 544 keeps the concenfration of ions at a very low level and thus virtually eliminates both back-diffusion processes. This low flow of purified water can serve as make up to the concentrate stream, and thus no decrease in water recovery results. The flow rate through the concentrating compartment positioned between the two guard compartments is designed to carry away the bulk of the ions removed from the diluting compartments.

A number of embodiments of the invention have been described. Nevertheless, it will be understood that various modifications may be made without departing from the spirit and scope of the invention. Accordingly, other embodiments are within the scope of the following claims. WHAT IS CLAIMED IS:

Claims

1. Electrodeionization apparatus for purifying water comprising a cathode, an anode spaced from said cathode, a plurality of alternating anion permeable membranes and cation permeable membranes between said cathode and anode defining concentrating and diluting flow channels, each channel being defined between an adjacent pair of membranes, the anion permeable membrane defining a diluting channel being closer to said anode than said diluting flow channel, the cation permeable membrane defining a diluting channel being closer to said cathode than said diluting flow channel, and cation exchange materials and anion exchange materials that are fixed in close contacting position with respect to each other in said diluting flow channels, said materials in each said diluting flow channel providing conductive paths for cations and anions to said adjacent membranes for said diluting flow channel and providing flow passages for water between said materials, said anion exchange materials and said cation exchange materials each having a characteristic dimension smaller than the characteristic dimensions of said flow passages.

2. The apparatus of claim 1, wherein individual particles of said cation exchange material and said anion exchange material are fixed together with a binder in sufficient particle concentration to provide conductive paths for cations and anions.

3. The apparatus of claim 2 wherein said particles of said cation exchange material and said anion exchange material and binder form larger combined particles packed into said diluting flow channel in contacting relation between said adjacent membranes.

4. The apparatus of claim 3 wherein the majority of said combined particles have dimensions greater than 0.1 mm.

5. The apparatus of claim 4 wherein the majority of said individual particles of said cation and anion exchange material have dimensions less than 0.1 mm.

6. The apparatus of claim 3 wherein said combined particles are sufficiently large so as to cause an acceptably low pressure drop in said diluting flow channel.

7. The apparatus of claim 6 wherein said pressure drop is less than 100 psig.

8. The apparatus of claim 2 wherein said particles of said cation exchange material and said anion exchange material and binder form filaments provided as a matrix between said adjacent membranes.

9. The apparatus of claim 8 wherein said filaments are provided as one or more layers of woven or nonwoven cloth between said adjacent membranes.

10. The apparatus of claim 9 wherein said filaments have diameters between 0.1 mm and 3.0 mm.

11. The apparatus of claim 9 wherein said cloth includes groups of generally parallel filaments, with filaments spaced center-to-center by a distance equal to or greater than the diameter of filaments.

12. The apparatus of claim 1, wherein said cation exchange materials are in the form of cation exchange material filaments, and said anion exchange materials are in the form of anion exchange material filaments, and said cation exchange material filaments and anion exchange material filaments are combined together in multifilament strands provided as a matrix between said adjacent membranes.

13. The apparatus of claim 12 wherein said multifilament strands are braided strands.

14. The apparatus of claim 12 wherein said sfrands are provided as one or more layers of woven or nonwoven cloth between said adjacent membranes.

15. The apparatus of claim 12 wherein said filaments have diameters between 0.01 mm and 0.1 mm, and said sfrands have diameters between 1.0 mm and 3.0 mm.

16. The apparatus of claim 12 wherein said cloth includes groups of generally parallel strands, with strands spaced center-to-center by a distance equal to or greater than the diameter of strands.

17. The apparatus of claim 2 wherein said particles of said cation exchange material and said anion exchange material and binder form an open cell foam between said adjacent membranes, the openings in the foam being sufficiently large to provide said flow passages with an acceptably low pressure drop in said diluting flow channel.

18. The apparatus of claim 17 wherein said pressure drop is less than 100 psig.

19. The apparatus of claim 3 wherein said binder is a member selected from the group consisting of thermoplastic polymers and thermosetting polymers.

20. The apparatus of claim 8 wherein said binder is a member selected from the group consisting of thermoplastic polymers and thermosetting polymers.

21. The apparatus of claim 17 wherein said binder is a polyurethane foam.

22. The apparatus of claim 1 wherein said cation exchange materials are made of a material selected from the group consisting of styrenic ion exchange resin, acrylic ion exchange resin, phenolic ion exchange resin, and carbohydrate ion exchange resin.

23. The apparatus of claim 1 wherein said anion exchange resins are made of a material selected from the group consisting of styrenic ion exchange resin, acrylic ion exchange resin, phenolic ion exchange resin, and carbohydrate ion exchange resin.

24. Electrodeionization apparatus for purifying water comprising a cathode, an anode spaced from said cathode, a plurality of alternating anion permeable membranes and cation permeable membranes between said cathode and anode defining concentrating and diluting flow channels, each channel being defined between an adjacent pair of membranes, the anion permeable membrane defining a diluting channel being closer to said anode than said diluting flow channel, the cation permeable membrane defining a diluting channel being closer to said cathode than said diluting flow channel, and cation exchange materials and anion exchange materials that are fixed in close contacting position with respect to each other in said diluting flow channels, said materials in each said diluting flow channel providing conductive paths to said adjacent membranes for said diluting flow channel and providing flow passages for water between said materials, individual particles of said cation exchange material and said anion exchange material being fixed together with a binder to form larger combined particles packed into said diluting flow channel in contacting relation between said adjacent membranes, said cation exchange material and anion exchange material being in sufficient particle concentration to provide conductive paths, said flow passages being provided in the spaces between and around said larger combined particles.

25. Electrodeionization apparatus for purifying water comprising a cathode, an anode spaced from said cathode, a plurality of alternating anion permeable membranes and cation permeable membranes between said cathode and anode defining concentrating and diluting flow channels, each channel being defined between an adjacent pair of membranes, the anion permeable membrane defining a diluting channel being closer to said anode than said diluting flow channel, the cation permeable membrane defining a diluting channel being closer to said cathode than said diluting flow channel, and cation exchange materials and anion exchange materials that are fixed in close contacting position with respect to each other in said diluting flow channels, said materials in each said diluting flow channel providing conductive paths to said adjacent membranes for said diluting flow channel and providing flow passages for water between said materials, individual particles of said cation exchange material and said anion exchange material being fixed together with a binder to form filaments provided as a matrix between said adjacent membranes, said cation exchange material and anion exchange material being in sufficient particle concenfration to provide conductive paths, said flow passages being provided in the space between and around said filaments.

26. Electrodeionization apparatus for purifying water comprising a cathode, an anode spaced from said cathode, a plurality of alternating anion permeable membranes and cation permeable membranes between said cathode and anode defining concentrating and diluting flow channels, each channel being defined between an adjacent pair of membranes, the anion permeable membrane defining a diluting channel being closer to said anode than said diluting flow channel, the cation permeable membrane defining a diluting channel being closer to said cathode than said diluting flow channel, and cation exchange materials and anion exchange materials that are fixed in close contacting position with respect to each other in said diluting flow channels, said materials in each said diluting flow channel providing conductive paths to said adjacent membranes for said diluting flow channel and providing flow passages for water between said materials, said cation exchange materials being in the form of cation exchange material filaments, said anion exchange materials are in the form of anion exchange material filaments, said cation exchange material filaments and anion exchange material filaments being combined together in multi-filament strands provided as a matrix between said adjacent membranes, said flow passages being provided in the space between and around said multifilament strands.

27. A method of purifying water comprising providing a cathode, an anode spaced from said cathode, and a plurality of alternating anion permeable membranes and cation permeable membranes between said cathode and anode defining concenfrating and diluting flow channels, each said channel being defined between an adjacent pair of said membranes, each said diluting flow channel having cation exchange materials and anion exchange materials that are fixed in close contacting position with respect to each other and provide flow passages for water between said materials, said anion exchange materials and said cation exchange materials each having a characteristic dimension smaller than the characteristic dimensions of said flow passages, the anion permeable membrane defining a dilutmg channel being closer to said anode than said diluting flow channel, the cation permeable membrane defining a diluting channel being closer to said cathode than said diluting flow channel, supplying feed water into each said diluting flow channel, applying a voltage between said anode and said cathode so that said materials in each said diluting flow channel provide conductive paths for cations and anions removed from said feed water to said adjacent membranes for said diluting flow channels, removing purified water from said diluting channels, and supplying concentrating water into said concenfrating channels and removing brine therefrom.

28. Electrodeionization apparatus for purifying water comprising a cathode, an anode spaced from said cathode, a plurality of alternating anion permeable membranes and cation permeable membranes between said cathode and anode defining concentrating and diluting flow channels, each channel being defined between an adjacent pair of membranes and having an inlet and an outlet, the anion permeable membrane defining a diluting channel being closer to said anode than said diluting flow channel, the cation permeable membrane defining a diluting channel being closer to said cathode than said diluting flow channel, and a recirculation loop to reintroduce a portion of the water from said diluting channel outlet to said diluting channel inlet so as to obtain an increased velocity through said diluting channel.

29. Electrodeionization apparatus for purifying water comprising a cathode, an anode spaced from said cathode, a plurality of alternating anion permeable membranes and cation permeable membranes between said cathode and anode defining concentrating and diluting flow channels, each channel being defined between an adjacent pair of membranes and having an inlet and an outlet, the anion permeable membrane defining a diluting channel being closer to said anode than said diluting flow channel, the cation permeable membrane defining a diluting channel being closer to said cathode than said diluting flow channel, and said diluting channel including flow diverters so as to obtain an increased velocity through said diluting channel.

30. A packing for an electrodeionization compartment comprising one or more macrostructural elements made up of smaller, microstructural elements, said microstructural elements being in fixed, close contacting position with respect to each other in said macrostructural elements so as to provide porosity in said macrostructural elements, a majority of said microstructural elements having a characteristic dimension between 5 and 50 micrometers, said macrostructural elements having a void fraction between about 25% and 50%.

31. The packing of claim 30 wherein said microstructural elements comprise particles held together in said one or more macrostructural elements by a binder.

32. The packing of claim 31 wherein said one or more macrostructural elements comprise beads, each said bead comprising a said macrostructural element.

33. The packing of claim 31 wherein said one or more macrostructural elements comprise strands, each said strand comprising a said macrostructural element.

34. The packing of claim 30 wherein said microstructural elements are fibers, and said one or more macrostructural elements comprise strands, each said strand comprising a said macrostructural element.

35. The packing of claim 33 or 34 wherein plural said strands are combined in a braid.

36. The packing of claim 31 wherein said binder comprises a porous polymer foam, and said one or more macrostructural elements comprises an integral foam element comprising said polymer foam and said particles.

37. The packing of claim 30 wherein said microstructural elements comprise cation exchange material.

38. The packing of claim 30 wherein said microstructural elements comprise anion exchange material.

39. The packing of claim 30 wherein said microstructural elements comprise a mixture of cation exchange material and anion exchange material. 40. The packing of claim 30 wherein said characteristic dimension is between 7 and

40 micrometers.

41. The packing of claim 30 wherein said void fraction is below 45%.

42. The packing of claim 30 wherein said one or more macrostructural elements comprise a plurality of macrostructural elements each having a characteristic dimension between about 0.3 and 3 mm.

43. Electrodeionization apparatus for purifying water comprising a cathode, an anode spaced from said cathode, a plurality of alternating anion permeable membranes and cation permeable membranes between said cathode and anode defining concentrating and diluting flow channels, each channel being defined between an adjacent pair of membranes, the anion permeable membrane defining a diluting channel being closer to said anode than said diluting flow channel, the cation permeable membrane defining a diluting channel being closer to said cathode than said diluting flow channel, and ion exchange packing in said diluting flow channels, said packing comprising one or more macrostructural elements made up of smaller, microstructural elements, said microstructural elements being in fixed, close contacting position with respect to each other in said macrostructural elements so as to provide porosity in said macrostructural elements, a majority of said microstructural elements having a characteristic dimension between 5 and 50 micrometers, said macrostructural elements having a void fraction between about 25% and 50%.

44. The apparatus of claim 43 wherein said microstructural elements comprise particles held together in said one or more macrostructural elements by a binder.

45. The apparatus of claim 43 wherein said one or more macrostructural elements comprise beads, each said bead comprising a said macrostructural element, said packing including micropassages between said microelements and macropassages between said macroelements.

46. The apparatus of claim 44 wherein said one or more macrostructural elements comprise strands, each said strand comprising a said macrostructural element, said packing including micropassages between said microelements and macropassages between said macroelements .

47. The apparatus of claim 43 wherein said microstructural elements are fibers, and said one or more macrostructural elements comprise sfrands including a plurality of said fibers, each said strand comprising a said macrostructural element, said packing including micropassages between said microelements and macropassages between said macroelements.

48. The apparatus of claim 46 or 47 wherein plural said strands are combined in a braid.

49. The apparatus of claim 46 or 47 wherein plural said strands are combined in a plurality of elongated braids in said diluting channel, and wherein said braids are oriented so that the longitudinal axes of the braids make an angle of between 30 and 60 degrees with the direction of water flow through said diluting channel.

50. The apparatus of claim 44 wherein said binder comprises a porous polymer foam, and wherein said one or more macrostructural elements comprises an integral foam element comprising said polymer foam and said particles.

51. The apparatus of claim 43 wherein said microstructural elements comprise cation exchange material.

52. The apparatus of claim 43 wherein said microstructural elements comprise anion exchange material.

53. The apparatus of claim 43 wherein said microstructural elements comprise a mixture of cation exchange material and anion exchange material.

54. The apparatus of claim 43 wherein said microstructural elements comprise a mixture of cation exchange material and anion exchange material in each said one or more macrostructural elements.

55. The apparatus of claim 43 wherein said characteristic dimension is between 7 and 40 micrometers.

56. The apparatus of claim 43 wherein said one or more macrostructural elements comprise a plurality of macrostructural elements each having a characteristic dimension between about 0.3 and 3 mm.

57. The apparatus of claim 43 wherein said void fraction is below 45%.

58. The apparatus of claim 46 or 47 wherein said sfrands are oriented so that the longitudinal axes of the strands make an angle of between 30 and 60 degrees with the direction of water flow through said diluting channel

59. Electrodeionization apparatus for purifying water including calcium and carbon dioxide and its hydrates comprising a first stage electrodeionization system followed by a second stage electrodeionization system, each said system including a cathode, an anode spaced from said cathode, a plurality of alternating anion permeable membranes and cation permeable membranes between said cathode and anode defining concentrating and diluting flow channels, each said channel being defined between an adjacent pair of said membranes, said first stage electrodeionization system includmg one member of the group consisting of a cation exchange resin and an anion exchange resin in a diluting flow channel, said second stage electrodeionization system including at least the other of said cation exchange resin and said anion exchange resin in a diluting flow channel, the outflow of water from said diluting flow channels in said first stage electrodeionization system being connected to be fed as the inflow for said dilutmg flow channels in said second stage electrodeionization system, the outflow of brine from each said concentrating flow channel in said first stage electrodeionization system being isolated from the inflow for any said concentrating flow channel in said second stage electrodeionization system, whereby said calcium and carbon dioxide and its hydrates tend to be removed in different stages so as to deter calcium carbonate precipitation in said concentrating flow channel.

60. The apparatus of claim 59 wherein said second stage electrodeionization system includes a mixed cation and anion exchange resin.

61. The apparatus of claim 59 wherein said first stage electrodeionization system includes cation exchange resin in the diluting compartments that causes removal of calcium and other cations in said first stage electrodeionization system.

62. The apparatus of claim 59 wherein said first stage includes anion exchange resin in the diluting compartments that causes removal of carbon dioxide and its hydrates and other anions in said first stage electrodeionization system.

63. Electrodeionization apparatus for purifying water including calcium and carbon dioxide and its hydrates comprising: a cathode, an anode spaced from said cathode, a plurality of alternating anion permeable membranes and cation permeable membranes between said cathode and anode defining concentrating and diluting flow channels, each said channel being defined between an adjacent pair of said membranes, each said diluting flow channel having ion exchange materials therein, each said concenfrating flow channel including cation exchange material nearer to said anion membrane than said cation membrane, whereby pH is lowered at the surface of said anion membrane in said concentrating flow channel so as to limit calcium carbonate precipitation in said concentrating flow channel.

64. The apparatus of claim 59 wherein each said concentrating flow channel of at least one of said first and second stage elecfrodeionization systems includes cation exchange material nearer to said anion membrane than said cation membrane.

65. The apparatus of claim 63 wherein each said concentrating flow channel includes anion exchange material between said cation membrane and said cation exchange material, whereby water splitting occurs between said anion exchange material and said cation exchange material.

66. The apparatus of claim 65 wherein each said diluting channel includes anion exchange resin and cation exchange resin.

67. The apparatus of claim 64 wherein each said concentrating flow channel includes anion exchange material between said cation membrane and said cation exchange material, whereby water splitting occurs between said anion exchange material and said cation exchange material.

68. The apparatus of claim 63 or 64 wherein said cation exchange material is in a fixed structure.

69. The apparatus of claim 65 or 66 wherein said anion exchange material is in a first fixed structure, and said cation exchange material is in a second fixed structure.

70. The apparatus of claim 61 wherein each said concentrating flow channel includes cation exchange material next to said anion membrane, and anion exchange material between said cation membrane and said cation exchange material, whereby water splitting occurs between said anion exchange material and said cation exchange material.

71. The apparatus of claim 62 wherein each said concentrating flow channel includes cation exchange material next to said anion membrane, and anion exchange material between said cation membrane and said cation exchange material, whereby water splitting occurs between said anion exchange material and said cation exchange material.

72. The apparatus of claim 65, 66, 70, or 71 wherein said anion exchange material and said cation exchange material in said concentrating channel directly contact each other.

73. The apparatus of claim 65, 66, 70, or 71 wherein a membrane is located between said anion exchange material and said cation exchange material in said concentrating channel.

74. The apparatus of claim 65, 66, 70, or 71 wherein a cation permeable membrane is located between said anion exchange material and said cation exchange material in said concentrating channel.

75. The apparatus of claim 65, 66, 70, or 71 wherein a cation permeable membrane and an anion permeable membrane are located between said anion exchange material and said cation exchange material in said concenfrating channel, said cation permeable membrane being next to said cation exchange material, said anion permeable membrane being next to said anion exchange material.

76. The apparatus of claim 65, 66, 70, or 71 wherein a bipolar membrane having an anion part and a cation part is located between said anion exchange material and said cation exchange material in said concentrating channel, said bipolar membrane being oriented such that said cation part is next to said cation exchange material and said anion part is next to said anion exchange material.

77. The apparatus of claim 65, 66, 70, or 71 wherein said anion exchange material has two layers, and further comprising a dialysis membrane located between said two layers.

78. The apparatus of claim 65, 66, 70, or 71 wherein said cation exchange material has two layers, and further comprising a dialysis membrane located between said two layers.

79. A method of purifying water including calcium and carbon dioxide and its hydrates comprising flowing said water into diluting channels of a first stage elecfrodeionization system having one member of the group consisting of a cation exchange resin and an anion exchange resin in said diluting flow channels to provide a partially treated effluent, flowing a first brine through concentrating channels of said first stage electrodeionization system, flowing said partially treated effluent into diluting channels of a second stage elecfrodeionization system including at least the other of said cation exchange resin and said anion exchange resin, and flowing a second brine through concenfrating channels of said second stage electrodeionization system, said first and second brines being isolated from each other, whereby said calcium and carbon dioxide and its hydrates tend to be removed in different stages so as to deter calcium carbonate precipitation in said concentrating flow channels.

80. The method of claim 79 wherein said diluting channels of said second stage electrodeionization system include a mixed cation and anion exchange resin.

81. The method of claim 79 wherein said diluting channels of said first stage electrodeionization system include cation exchange resin that causes removal of calcium and other cations in said first stage elecfrodeionization system.

82. The method of claim 79 wherein said diluting channels of said first stage include anion exchange resin that causes removal of anions carbon dioxide and its hydrates and other anions in said first stage electrodeionization system.

83. A method of purifying water including calcium and carbon dioxide and its hydrates comprising flowing said water into diluting channels of an electrodeionization system, flowing a first brine through concentrating channels of said electrodeionization system, and lowering pH at the surfaces of anion exchange membranes in said concentrating flow channels so as to limit calcium carbonate precipitation in said concentrating flow channels.

84. The method of claim 83 wherein each said concentrating flow channel includes cation exchange material next to said anion membrane.

85. The method of claim 79 wherein each said concentrating flow channel includes cation exchange material next to said anion membrane.

86. The method of claim 83 wherein each said concentrating flow channel includes anion exchange material between said cation membrane and said cation exchange material, and further comprising water splitting between said anion exchange material and said cation exchange material.

87. The method of claim 79 wherein each said concentrating flow channel includes anion exchange material between said cation membrane and said cation exchange material, and further comprising water splitting between said anion exchange material and said cation exchange material.

88. Electrodeionization apparatus for purifying water including calcium and carbon dioxide and its hydrates comprising a first stage electrodeionization system followed by a second stage elecfrodeionization system, each said system including a cathode, an anode spaced from said cathode, a plurality of alternating anion permeable membranes and cation permeable membranes between said cathode and anode defining concentrating and diluting flow channels, each said channel being defined between an adjacent pair of said membranes, said first stage elecfrodeionization system including one member of the group consisting of a cation exchange resin and an anion exchange resin in a diluting flow channel, said second stage electrodeionization system including at least the other of said cation exchange resin and said anion exchange resin in a diluting flow channel, the outflow of water from said dilutmg flow channels in said first stage elecfrodeionization system being connected to be fed as the inflow for said diluting flow channels in said second stage electrodeionization system, the outflow of brine from each said concentrating flow channel in said first stage electrodeionization system being isolated from the inflow for any said concentrating flow channel in said second stage electrodeionization system, whereby said calcium and carbon dioxide and its hydrates tend to be removed in different stages so as to deter calcium carbonate precipitation in said concentrating flow channel.

89. The apparatus of claim 88 wherein said second stage elecfrodeionization system includes a mixed cation and anion exchange resin.

90. The apparatus of claim 88 wherein said first stage electrodeionization system includes cation exchange resin in the diluting compartments that causes removal of calcium and other cations in said first stage electrodeionization system.

91. The apparatus of claim 88 wherein said first stage includes anion exchange resin in the diluting compartments that causes removal of carbon dioxide and its hydrates and other anions in said first stage electrodeionization system.

92. Electrodeionization apparatus for purifying water including calcium and carbon dioxide and its hydrates comprising: a cathode, an anode spaced from said cathode, a plurality of alternating anion permeable membranes and cation permeable membranes between said cathode and anode defining concentrating and dilutmg flow channels, each said channel being defined between an adjacent pair of said membranes, each said diluting flow channel having ion exchange materials therein, each said concentrating flow channel including cation exchange material nearer to said anion membrane than said cation membrane, whereby pH is lowered at the surface of said anion membrane in said concentrating flow channel so as to limit calcium carbonate precipitation in said concentrating flow channel.

93. The apparatus of claim 88 wherein each said concentrating flow channel of at least one of said first and second stage electrodeionization systems includes cation exchange material nearer to said anion membrane than said cation membrane.

94. The apparatus of claim 92 wherein each said concentrating flow channel includes anion exchange material between said cation membrane and said cation exchange material, whereby water splitting occurs between said anion exchange material and said cation exchange material.

95. The apparatus of claim 94 wherein each said diluting channel includes anion exchange resin and cation exchange resin.

96. The apparatus of claim 93 wherein each said concentrating flow channel includes anion exchange material between said cation membrane and said cation exchange material, whereby water splitting occurs between said anion exchange material and said cation exchange material.

97. The apparatus of claim 92 or 93 wherein said cation exchange material is in a fixed structure.

98. The apparatus of claim 94 or 95 wherein said anion exchange material is in a first fixed structure, and said cation exchange material is in a second fixed structure.

99. The apparatus of claim 90 wherein each said concentrating flow channel includes cation exchange material next to said anion membrane, and anion exchange material between said cation membrane and said cation exchange material, whereby water splitting occurs between said anion exchange material and said cation exchange material.

100. The apparatus of claim 91 wherein each said concentrating flow channel includes cation exchange material next to said anion membrane, and anion exchange material between said cation membrane and said cation exchange material, whereby water splitting occurs between said anion exchange material and said cation exchange material.

101. The apparatus of claim 74, 95, 99, or 100 wherein said anion exchange material and said cation exchange material in said concentrating channel directly contact each other.

102. The apparatus of claim 74, 95, 99, or 100 wherein a membrane is located between said anion exchange material and said cation exchange material in said concentrating channel.

103. The apparatus of claim 74, 95, 99, or 100 wherein a cation permeable membrane is located between said anion exchange material and said cation exchange material in said concenfrating channel.

104. The apparatus of claim 74, 95, 99, or 100 wherein a cation permeable membrane and an anion permeable membrane are located between said anion exchange material and said cation exchange material in said concentrating channel, said cation permeable membrane being next to said cation exchange material, said anion permeable membrane being next to said anion exchange material.

105. The apparatus of claim 74, 95, 99, or 100 wherein a bipolar membrane having an anion part and a cation part is located between said anion exchange material and said cation exchange material in said concentrating channel, said bipolar membrane being oriented such that said cation part is next to said cation exchange material and said anion part is next to said anion exchange material.

106. The apparatus of claim 74, 95, 99, or 100 wherein said anion exchange material has two layers, and further comprising a dialysis membrane located between said two layers.

107. The apparatus of claim 74, 95, 99, or 100 wherein said cation exchange material has two layers, and further comprising a dialysis membrane located between said two layers.

108. A method of purifying water including calcium and carbon dioxide and its hydrates comprising flowing said water into diluting channels of a first stage electrodeionization system having one member of the group consisting of a cation exchange resin and an anion exchange resin in said diluting flow channels to provide a partially treated effluent, flowing a first brine through concentrating channels of said first stage elecfrodeionization system, flowing said partially treated effluent into diluting channels of a second stage elecfrodeionization system including at least the other of said cation exchange resin and said anion exchange resin, and flowing a second brine through concentrating channels of said second stage elecfrodeionization system, said first and second brines being isolated from each other, whereby said calcium and carbon dioxide and its hydrates tend to be removed in different stages so as to deter calcium carbonate precipitation in said concenfrating flow channels.

109. The method of claim 108 wherein said diluting channels of said second stage elecfrodeionization system include a mixed cation and anion exchange resin.

110. The method of claim 108 wherein said diluting channels of said first stage electrodeionization system include cation exchange resin that causes removal of calcium and other cations in said first stage electrodeionization system.

111. The method of claim 108 wherein said dilutmg channels of said first stage include anion exchange resin that causes removal of anions carbon dioxide and its hydrates and other anions in said first stage electrodeionization system.

112. A method of purifying water including calcium and carbon dioxide and its hydrates comprising flowing said water into diluting channels of an electrodeionization system, flowing a first brine through concenfrating channels of said elecfrodeionization system, and lowering pH at the surfaces of anion exchange membranes in said concentrating flow channels so as to limit calcium carbonate precipitation in said concentrating flow channels.

113. The method of claim 112 wherein each said concentrating flow channel includes cation exchange material next to said anion membrane.

114. The method of claim 108 wherein each said concenfrating flow channel includes cation exchange material next to said anion membrane.

115. The method of claim 1 12 wherein each said concentrating flow channel includes anion exchange material between said cation membrane and said cation exchange material, and further comprising water splitting between said anion exchange material and said cation exchange material.

116. The method of claim 108 wherein each said concenfrating flow channel includes anion exchange material between said cation membrane and said cation exchange material, and further comprising water splitting between said anion exchange material and said cation exchange material.

117. Electrodeionization apparatus for purifying water including calcium and carbon dioxide and its hydrates comprising: a cathode, an anode spaced from said cathode, a plurality of alternating anion permeable membranes and cation permeable membranes between said cathode and anode defining concenfrating flow channels and diluting flow channels, each said channel being defined between a pair of said membranes, said concentrating flow channels each having a concenfrating inlet and a concentrating outlet, said diluting flow channels each having a diluting inlet and a diluting outlet, said concentrating inlets being adjacent to said diluting outlets, and said diluting inlets being adjacent to said concentrating outlets, whereby there is countercunent flow in said diluting flow channels and said concentrating flow channels, such that calcium is avoided at the concenfrating side of the anion membranes in the region where scaling is likely to occur so as to limit calcium carbonate precipitation in said concentrating flow channel.

118. Electrodeionization apparatus for purifying water including calcium and carbon dioxide and its hydrates comprising: a cathode, an anode spaced from said cathode, a plurality of alternating anion permeable membranes and cation permeable membranes between said cathode and anode defining concentrating flow channels and diluting flow channels, each said channel being defined between an adjacent pair of said membranes, said concentrating flow channels each having a concentrating inlet and a concenfrating outlet, said diluting flow channels each having a diluting inlet and a diluting outlet, said concentrating flow channels having first and second regions, said first region being upstream of said second region and beginning at said concentrating inlet and rendering feed therein substantially acidic, said second region being downstream of said first region and ending at said concentrating outlet and containing calcium, whereby calcium is introduced into said concentrating flow channels under acidic conditions so as to limit calcium carbonate precipitation in said concenfrating flow channels.

119. The apparatus of claim 117 wherein said diluting flow channels include a mixture of cation and anion resins.

120. The apparatus of claim 117 wherein said diluting flow channels include cation resin only at the diluting inlets.

121. The apparatus of claim 118 wherein said concentrating flow channel includes first and second flow channel portions in overlying relation, said first flow channel portion having a first flow channel inlet being said concentrating inlet and a first flow channel outlet, said second flow channel portion having a second outlet being said concentrating outlet and a second flow channel inlet, said first flow channel outlet being connected to said second flow channel inlet.

122. The apparatus of claim 121 wherein said first flow channel portion includes a cation resin, and said second flow channel portion includes an anion resin.

123. The apparatus of claim 121 wherein said first flow channel portion and said second flow channel portion are separated by a cation membrane.

124. The apparatus of claim 121 wherein said first flow channel portion and said second flow channel portion are separated by a bipolar membrane.

125. The apparatus of claim 122 wherein said first charmel inlets and said second channel outlets are adjacent to said diluting outlets, and said diluting inlets are adjacent to said first channel outlets and said second channel inlets.

126. The apparatus of claim 122 wherein said first channel inlets and said second channel inlets are adjacent to said diluting outlets, and said diluting inlets are adjacent to said first channel outlets and said second channel outlets.

127. The apparatus of claim 122 wherein each said second channel outlet is connected to divert a portion of its effluent to said first channel inlet.

128. A method of purifying water including calcium and carbon dioxide and its hydrates comprising flowing said water into diluting channels of an electrodeionization system, and flowing feed through concentrating channels of said electrodeionization system in countercunent flow with respect to flow in said diluting channels.

129. The method of claim 127 wherein said diluting flow channels include a mixture of cation and anion resins.

130. The method of claim 127 wherein said diluting flow channels include cation resin only at the diluting inlets.

131. A method of purifying water including calcium and carbon dioxide and its hydrates comprising flowing said water into diluting channels of an elecfrodeionization system, and flowing feed through concentrating channels of said electrodeionization system having concentrating inlets and concenfrating outlets and first and second regions, each said first region being upsfream of said second region and beginning at said concentrating inlet and rendering feed therein substantially acidic, said second region being downstream of said first region and ending at said concentrating outlet and containing calcium, whereby calcium is introduced into said concentrating flow channels under acidic conditions so as to limit calcium carbonate precipitation in said concenfrating flow channels.

132. The method of claim 131 wherein said concentrating flow channel includes first and second flow channel portions in overlying relation, said first flow channel portion having a first flow channel inlet being said concentrating inlet and a first flow channel outlet, said second flow channel portion having a second outlet being said concenfrating outlet and a second flow channel inlet, said first flow channel outlet being connected to said second flow channel inlet.

133. The method and of claim 132 wherein said first flow channel portion includes a cation resin, and said second flow channel portion includes an anion resin.

134. The method of claim 132 wherein said first flow channel portion and said second flow channel portion are separated by a cation membrane.

135. The method of claim 132 wherein said first flow channel portion and said second flow channel portion are separated by a bipolar membrane.

136. The method of claim 133 wherein said first channel inlets and said second channel outlets are adjacent to said diluting outlets, and said diluting inlets are adjacent to said first channel outlets and said second channel inlets.

137. The method apparatus of claim 133 wherein said first channel inlets and said second channel inlets are adjacent to said diluting outlets, and said diluting inlets are adjacent to said first channel outlets and said second channel outlets.

138. Electrodeionization apparatus for purifying water comprising a cathode, an anode spaced from said cathode, a plurality of alternating anion permeable membranes and cation permeable membranes between said cathode and anode defining concentrating and diluting flow channels, each said channel being defined between an adjacent pair of said membranes, each said diluting flow channel having ion exchange materials therein, each said concentrating flow channel including a first guard channel adjacent said anion permeable membrane, a second guard channel adjacent said cation permeable membrane, and a brine channel between said first and second guard channels, said first and second guard channels having water with lower concentration of dissolved ions than water in said brine channel so as limit fransfer from a said concentrating flow channel to a said diluting flow channel.

139. The apparatus of claim 138 wherein a further anion permeable membrane separates said first guard channel from said brine channel, and a further cation permeable membrane separates said second guard channel from said brine channel.

140. The apparatus of claim 139 wherein said first guard channel includes anion exchange material, and said second guard channel includes cation exchange material.

141. A method of purifying water comprising providing a cathode, an anode spaced from said cathode, and a plurality of alternating anion permeable membranes and cation permeable membranes between said cathode and anode defining concenfrating and diluting flow channels, each said channel being defined between an adjacent pair of said membranes, each said diluting flow channel having ion exchange materials therein, supplying feed water into each said diluting flow channel and removing purified water therefrom, supplying concentrating water into said concentrating channels and removing brine therefrom, and providing water with lower concentration of dissolved ions than said brine at surfaces of said anion permeable membrane and cation permeable membrane at said concentrating charmel so as limit transfer from said concentrating flow channel to said diluting flow channel.

142. The method of claim 141 wherein said providing low concentration water step includes flowing low concenfration water into a first guard channel adjacent said anion permeable membrane and into a second guard channel adjacent said cation permeable membrane, and wherein said supplying concentrating water step includes supplying said concenfrating water into a brine channel between said first and second guard channels and removing said brine from said brine channel.

143. The method of claim 142 wherein a further anion permeable membrane separates said first guard channel from said brine channel, and a further cation permeable membrane separates said second guard channel from said brine channel.

144. The method of claim 143 wherein said first guard channel includes anion exchange material, and said second guard channel includes cation exchange material.