Electrodialysis unit for water treatment

Abstract

An electrodialysis unit 8 comprises a plurality of cathodes 68, a plurality of anodes 70 and a plurality of membranes 71; wherein the cathodes 68 and anodes 70 are arranged alternately in an electrode stack, with membranes 71 in between each cathode 68 and anode 70; and wherein the cathode 68 and the anode 70 are each formed of a single conductive plate such that both surfaces of the cathode plates and anode plates enclosed within the electrode stack are, in use, in conductive contact with the water being treated.

Claims

1. An electrodialysis unit comprising: a plurality of cathodes, a plurality of anodes and a plurality of membranes; wherein the cathodes and anodes are arranged alternately in an electrode stack, with a membrane in between each cathode and anode; and wherein the cathode and the anode are each formed of a single conductive plate such that both surfaces of the cathode plates and anode plates enclosed within the electrode stack are, in use, in conductive contact with the water being treated, wherein the conductive plates that form electrodes are clamped between and supported by non-conductive separators for separating the fluid flow into and out of the cathode flow paths and anode flow paths; and wherein the conductive plates are each provided with a seal that is bonded to a portion of the conductive plate and forms a shape corresponding to the shape of the outer edge of the separators, wherein the seal is bonded to two opposite sides of the conductive plate and not bonded to two other opposite sides of the conductive plate, and wherein the seal contacts the separators on either side of the conductive plate in order to form an enclosed electrode chamber around the conductive plate.

2. An electrodialysis unit as claimed in claim 1, wherein the electrode stack comprises multiple sets of electrodes forming multiple membrane cells, wherein each electrode in a single set is electrically connected in parallel, wherein each set of electrodes is electrically connected in series.

3. An electrodialysis unit as claimed in claim 1, wherein the electrical connections for the electrodes are made directly to the conductive material of the conductive plates.

4. An electrodialysis unit as claimed in claim 1, wherein the conductive plates extend outside of the reaction area of the electrodialysis unit to provide electrical connection points.

5. An electrodialysis unit as claimed in claim 1, wherein the separators include openings exposing the conductive plates in the reaction zones, and wherein the membranes are placed across these openings between the anode and the cathode to complete membrane cells.

6. An electrodialysis unit as claimed in claim 1, wherein the separators include inlet flow passages for incoming water and outlet flow passages for outgoing diluate and concentrate.

7. An electrodialysis unit as claimed in claim 1, wherein the separators include flow guide features for providing uniform and/or laminar flow to cathode and/or anode flow paths.

8. An electrodialysis unit as claimed in claim 7 wherein alternate cathode and anode separators comprise different flow guide features and/or provide for different flow rates for the cathode and the anode flow paths.

9. An electrodialysis unit as claimed in claim 8, wherein different separator designs support the anode and cathode plates at different positions when assembled in the electrode stack.

10. An electrodialysis unit as claimed in claim 1, including cathode chambers comprising first and second cathode separators located on either side of the cathode in the form of a conductive plate and anode chambers comprising first and second anode separators located on either side of the anode in the form of a conductive plate, wherein the electrodialysis unit comprises a sequence of anode and cathode chambers, with membranes between each of the cathode and anode chambers.

11. An electrodialysis unit as claimed in claim 1, wherein the separators include through holes that align in the electrode stack to form water inlet and outlet passages.

12. An electrodialysis unit as claimed in claim 11, comprising two cathode inlet passages, one anode inlet passage, two cathode outlet passages and one anode outlet passage, each of about the same size in order to provide an increased flow rate through the cathode flow paths as compared with the flow rate through the anode flow paths.

13. An electrodialysis unit as claimed in claim 11, wherein seals are provided about the through holes to maintain separation of anode fluid and cathode fluids.

14. A method of manufacturing an electrodialysis unit comprising a plurality of cathodes, a plurality of anodes and a plurality of membranes; the method comprising: arranging the cathodes and anodes alternately in an electrode stack, with membranes in between each cathode and anode; wherein the cathode and the anode are each formed of a single conductive plate such that within the electrode stack both surfaces of the cathode plates and anode plates are, in use, in conductive contact with the water being treated; wherein the conductive plates that form electrodes are clamped between and supported by non-conductive separators for separating the fluid flow into and out of the cathode flow paths and anode flow paths; and wherein the conductive plates are each provided with a seal that is bonded to a portion of the conductive plate and forms a shape corresponding to the shape of the outer edge of the separators, wherein the seal is bonded to two opposite sides of the conductive plate and not bonded to two other opposite sides of the conductive plate, and wherein the seal contacts the separators on either side of the conductive plate in order to form an enclosed electrode chamber around the conductive plate.

15. A method as claimed in claim 14, wherein a thermosetting or vulcanising rubber is used for the seal, and the method comprises applying the rubber to the electrode prior to heat treatment, and then bonding the rubber to the electrode by performing a thermosetting or vulcanising process whilst it is in contact with the electrode.

16. A method as claimed in claim 14, wherein the method includes clamping each conductive plate and seal between non-conductive separators.

17. A method as claimed in claim 16, comprising providing openings in the separators exposing the conductive plates in the reaction zones and placing membranes across these openings between the anode and the cathode, such that the membranes are sandwiched between adjacent electrodes.

18. An electrodialysis unit as claimed in claim 2, wherein the electrodes at the outer ends of each set of electrodes in the electrode stack are both cathodes.

19. An electrodialysis unit as claimed in claim 18, wherein the seals comprises a thermosetting or vulcanising rubber bonded to the electrode by a thermosetting or vulcanising process.

20. An electrodialysis unit comprising: a plurality of cathodes, a plurality of anodes and a plurality of membranes; wherein the cathodes and anodes are arranged alternately in an electrode stack, with a membrane in between each cathode and anode; and wherein the cathode and the anode are each formed of a single conductive plate such that both surfaces of the cathode plates and anode plates enclosed within the electrode stack are, in use, in conductive contact with the water being treated, wherein the conductive plates that form electrodes are clamped between and supported by non-conductive separators for separating the fluid flow into and out of the cathode flow paths and anode flow paths; wherein the separators include through holes that align in the electrode stack to form water inlet and outlet passages comprising two cathode inlet passages, one anode inlet passage, two cathode outlet passages, and one anode outlet passage, each of about the same size in order to provide an increased flow rate through the cathode flow paths as compared with the flow rate through the anode flow paths.

Description

(1) Preferred embodiments of the invention will now be described by way of example only and with reference to the accompanying drawings in which:

(2) FIG. 1 shows a ballast water treatment system with an electrodialysis unit,

(3) FIG. 2 illustrates an electrodialysis unit including a stack of electrodes,

(4) FIG. 3 shows a single electrode chamber as used in the unit of FIG. 2,

(5) FIG. 4 shows an electrode plate and seal,

(6) FIG. 5 is a partial cutaway view of an electrodialysis unit in which the flow distributor can be seen,

(7) FIG. 6 is a perspective view of the internal tube of the flow distributor,

(8) FIG. 7 is a partial view of a separator showing the flow conditioning elements,

(9) FIG. 8 is a schematic wireframe drawing showing further detail of the flow distributor and flow conditioning elements,

(10) FIG. 9 is a cross-section through a portion of two cathode chambers and one anode chamber showing the leading edges of the electrodes,

(11) FIG. 10 shows a plot of velocity across each of the cathode chambers along the electrode stack in a computer model when the preferred flow distributor is not used,

(12) FIG. 11 shows a plot of velocity across each of the cathode chambers along the electrode stack in a computer model when the preferred flow distributor is used,

(13) FIG. 12 shows a plot of velocity across the width of a cathode flow path in a computer model when the preferred flow conditioning elements are not used, and

(14) FIG. 13 shows a plot of velocity across the width of a cathode flow path in a computer model when the preferred flow conditioning elements are used.

(15) The arrangement of FIG. 1 utilises an electrodialysis unit within a ballast water treatment system, but it will be appreciated that other uses for the preferred electrodialysis unit exist, and that the electrodialysis unit can be adapted to suit different requirements. In particular, it should be understood that the electrodialysis unit described herein can be used in ballast water treatment, or in other water treatment applications, without the need for combination with other treatment types as shown in the exemplary arrangement of FIG. 1.

(16) FIG. 1 thus illustrates a ballast water treatment system that includes an electrodialysis unit 8. In this example, the water is filtered and then treated by a cavitation unit 10, a gas injection unit 14 and the electrodialysis unit 8. This series of treatments causes damage and death to the organisms in the water. As well as affecting organisms in the water, nitrogen added to the water at the injection unit 14 reduces the level of dissolved oxygen in the water and reduces the potential of re-growth of organisms as well as reducing the weathering of coatings and the speed of corrosion. Furthermore, the reduction in oxygen is thought to prolong the effect of oxidants introduced into the water via the product of the electrodialysis unit 8. By controlled atmosphere management when the ballast tanks are empty by using nitrogen, these effects are enhanced further.

(17) During filling of the ballast tanks, ballast water is pumped from the sea through an inlet pipe 1 by the use of the ship's ballast pump system 2. After the pump 2, water flows through a pipe and is filtered through a first filter 4, which filters larger particles from the water. These form a sludge which is discharged at the point of ballast uptake.

(18) Downstream of the first filter 4, a pressure booster may optionally be installed. The pressure booster can be used to maintain the level of water pressure needed for successful treatment in the units further downstream.

(19) In this example, water then continues to flow into the cavitation unit 10. In the cavitation unit 10 hydrodynamic cavitation is induced by a rapid acceleration of the fluid flow velocity, which allows the fluid static pressure to rapidly drop to the fluid vapour pressure. This then leads to the development of vapour bubbles. After a controlled period of time which allows bubble growth, a rapid controlled deceleration then follows. This causes the fluid static pressure to rise rapidly which causes the vapour bubbles to violently collapse or implode exposing any organisms or the like in the water to the high intensity pressure and temperature pulses, which breaks down the organisms in the water.

(20) After the cavitation unit 10, a part of the water flows through the electrodialysis unit 8. The remainder of the water is not treated by the electrodialysis unit 8, and can simply continue to flow along a pipe or conduit to the later treatment stages. In the embodiment of FIG. 1 the electrodialysis unit is fitted externally to the main flow conduit, and thus could be retro-fitted to an existing treatment system.

(21) In an alternative embodiment, instead of or in addition to the treatment of incoming ballast water by the electrodialysis unit 8, another source of brine or saltwater 24 can be used as the input electrolyte for the electrodialysis unit 8. This could, for example, be brine produced as a by-product in a ship's freshwater production.

(22) The electrodialysis unit 8 of the preferred embodiment is provided with a temperature control system 9. This is used to ensure that the water utilised by the electrodialysis unit 8 does not drop below a set temperature. The temperature control system 9 includes a temperature monitoring device 9a for monitoring the temperature of incoming water and a heater 9b for increasing the temperature of the incoming water before it reaches the membrane cell of the electrodialysis unit 8. The heater 9b is arranged to operate to increase the temperature of the incoming water when the original water temperature is below a predetermined level. In this embodiment the predetermined level is 16 C. If the temperature of the incoming water is below 16 C. then the water is heated up to about 20 C. using the heater. The heater 9b uses waste heat from the ship's engines.

(23) The electrodialysis unit 8, which is described in more detail below with reference to FIGS. 2 to 9, produces a diluate stream 11 and a concentrate stream 12. These two streams progress to a pH balancer or mixing unit 13, which produces a product 17 of the electrodialysis unit 8 that is directed back into the main water flow, and depending on the composition of the product 17, the mixing unit 13 may also give out a residue of diluate 18. The mixing unit 13 includes a pump or the like to control the amount of diluate 11 which is added to the concentrate 12 to form the optimum product 17 of the electrodialysis unit 8.

(24) Downstream of the point of injection of the product 17 of the electrodialysis unit 8 there is a sampling and measurement point 15, which measures ORP and/or TRO and communicates the measured values to the mixing unit 13. These measurements monitor the effect of the electrodialysis unit 8 on the water and are used to control the mixing unit 13, for example by controlling a dosing pump.

(25) The diluate residue 18 may be reinjected into the incoming water prior to all treatment steps, and preferably also before the filter 4 and/or the ballast water pump 2. Alternatively, it may be stored in a holding tank 25 or ship's bilge water tank 26.

(26) In the arrangement shown, the gas injection unit 14 treats the water after the product 17 of the electrodialysis unit 8 is returned to the main flow. However, in alternative arrangements the product 17 is returned to the main flow downstream of the gas injection unit 14, with the monitoring unit 15 likewise downstream of the gas injection unit 14, monitoring the water conditions after the product 17 has been mixed in.

(27) In the gas injection unit 14, nitrogen gas 16 is injected into the incoming water using a steam/nitrogen injector or a gas/water mixer in order to achieve the desired level of nitrogen super-saturation in the water, which kills organisms and reduces corrosion by reducing the oxygen level. This also prolongs the treatment effect of the oxidants in the water.

(28) Downstream of the treatment units, treated water is distributed by the ship's ballast water piping system 23 to ballast water tanks. Here, excess gas is evacuated until a stable condition is achieved. This is regulated by means of valves integrated with the tanks ventilation system. These valves ensure stable conditions in the tank during the period the ballast water remains in the tank, in particular a high level of nitrogen super-saturation and a low level of dissolved oxygen in the water. Maintaining the level of super-saturation leads to an ongoing water treatment both by the super-saturation itself and also by oxidants introduced by the electrodialysis unit 8. The treatment thus results in treated water that continues to kill or disable any surviving organisms whilst the water is stored in the ballast tanks.

(29) Water is then left to rest in the ballast water tanks. When the ballast water is discharged, water flows through a discharge treatment process that returns the oxygen content of the water to an environmentally acceptable level for discharge. The water is pumped from the ballast tanks and passes through at least the gas injection unit 14. This is used to return oxygen to the water as air replaces nitrogen as the injection gas. Optionally, the water may be re-treated by the cavitation unit 10 as it is discharged.

(30) The operation of the electrodialysis unit 8 will now be explained. An embodiment of the structural arrangement of electrodialysis unit 8 is described below with reference to FIGS. 2 to 9. As discussed above, electrodialysis is an electro-membrane process where ions are transported through ion exchange membranes in a fluid system. In the simplest implementation of an electrodialysis unit a single membrane is placed between two electrodes. An electric charge established by applying a voltage between two electrodes allows ions to be driven through the membrane provided the fluid is conductive. The voltage is applied by power connection points of a conventional type, which are not shown in the drawings. The two electrodes represent respectively the anode and the cathode. The electric charge creates different reactions at the different electrodes. At the anode, the electrolyte will have an acidic characteristic whilst at the cathode, the electrolyte will be characterised by becoming alkaline. Membranes used in electrodialysis are chosen for the ability to allow ion exchange whilst being liquid impermeable. This allows the alkaline solution to be kept separate from the acidic solution.

(31) Various reactions which occur in an electrodialysis membrane cell where the incoming electrolyte is ballast water taken from a ballast water pipeline (i.e. sea water) are shown in Table 1 below. This includes a reaction on the cathode side that produces brucite (Mg(OH).sub.2). Other reactions will also occur since various compounds may be present in the water in addition to sodium and magnesium salts.

(32) TABLE-US-00001 TABLE 1 Reactions at the anode: Reactions at the cathode: 2Cl.sup. 2e .fwdarw. Cl.sub.2 2H.sub.2O + 2Na.sup.+ + 2e .fwdarw. 2NaOH + H.sub.2 2H.sub.2O 4e .fwdarw. 4H.sup.+ + O.sub.2 2H.sub.2O + 2e .fwdarw. H.sub.2 + 2OH.sup. Cl.sub.2 + H.sub.2O .fwdarw. HClO + HCl O.sub.2 + e .fwdarw. O.sub.2.sup. HCl + NaOH .fwdarw. NaCl + H2 O.sub.2.sup. + H.sup.+ .fwdarw. HO.sub.2 Cl.sup. + 2OH.sup. 2e .fwdarw. ClO.sup. + H.sub.2O O.sub.2 + H.sub.2O + 2e .fwdarw. HO.sub.2.sup. + OH.sup. 3OH.sup. 2e .fwdarw. HO.sub.2.sup. + H.sub.2O O.sub.2 + 2H.sub.2 + 2e .fwdarw. H.sub.2O.sub.2 + 2OH.sup. HO.sub.2.sup. e .fwdarw. HO.sub.2 H.sup.+ + e .fwdarw. H.sup. OH.sup. e .fwdarw. OH.sup. H.sup. + H.sup. .fwdarw. H.sub.2 OH.sup. + OH.sup. .fwdarw. H.sub.2O.sub.2 OH.sup. + OH.sup. .fwdarw. H.sub.2O.sub.2 HClO + H.sub.2O.sub.2 .fwdarw.HCl + O.sub.2 + H.sub.2O H.sub.2O.sub.2 + OH.sup. .fwdarw. HO.sub.2 + H.sub.2O ClO.sup. + H.sub.2O.sub.2 .fwdarw. .sup.1O.sub.2 + Cl.sup. + H.sub.2O H.sub.2O.sub.2 H.sup.+ + HO.sub.2.sup. H.sub.2O.sub.2 + OH.sup. HO.sub.2.sup. + H.sub.2O OH.sup. + HO.sub.2.sup. O.sub.2.sup.2 + H.sub.2O O.sub.2.sup.2 + H.sub.2O.sub.2 .fwdarw. O.sub.2.sup. + OH.sup. + OH OH + H.sub.2O.sub.2 .fwdarw.H.sub.2O OH.sup. + HCO.sub.3.sup. + Ca.sup.2+ = CaCO.sub.3 + H.sub.2O 2OH.sup. + Mg.sup.2+ = Mg(OH).sub.2

(33) Table 2 below illustrates typical properties for an acidic solution produced at the anode and an alkaline solution produced at the cathode. The acidic solution forms the concentrate stream and the alkaline solution forms the diluate stream.

(34) TABLE-US-00002 TABLE 2 pH TRO (mg Cl/L) ORP (mV) Acidic solution (at the anode) 2-4 400-1200 1100-1200 Alkaline solution (at the cathode) 11-14 800-900

(35) The two separated streams are mixed in a ratio providing a product of the electrodialysis unit and optionally a residue with typical characteristics shown in Table 3. The product is mainly concentrate from the anode, possibly with the addition of diluate to control the pH level. The residue will be formed of any diluate that is not mixed in to the product. Typically the pH of the product in preferred implementations of the electrodialysis treatment is between 4-6, but treatment of the water will also occur within the broader pH range given below.

(36) TABLE-US-00003 TABLE 3 pH TRO (mg Cl/L) ORP (mV) Product 2-8.5 400-1000 750-800 Residue 8.5-14 800-900

(37) In order to tailor the chemical characteristics of the two streams, cross-treatment may be applied. This may constitute of an arrangement allowing all of or a portion of one or both streams to be re-injected at the entrance to the opposite compartment to the compartment from which it arrived from. Thus, the concentrate stream produced by the anode could be cross-treated by re-injection into the cathode side of the electrodialysis unit. The characteristics of the stream(s) expressed by pH, ORP and TRO may be further tailored by this method and enable the amount of residual diluate after mixing to be reduced if mixing is applied in addition.

(38) The mixing ratio will depend on the quality of the raw electrolyte, the size of the electrodes and the power applied.

(39) The product of the electrodialysis unit enters the ballast water flow in conjunction with the point of injection of the N.sub.2, preferably immediately behind, and thus is introduced into the water in conjunction with the process of super-saturation/oxygen removal. The residue, if any, is injected upstream in the main flow immediately in front of the filter.

(40) FIGS. 2 to 9 illustrate an embodiment of an electrodialysis unit 8 that can be used to treat water. The electrodialysis unit may be used in the ballast water treatment system of FIG. 1 or in any other appropriate water treatment system. It can be used alone to provide a treatment effect, or alternatively it can be used in combination with other water treatment devices.

(41) FIG. 2 illustrates an electrodialysis unit 8 including a stack of electrode chambers 30 sandwiched between two end plates 32. The electrode stack is clamped between the end plates 32 by screws 34. The electrode chambers 30 are placed together in sets of ten membrane cells separated by insulating layers. The sets of electrode chambers 30 and plastic insulating layers can be seen more clearly in FIG. 5. The electrode chambers 30 are arranged in sets in this fashion to enable a series connection of multiple sets of chambers 30. Water enters the electrode stack via cathode water inlets 50 and an anode water inlet 52 at the base of the electrode chambers 30 and then flows upward through the anode and cathode chambers. The water inlets 50, 52 are at the reverse side of the electrodialysis unit 8 in FIG. 2, but can be seen in FIG. 5 in which the unit 8 is shown from the opposite side. The diluate stream 11 from the cathode reaction and the concentrate stream 12 from the anode reaction exit the electrode stack via a concentrate outlet 36 and diluate outlets 38. As discussed above, it is advantageous to have a higher flow rate on the cathode side and so the preferred embodiment includes two water inlet pipes for the cathode side and consequently two outlet pipes 38 for the diluate, with only one concentrate outlet 36. Also shown in FIG. 2 are exposed ends 40 of the electrodes and the electrical connection board 42 for the electrical supply to the electrodes.

(42) FIG. 3 shows a single electrode chamber 30. The unit 8 of FIG. 2 consists of a large number of these electrode chambers 30 stacked together. The electrode chamber 30 includes a titanium electrode plate 44 supported by and within two separators 46, which are placed one on either side of the electrode 44. A rubber seal 48 extends around the outer edge of the separators 46 and provides a water tight barrier enclosing the electrode chamber 30. The exposed ends 40 of the electrodes extend beyond the rubber seal 48 so that the electrical connections 42 can be made outside of the reaction zone.

(43) Water enters the electrode chamber 30 via through holes 54 at one end and exits via through holes 54 at the other end. The through holes 54 are in fluid communication with the corresponding water inlets 50, 52 and water outlets 36, 38. Each separator 46 has through holes 54 for each of the three inlets 50, 52 and outlets 36, 38. Within the electrode chamber 30 the separators 46 are provided with flow guides for passage of water from the appropriate water inlet to the appropriate water outlet. Thus, the cathode electrode chamber will have flow guides to take water from the cathode water inlets 50 via the two outer through holes 54 at the inlet side, direct it to pass across the cathode, and then pass the diluate from the cathode reaction via further flow guides to the outer through holes 54 on the outlet side and hence to the diluate outlets 38. The anode electrode chamber will have flow guides to take water from the anode water inlet 52 via the central through hole 54 at the inlet side, direct it to pass across the anode, and then pass the concentrate from the anode reaction via further flow guides to the central through hole 54 on the outlet side and hence to the diluate outlet 36.

(44) FIG. 4 shows an electrode plate 44 and seal 48 prior to attachment of the separators 46. The rubber seal 48 is bonded to the electrode plate 44 along two sides as shown in the Figure. The seal 48 is also on both front and back surfaces of the electrode plate 44. The exposed end 40 of the electrode plate 44 extends beyond the seal along one side of the electrode plate to permit electrical connection as set out above.

(45) FIG. 5 is a partial cutaway view of an electrodialysis unit showing details of the flow distributor 56 for one of the cathode water inlets 52. FIG. 5 also more clearly shows the five sets of membrane cells separated by plastic insulating layers. The construction of the membrane cells is described in more detail below with reference to FIG. 9. In FIG. 5 one of the end plates 32 and each of the electrode chambers 30 are partially cut away to expose a circular passage formed by aligned through holes 54 (also partially cut away). This circular passage forms a first tube 58 of the flow distributor 56. The first tube 58 can be seen more clearly in the wireframe diagram of FIG. 8, which shows more detail of the fluid flow arrangement for the cathodes. The flow distributor 56 also includes a second tube 60, located concentrically within the through holes 54. In FIG. 5 this second tube 60 is inserted for one of the cathode inlets 50, but it is not shown for the other cathode inlet 50 or for the anode inlet 52. When the electrodialysis unit 8 is complete there is a second tube 60 in each water inlet, fitted concentrically with each set of through holes 54.

(46) The second tube 60 includes holes 62 along its length. These holes 62 take the form of transverse slits cut on two sides of the second tube 60, and placed at the upper and lower sides of the second tube 60 when it is inserted in the first tube 58. FIG. 6 is a perspective view of the second tube 60 of the flow distributor 56 and shows further detail, including the holes 62 on the second, lower, side of the second tube 60.

(47) Flow conditioning elements 64 on the separator 46 for the cathode chamber are shown in FIG. 7A, which is a partial view of the lower part of a cathode separator 46. The flow conditioning elements 64 are for evenly distributing the flow across the width W of the cathode flow path.

(48) The three through holes 54 would align with through holes 54 in other separators 46 in the electrode stack to form the first tubes 58 of the flow distributors. The second tubes 60, which are not shown in FIG. 7, would be inserted into the aligned through holes 54, with holes 62 in the second tubes 60 allowing water to pass into the first tubes 58. In FIG. 7A since the separator 46 is for the cathode chamber the outer through holes 54 would be open to the cathode flow paths whereas the central through hole 54 would be sealed to prevent water from the anode inlet 52 entering the cathode chamber. This sealing may be achieved by an O-ring seal placed about the central through hole. Holes would hence be formed in the first tubes 58 at the two outer through holes 54 to permit water to pass from the water inlets 50, along the tubes 60, 58 and then to the cathode reaction area via the flow conditioning elements 64.

(49) The flow conditioning elements 64 take the form of channels extending away from the through holes 54 in a fan shape in order to distribute water evenly across the entire width W of the cathode flow path. The channels are recessed into the separator 46 and separated from each other by walls 66. When the two separators 46 that form the cathode chamber are joined together the walls 66 on each separator 46 face each other and come into contact so that the channels are sealed. Each channel has an end portion that is parallel with the flow direction through the cathode flow path. This helps reduce turbulence and promotes laminar flow.

(50) FIG. 7B is a similar partial view of a separator 46 for the anode chamber. This anode separator includes flow conditioning elements 65 for the anode flow path. As with the cathode flow conditioning elements 64 the anode flow conditioning elements 65 take the form of channels extending away from the through hole 54 in a fan shape in order to distribute water evenly across the entire width W of the anode flow path. Since the anode flow path is supplied with water from only the single central through hole 54 the anode flow conditioning elements 65 fan out over a larger angle than the cathode flow conditioning elements 64. This allows water from flow distributor 56 in the central through hole 54 to be evenly distributed over the anode flow path. The two outer through holes would be sealed, e.g. by an O-ring seal, to prevent water ingress from the cathode water supply. The anode flow conditioning elements 65 are recessed channels divided by walls 67. The flow conditioning part of the anode separator 46 extends for a greater distance away from the through holes 54, since the leading edge of the anode is located at a greater distance from the water inlet, as discussed in more detail below with reference to FIG. 9.

(51) FIG. 8 is a schematic wireframe drawing showing further detail of the flow distributor 56 and flow conditioning elements 64 for the cathode flow paths in the electrode stack. The detail of the flow conditioning elements 64 is omitted for clarity, but the fan shapes can be seen. Each cathode chamber has two symmetrical sets of flow conditioning elements 64 that join in similar fashion to two flow distributors 56 in the two outer through holes 54 of the separators 46. As discussed above, the through holes 54 are aligned to produce a first tube 58 of the flow distributor 56. The first tube 58 connects to each of the sets of flow conditioning elements 64 via holes on an upper side. A second tube 60 located concentrically within the first tube 58 supplies water to the first tube 58 from the two cathode inlets 50. Water passes between the first tube 58 and the second tube 60 via slit shaped holes 62 in upper and lower surfaces of the second tube.

(52) The two tube flow distributor 56 acts to distribute water equally to each cathode chamber along the length of the electrode stack 30. The flow conditioning elements 64 provide even distribution of the water across the width W of each cathode flow path, and also promote laminar flow in the cathode flow paths.

(53) For the anode chamber there is an arrangement similar to that shown in FIG. 8, but with water being distributed from only the central through hole 54 instead of from the two outer holes 54. The anode water flow path passes through a flow distributor 56 of identical design to the flow distributor 56 described above, using first and second tubes 58, 60. This flow distributor 56 would be formed using a first tube 58 created by the aligned central through holes 54 that connect to the anode water inlet 52.

(54) After the incoming water passes through the flow distributors 56 and exits the flow conditioning elements 64, 65 it flows into the cathode and anode flow paths within the cathode and anode chambers. At this point, as explained below with reference to FIGS. 10 to 13, the water is equally distributed to each flow path along the electrode stack and evenly distributed across the width W of each flow path. The equal distribution of the water ensures an equal rate of reaction across each membrane cell in the electrode stack. The even distribution of water across each flow path width W means that the reaction occurs evenly over the width of the electrodes, and also promotes laminar flow in the cathode flow paths.

(55) FIG. 9 is a cross-section through a portion of two cathodes 68 and one anode 70 at the point where water enters the cathode chambers and electrode chamber. A membrane 71 is located between the electrodes to form the membrane cells. The Figure shows a partial cross-section through two complete membrane cells (one either side of the anode 70) and two partial membrane cells (at the outside portions of the two cathodes 68).

(56) FIG. 9 illustrates further features used to promote laminar flow through the electrode chambers, especially in the reaction zone of the cathode flow path. Incoming water for the cathode flow paths 72 arrives from the flow conditioning elements 64 of the separators 46 as indicated by the arrow C. Water for the anode flow paths 74 arrives from the flow conditioning elements 65 as indicated by the arrow A. The water flow through the flow conditioning elements 64, 65 supplies two flow paths 72, 74 that pass along each of the two sides of the respective cathode 68 or anode 70.

(57) The water exiting the flow conditioning elements 64, 65 is allowed to flow a fixed distance where the flow is undisturbed before the flow is divided gently into two equal flows that enter the flow paths 72, 74 on either side of the electrodes. This fixed distance helps the flow to recover from any disruptive effects that may have arisen from the previous flow guides. A gentle division of the flow is achieved through the shape of the electrode leading edge 76, which is wedge-shaped to minimise turbulence. The fixed distance of undisturbed flow in the preferred embodiment is around 10 mm.

(58) It will be noted that the leading edge 76 of the anode 70 is placed at a larger distance away from the water inlet than the leading edge 76 of the cathode 68. The electrodialysis unit is designed such that water flows an additional fixed distance X over the cathode before being subjected to electrical treatment in the reaction zone. This further distance X allows any residual turbulence to dissipate and helps the flow to develop into a laminar flow before the seawater is subjected to any electrical current. This is achieved through the use of different lengths of anode 70 and cathode 68 which permits an offset cathode/anode configuration. In the preferred design shown herein this fixed distance X is around 30 mm with a gap of 2 mm between cathode 68 and membrane. The reaction zone begins when both the anode 70 and cathode 68 are present in sufficient proximity, in this case this will be after the distance X as marked on the Figure. In the reaction zone electrodialysis occurs and as the water passes along the anode flow paths 74 and cathode flow paths 72 in the reaction zone ion exchange occurs across the membranes 71, generating an acidic concentrate on the anode side and alkaline diluate on the cathode side as described above. The concentrate and diluate exit the electrodialysis unit via outlets 36, 38 and are used to treat water by mixing the concentrate with some or all of the diluate to provide a product of the electrodialysis unit, which is harmful to micro organisms.

(59) On each side of the anode 70 a spacer element 78 is included in the anode flow paths 74. To avoid turbulence there are no spacer elements on the cathode flow paths 72. In the cathode flow paths 72 conditioned flow is provided by the flow conditioning elements 64. This flow becomes more laminar as it passes across the 10 mm region of undisturbed flow, after which it is divided by the wedge shaped end 76 of the cathode 68. The water then flows along two cathode flow paths 72 for a further distance of 30 mm, which acts to further promote laminar flow. By the time the incoming water enters the reaction zone in the cathode flow paths 72 the flow is generally laminar. As discussed above, this laminar flow avoids the build-up of brucite deposits and also helps avoid build-up of other contaminants.

(60) As discussed above, the preferred electrodialysis unit is made up of several sets of membrane cells, with each set of cells being formed by five anodes and six cathodes, with cathodes being placed at the outer ends. With this arrangement the outer cathodes would only have one active side, with one flow path along the inner side of the cathodes. The outer surfaces of the outer cathodes would not be active and would be blocked to prevent water flowing.

(61) Computer modelling has been used to illustrate the advantageous effects of the preferred embodiment.

(62) FIGS. 10 and 11 show the effect of the two tube flow distributor system. FIG. 10 shows a plot of velocity across each of the cathode chambers along the electrode stack in a computer model when the preferred flow distributor 56 is not used, whereas FIG. 11 shows a plot of velocity across each of the cathode chambers along the electrode stack in a computer model when the preferred flow distributor 56 is used. The plots show flow velocity on the vertical axis with the horizontal axis showing the distance of the cathode flow path 72 from the cathode water inlet 50 at the end of the electrode stack. As can be seen by a comparison of the Figures when the flow distributor 56 is not used there is a considerably higher velocity in the cathode flow paths 72 at greater distances from the water inlet 50. When the flow distributor 56 is used the water is significantly more evenly distributed along the length of the electrode stack.

(63) FIGS. 12 and 13 show the effect of the flow conditioning elements 64 on water flow across the cathode flow paths 72. FIG. 12 shows a plot of velocity across the width of a cathode flow path in a computer model when the preferred flow conditioning elements 64 are not included, and the water instead passes through a fan shaped region without the channels 64 or walls 66. FIG. 13 shows a plot of velocity across the width of a cathode flow path in a computer model when the preferred flow conditioning elements 64 are present. The vertical axis shows flow velocity and the horizontal axis shows the distance across the width of a cathode flow path 72. The peaks in each plot illustrate the likely velocity at points across the width W of the cathode flow path 72. The sharp troughs are due to the effect of the flow conditioning elements at the exit of the chamber which soon dissipate away. As can be seen, when the average flow across the chamber is studied, the channels 64 and walls 66 provide for a more even distribution of velocity and thus flow across the width W of the cathode flow path 72. When they are not present the velocity and thus flow is less even and this would lead to turbulence and secondary flows in subsequent parts of the cathode flow path 72.