Electrodeionization is a deep demineralisation stage in which ion exchange proceeds continuously and the resin is regenerated not by a reagent but by a direct-current electric field. Dissolved salts leave the stream through ion-exchange membranes, so no acid or alkali is needed on site for regeneration and no saline effluent is produced.
Principle
The unit is assembled from alternating cation- and anion-exchange membranes. Between them two kinds of chamber are formed: diluting chambers filled with a mixed bed of ion-exchange resin, and concentrate chambers that receive the removed salts. A direct-current electric field is applied across the flow — cathode on one side of the stack, anode on the other. Water travels along the chambers and ions travel across them, which is the essential difference from a filter, where what is retained stays in the bed until regeneration.
Dissolved salts are picked up by the resin exactly as in an ordinary ion-exchange filter, but they do not stay on it. Under the field, cations move towards the cathode through the cation-exchange membrane and anions towards the anode through the anion-exchange one; each membrane passes ions of its own sign only, so impurities do not return to the diluting stream. The resin here works less as a capacity than as a conducting medium along which an ion reaches the membrane: without it, water of such low salinity would hardly conduct current at all.
The resin is restored by H⁺ and OH⁻ ions, and these do not come from a reagent. At the boundaries between cation and anion resin beads, a sufficient voltage splits the water itself, and the ions formed convert the resin back into its working form continuously. Regeneration is therefore combined with demineralisation and never stops. Hence the principal consequence for operation: no acid and no alkali is stored or dosed on site, and no saline regeneration effluent is produced at all.
Module capacity is stated as an interval, and both of its limits carry meaning. Above the upper one the water has no time to give up its ions to the resin, and the resistivity at the outlet falls. Below the lower one the velocity in the chambers drops while the share of current per unit volume rises: concentration in the concentrate chambers increases and deposits on the membranes become possible. Flow is therefore set by calculation on a par with current, and a scheme with a variable draw-off is built around a storage tank and a recirculation loop. Outlet quality itself is customarily expressed as resistivity in MΩ·cm rather than as salinity, and readings are corrected to 25 °C: without that conversion, measurements at different points cannot be compared with one another.
Principle
Part of the current in the unit is spent not on transporting impurities but on splitting water. At first sight this is a loss to be eliminated by lowering the voltage. In reality it is precisely this share of the current that provides continuous regeneration: without the H⁺ and OH⁻ produced, the resin would pass into its salt form and the unit would turn into an ordinary ion-exchange filter that nobody regenerates. The working current is therefore set above what ion transport alone requires, and the unit cannot be «tuned to run more economically» by reducing the voltage: the saving is paid for in water quality.
Resistivity is the reciprocal of conductivity and is therefore strongly non-linear with respect to salt content. One MΩ·cm corresponds to a conductivity of 1 µS/cm, that is roughly half a milligram of dissolved salts per litre; 18.2 MΩ·cm is the limiting value for water at 25 °C in which the only charge carriers left are the products of its own dissociation. The whole difference between «one» and «eighteen» therefore amounts to fractions of a milligram per litre, and a requirement of «not below 15 MΩ·cm» differs from «not below 1 MΩ·cm» in the cost of the scheme, not by fourteen units of a scale.
Reverse osmosis retains almost no carbon dioxide: the molecule is electrically neutral and passes the active layer of the membrane practically unhindered. In water downstream of osmosis it is therefore present in its original amount, and inside the diluting chamber, at the high pH near the resin surface, it converts to bicarbonate and is carried by the field like any other anion. The unit does remove it, but pays in current: at an elevated carbon dioxide content, less of the current load is left for the target impurities. For this reason deep schemes place degassing between the membrane stage and electrodeionization, and where the content is persistently high the water is alkalised ahead of the membrane so that the carbon dioxide converts to bicarbonate and is retained by osmosis itself.
Practice
Electrodeionization works on reverse-osmosis permeate and on nothing else. This is not a preference in schematic design but a condition of operability: raw water at full salinity creates a current load the stack is not designed for, and hardness salts precipitate inside the chambers at the high pH near the membrane surfaces. The order of stages here admits no rearrangement.
The main argument for the stage is not the depth of demineralisation as such but the absence of a reagent facility. A mixed-bed ion-exchange filter gives comparable water quality, yet it calls for acid and alkali stored on site, a room and permits for them, trained personnel and neutralisation of regeneration effluent. Electrodeionization removes all four items entirely: there are no reagents at all.
The unit neither disinfects the water nor removes organic matter. In pharmaceutical and microelectronic schemes ultraviolet treatment follows it, and where total organic carbon is tightly limited, sorption and ultraviolet oxidation precede it. Water downstream of the unit is practically free of salts and therefore aggressive: the loop is built from stainless steel of a suitable grade or from polymers, not from carbon steel.
We do not manufacture the unit. Our work is the water analysis and the calculation of the whole scheme, from pretreatment to the point of use, the selection of modules and their number, the design of the pipework and the recirculation loop, supply, commissioning and service.
Limits
Practice
The cases are typical examples, not site reports.
Scheme
Electrodeionization is never a stand-alone stage. Reverse osmosis is mandatory ahead of it, and ahead of that comes pretreatment removing suspended solids, iron, manganese, free chlorine and hardness. A scheme in which the unit stands second from the source is inoperable.
The requirements for the inlet water are stated quantitatively, not as the words «osmosis permeate». What is limited is the conductivity of the feed, the hardness — to a value of the order of 1 mg/l as calcium carbonate, that is roughly 0.02 meq/l, — free chlorine, which must be absent, iron and manganese at hundredths of a milligram per litre, and the content of carbon dioxide and total organic carbon. These figures are set by the module manufacturer, and it is they that determine the stages placed ahead of it.
Carbon dioxide is handled as a separate decision in the scheme. It is removed either by degassing between the membrane stage and the unit, or by alkalising the water ahead of the membrane, whereupon it converts to bicarbonate and is retained by osmosis itself. The choice is made by calculation: the first option adds no salts to the water, the second needs neither a separate apparatus nor ventilation.
Downstream of the unit a storage tank with a vent filter and ultraviolet disinfection are provided, and where requirements are tight, a permanent recirculation loop. Water of this quality is not stored motionless: it absorbs carbon dioxide from the air, loses resistivity and becomes colonised by biofilm. Loop materials are chosen for the aggressiveness of the water, not by the practice customary in water treatment.
Scheme
Operation
Energy
The specific electricity consumption of the unit itself is small and is governed by the salinity of the feed: working on reverse-osmosis permeate of the specified quality it amounts to some 0.1–0.3 kWh per cubic metre of demineralised water. Judging the stage by this figure in isolation from the scheme would nevertheless be wrong: in a deep demineralisation scheme the main energy is drawn by the high-pressure pump of the reverse-osmosis unit, and it is that pump which determines total consumption per cubic metre. The module is fed with direct current from a rectifier, and voltage and current are set by calculation from the water composition and the flow.
Consumables
What goes to drain
The removed salts leave with the flow from the concentrate chambers. In volume this is the smaller part of the feed, and in composition it is water that has taken up the impurities from the diluting chambers. Since the feed is reverse-osmosis permeate, the salinity of that flow remains low in absolute terms and bears no comparison with the regeneration effluent of an ion-exchange filter: the stage produces no saline effluent requiring neutralisation or separate agreement at all. In certain designs an additive raising conductivity is introduced into the concentrate loop; whether this is needed follows from the design of the module and is stated by the manufacturer.
Recovery
The share of water delivered to the consumer is set by the flow of the concentrate loop. With modules of this class the larger part of the feed goes to the consumer and the smaller part to the concentrate; the actual value depends on the design of the module, on the quality of the feed water and on the quality specified at the outlet, so it is arrived at by calculation rather than assigned in advance. This water need not be lost: in composition the concentrate is close to the feed, and it is usually returned to the inlet of the reverse-osmosis unit, where it mixes with the raw stream. The decision is taken at the design stage, since it calls for space, a pump and for the return to be accounted for in the balance of the membrane stage.
Sizing
What we account for
What a wrong choice costs
Sizing
Questions
Yes, and this is not a matter of economy. The unit is designed for water at the residual salinity left by the membrane stage: raw water creates a current load the stack is not built for, and hardness salts precipitate right inside the chambers at the high pH near the membrane surfaces. The order of stages admits no rearrangement here: without reverse osmosis the stage does not work worse — it does not work.
Water quality is comparable; the difference lies in operation. A mixed bed is regenerated with acid and alkali, which means storing them on site, a room and permits for them, trained personnel and neutralisation of saline effluent. Here regeneration proceeds continuously and by an electric field: there are no reagents at all and no effluent to neutralise. Quality also stays level rather than varying over a cycle — with a mixed bed it is at its best immediately after regeneration, whereas here it remains constant as long as the feed water and the current do not change.
No, and the reason lies in the mechanism. Part of the current is spent on splitting water, producing H⁺ and OH⁻ ions, and it is those ions that regenerate the resin continuously. Lowering the voltage reduces that share first: the resin begins to pass into its salt form and the resistivity at the outlet falls. The operating point is set by calculation from the water composition and the flow, and in a deep demineralisation scheme savings should be sought at the high-pressure pump of the reverse-osmosis unit, where the main energy is concentrated.
A resistivity of 18.2 MΩ·cm at 25 °C is the limiting value for water in which the only charge carriers left are the products of its own dissociation. The scale is strongly non-linear: 1 MΩ·cm corresponds to roughly half a milligram of salts per litre, and the whole difference between one and eighteen amounts to fractions of a milligram. Requirements differ with the purpose: for boiler make-up and for electroplating a few MΩ·cm are usually enough, whereas microelectronics and part of pharmaceutical practice call for the upper end of the scale, and that end is reached by a scheme with recirculation, ultraviolet treatment and a polishing mixed bed rather than by a single stage.
Service life is governed by compliance with the feed-water requirements rather than by the calendar. The module is not dismantled on site: resin and membranes are replaced together with it as a whole. Its life is shortened by hardness and iron that have slipped through the membrane stage, by free chlorine, by organic matter and by prolonged operation outside the rated interval of flow. The approach of the limiting condition is visible in advance — from a rising voltage at unchanged current and flow, and from a falling resistivity at the outlet.
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