Voltage is the driving force, which pulls the impurity ions from the feed streams into the concentrate streams. The local voltage gradients also cause H2O to split into H+ and OH- ions. The constant formation and high local concentration of these ions allows the state of the resins in the polishing section within the EDI module to be in the full hydrogen and hydroxyl form, and fully able to remove weakly ionized species such as CO2 and silica. These also prevent the growth of bacteria within the EDI module. Excess H+ and OH- ions are pulled from the feed streams into the concentrate streams, which also compete with any impurity ions for transport sites.
There is an optimum voltage to achieve the highest quality water. At voltages lower than this, the driving force is inadequate to move the ions across the purifying chamber resin bed and then across the membranes before the product stream exits the module. At voltages higher than optimum, the overvoltage creates excess water splitting and therefore excess current, and also causes ion polarization and thus back diffusion, which lowers product water resistivity.
Within the range set for each module type, the optimum voltage depends on the ion load and on the water recovery rate. Higher ion loads in the feed and higher recovery rates lead to a higher ion concentration in the concentrate chambers, which lowers the resistance of the module the lower stack resistance leads to a lower optimum voltage.
Typical current draw for an EDI module at nominal voltage is 2-4 amps with a feed conductivity of 4-10 uS/cm. Current may be as low as 1 amp or less. Current at high feed conductivities (20-30 uS/cm) will lead to currents as high as 8 amps, or higher. Fundamentally, current is proportional to the total number of ions moved. These ions include the impurity ions in the RO permeate, such as Na+ and Cl-, and the ions caused by water splitting, H+ and OH-. The water splitting rate depends on the local voltage gradients, so that higher voltages across the resin chambers leads to higher quantities of H+ and OH- to be moved.
A portion of the current is then directly proportional to the ion content of the feed (TDS, or uS/cm). The other portion of the current, proportional to water splitting, increases non-linearly with overvoltage. The current efficiency is the fraction of the total current that is required to move the impurity ions in the EDI feed.
If the module current is higher than expected, it could be because the voltage is set higher than optimum, and excess water splitting results in the excess current.
Current also depends on the concentrate flow, and therefore on the module water recovery. The nominal concentrate flow is 10% of the feed flow. If the concentrate flow is lower than recommended, the concentrate will be more conductive and the current will increase.
Normally, an EDI module will start up with high quality product water. This is because the EDI module has excess mixed bed ion-exchange resins in it in the H+ and OH- form, in the polishing section.
However, after operating conditions have changed, a module will take between 8 and 24 hours to reach a new steady state. The true steady state is defined as reaching a mass balance on the ions entering and leaving the module. At steady state, the kinetics of ion migration match the ion feed rate. Steady state for trace ions such as silica may take as long as 2-4 weeks.
If voltage is lowered or ion load is raised, the internal ion-exchange resins will begin to adsorb the excess ions. In this condition, fewer ions leave the module than enter. Eventually a new steady state is reached. During this time, the working ion front progresses in the resin bed from near the bottom of the module upward.
If voltage is raised or ion load is lowered, the resins will lose some of their excess ions to the concentrate stream, and more ions will be exiting the module than enter it. During this time, the location of the working ion front grows closer to the inlet of the module. This latter is the mechanism of the regeneration procedure.
An ion balance done on the module(s) during operation is a valuable tool in determining if the EDI system is operating at steady state.
The ability for EDI to remove ions from a stream depends in part on the properties of the ionic species. In a standard resin bed, the adsorption strength and kinetics depend on the ionic size, the degree of hydration, and on the type of resin.
In EDI, the ionic charge is even more important since this is the driving force to move the ions along the resin surfaces to the membrane, and through it.
The following ionic sizes are the effective sizes in aqueous solution at 25oC. These sizes include full hydration. The larger the effective size the slower the diffusion rate larger ions are removed by EDI less well. The larger the effective size, the more distributed the charge and the less well adsorbed by the resin.
The higher the ionic charge the stronger the applied voltage will pull the ion through the membrane. This is counterbalanced by higher degrees of hydration and larger, heavier molecules which slow diffusion.
The table below shows the selectivity of different ions for resin. This is a measure of their adsorption strength to the resin. Strong adsorption means low leakage through a resin bed or a EDI module.
Sodium (Na+), Chloride (Cl-), Calcium (Ca+2), Hydrogen ion (H+), and Hydroxyl ion (OH-) are considered easy ions for EDI. All of these ions are adsorbed well by the resin and have a charge that is definite and difficult to polarize. These ions are fairly easy to remove in the “working” section of the EDI module.
Carbon Dioxide (CO2), Silica (SiO2), and Boron (H3BO3) all have weak anionic charge under normal operation and pH. Because of this, they are weakly adsorbed to the resins and the applied voltage has little driving force.
To effectively remove these ions, other system strategies are used.
If the total ion load to the EDI is lowered, the working section of the module will be small, and the polishing section relatively larger. The larger polishing section will aid the removal of the hard-to-remove ions.
CO2 can be removed by the RO if the pH of the feed is raised (the pK1 of carbonic acid (H2CO3) is 6.35). Hence, with moderately high pH, bicarbonate ion can be removed, Of course, hard cations (Ca+2, Mg+2) must be removed first to operate the RO at high pH. CO2 can be removed as a gas after the RO (see Liqui-Cel®), which is helped with low pH (so the CO2 is not in ionic form). The pK1 of silicic acid (H2SiO3) is 9.77. The pK1 of orthoboric acid (H3BO3) is 9.28. only with pH > 10 can silica and boron be removed effectively. This is the theory behind the HEROtm process.
Raising the pH of the EDI feed is counter-productive. Since Na+ and OH- ions are "easy” ions to remove, the addition of NaOH before the EDI simply raises the ion load for the working section of the EDI, and the pH returns to 7.0 by the end of the working section. Further, the size of the polishing section is now smaller.
Pressure drop depends on temperature mostly due to the effect on the viscosity of water. The table below shows the absolute viscosity of water (cP) at temperatures of interest, and the relative viscosity (based at 25oC). Pressure drop will increase or decrease proportionally to the viscosity. Note that at 5oC the viscosity of water is 70% higher than at 25oC.
As temperature increases, the resistance of the stack will decrease. At a given voltage, current will increase. One cause of this phenomenon is increased ionic activity at higher temperatures. All other things being equal, the stack resistance will change about 2% per 1oC.
The quality optimization will depend on other factors (below), and so the optimum setting of the voltage will change with temperature.
There is an optimum temperature for operation.
As temperature increases to 35oC, product quality will generally increase since ions are more mobile and move more easily. Higher than this, quality will lessen due to increases in ionic “leakage.” This is caused by lower adsorption of the ions to the internal ion exchange resins. In addition, the actual ionic resistivity, uncompensated for temperature, will increase and there is less accuracy in the reading (see section below).
A lower voltage is required at higher temperatures to move the ions into the concentrate.
As temperature decreases toward 15oC, product quality may lessen. Some of this is due to errors in resistivity measurement temperature compensation; some improvement is due to the greater adsorption of the ions to the internal ion exchange resin. As temperature decreases further, the activation of diffusion through the membrane will become larger and quality will decrease.
At low temperatures, a higher voltage will be needed to continue to split water effectively, and move sluggish ions faster.
Resistivity measurements change strongly with temperature, and are normally corrected to a standard temperature (25oC). Impurity ions in water have a higher electrical conductance at higher temperatures because the ions are more mobile. Similarly, Ultrapure water has a lower electrical resistance as temperature is raised because water dissociates into H+ and OH- more.
The correction for temperature in the meters is large, and is subject to errors. A high quality resistivity meter is recommended.
Hot Dl water is the most difficult to measure accurately.
The pressure drop will increase on each of these streams as the flow to each is increased. Pressure drop is defined here as measured near the inlet and outlet fittings of the module.
Electrolyte Pressure Drop: at 0.05 gpm (10 lph) the pressure drop will be approximately 20 psi (1.4 bar). If pressure drop rises above this, then the inlet may be fouled or blocked with debris. The inlet water must be finely filtered. This flow should be independent of the size of the module and the number of cells since there is only one anode/cathode per module.
Concentrate Pressure Drop: The concentrate flow will be different for each design, for each operation, and for each EDI model. Typically, the concentrate outlet be set between 5% and10% of the EDI product flow.
If the concentrate pressure drop increases during operation, it may need cleaning or it may have debris in the concentrate inlet. The inlet water must be finely filtered (RO effluent). Feed-Product Pressure Drop: The Feed-Product pressure drop increases with flow. The pressure drop is close to being linear (first-order) with flow; that is, twice the flow will cause twice the pressure drop.
There can be substantial pressure drop in manifold lines. valves, flowmeters, solenoid valves, elbows, and tees.
Plate-and-frame modules are sealed with internal gaskets, and there will be some internal leakage. In an EDI module, if the concentrate leaks into the product stream then the product resistivity will suffer.
Product Outlet pressure must be greater than Concentrate Outlet pressure.
To ensure that internal leakage does not impact product quality, the product outlet must have a higher pressure than the concentrate or electrolyte streams outlets. This way, any internal leakage will not add ions to the product stream.
For the simplest, easiest system, there should be no back-pressure applied to the concentrate stream outlet. In systems with valves to manually control the concentrate backpressure, the result is often complication and operator error.
To send the concentrate stream to the inlet of the RO, the outlet is ideally first plumbed into an ambient “break” tank then pumped independently into the RO inlet pretreatment line. When this is done, the EDI module can approach 99% recovery.
The product quality depends on the ability of the module to remove ions from the purifying chamber before they exit the module. More feed ions will result in lower product quality. This is true for both general ionic conductivity (NaCl) and weak ions (silica, boron, and bicarbonate).
Additional ions add a load that has two results. The first is that the depth of the working bed within the EDI module lengthens—causing the polishing bed to shorten. The first quality deterioration occurs in lower removal of weakly charged species.
Reducing feed conductivity helps improve silica and CO2 removal.
The second result is that the module current draw increases as feed conductivity increases. Moving more ions takes more electrons. The current increase is not linear because the current also moves water, which has been split.
Increased feed conductivity increases current draw.