Electrodeionisation: high-purity water without chemical regeneration

There are tasks where reverse osmosis is not the end of the scheme but its middle. A high-pressure boiler, a rinsing stage in electronics, a laboratory loop: these need water with practically no salts left in it. The classic answer is an ion exchange filter with resin, but it has a price, and it is paid not in money but in downtime and a chemical inventory.

What an EDI module does

Electrodeionisation combines two processes: ion exchange and electrodialysis. Inside the module, ion exchange membranes alternate with layers of ion exchange resin, and an electric field is applied across the whole stack.

The resin, as in an ordinary filter, captures dissolved ions from the flow. But then comes the main thing: the field continuously pulls the captured ions through the membranes into a separate concentrate channel. The resin is freed at exactly the same rate at which it takes on load, and regenerates itself. No reagents for its regeneration are needed at all.

The result is the removal of practically all dissolved salts and impurities and high-purity water at the outlet. The process is continuous: the module has no "service — exhaustion — regeneration — service" cycle.

Resin with regeneration and resin without it

A mixed-bed filter with resin solves the same task differently. It works until the exchange capacity of the media is used up, after which it is taken out of service and regenerated with acid and alkali. That pulls a whole chain of requirements behind it — the kind that usually come to mind only once you are already on site.

The difference shows up not in the laboratory but in the production schedule. A resin unit requires either a standby filter or a window in the shop's operation. An EDI module needs no such window, and it brings no acid or alkali onto the site — chemicals that have to be stored somewhere, dosed by something and then neutralised in the effluent.

Why EDI is installed only after reverse osmosis

The temptation is understandable: if the module removes practically all salts, why not feed raw water straight to it? Because EDI is designed for water that has already been desalinated. It is a polishing stage, not the main one.

The logic is the same as for any deep treatment unit. The module removes residual ions — those that passed through the osmotic membrane. If raw water is fed to it, the ionic load will be orders of magnitude above the design figure: the resin will not have time to free itself, the field will not pull everything into the concentrate, and permeate quality will drop. Electric field strength does not help here — it is designed for a specific load range.

That is why the scheme is always two-stage: reverse osmosis removes the bulk of the salts, EDI polishes off the remainder. Where two osmosis stages in series used to be installed, osmosis plus EDI is now more common — because a second membrane stage lowers the salt content but does not give water of that purity.

What this demands of the feed water

The module's key property is the same as its main advantage: the resin inside is not regenerated with reagents. There is nothing to restore it with if it fails. Hence the requirements for what arrives at the inlet:

• ✓Hardness. Residual calcium and magnesium salts are capable of precipitating inside the module in the zone where the ion concentration is highest. The resin does not give those deposits back, and the field does not carry them out.
• ✓Free chlorine and oxidants. Ion exchange membranes are sensitive to them, and replacing the membrane stack means replacing the module.
• ✓Dissolved carbon dioxide. The osmotic membrane passes it as a gas, but inside the module it converts to ionic form and creates a load that is invisible in a calculation based on salt content. It is removed by degassing between the stages.
• ✓Suspended matter and organics. Anything that mechanically clogs the channels reaches the module only when the preceding stage is faulty, but the consequences are the same — a drop in capacity with no way to flush the module with a reagent.

The practical conclusion: EDI is not a standalone unit but part of a line, and the reliability of the first stage becomes the condition for the second to work. Conductivity monitoring after the osmosis is not an optional extra here: it is how you spot in good time that the first-stage membranes are on the way out.

Where water of this purity is needed

The range of applications is narrow and quite specific: power generation, electronics, pharmaceuticals and laboratories.

In power generation this is boiler feed water — salts in it turn into deposits on heating surfaces and carry-over with the steam. In electronics the water goes to rinsing, and any remaining ion is a defect on the product. In pharmaceuticals and laboratories the requirements for water are set by the process itself, and ordinary demineralised water does not meet them.

If the task is phrased as "remove the scale" or "bring the salt content down to something acceptable", EDI is overkill — softening and reverse osmosis do that job. Electrodeionisation is chosen when there is a stated requirement for the residual conductivity at the outlet.

What next

Modules are selected by the required capacity of the line, and that is the only selection parameter within the range. An important detail: the capacity of each module is given as a range, not as a single figure. For the EDI-P-50L it is 30–60 l/h, for the EDI-HP-0.5 it is 300–700 l/h, for the EDI-LX-2.0 it is 1.1–2.8 m³/h, and for the largest EDI-LX-8.0 it is 6.5–9.5 m³/h. The line's operating point must fall inside the range and not at its edge: a module running at the upper limit leaves no margin for seasonal changes in load.

The ranges differ in the intended duty of the versions: EDI-P covers small flows from 30 l/h, EDI-HP covers from 300 l/h to 4.8 m³/h, and EDI-LX is the widest, from 220 l/h to 9.5 m³/h.

Before getting in touch, collect three things: the water quality requirement your process imposes at the outlet, the line flow allowing for unevenness, and the details of the first stage — which osmosis unit is installed or planned and whether there is degassing between the stages. With this you can look at electrodeionisation modules and select a version for the line. If there is no first stage yet, that is where to start: EDI does not work without osmosis ahead of it.