Producing high quality and efficient Continuous Electro De-Ionisation modules

EDI is a chemical-free process to produce Ultra-Pure Water by removing ions
from Reverse Osmosis product water via electric potential.
Electrodeionisation is a logical evolution of conventional ion exchange
technology. Equal to conventional ion exchange, in the ion exchange resins
cations and anions from the feedwater are exchanged for hydrogen and
hydroxyl ions, producing high purity demineralized water. The key operational
difference and a main feature of EDI are that the resins are regenerated
continuously without the use of chemicals. This electrochemical process is
achieved by ion-conducting membranes under a DC current. Hydrogen and
hydroxyl ions are formed by the water dissociation reaction (water splitting) and
regenerate the resins continuous without adding any chemical reagents.

H O ↔ H + OH

The electric potential at each end of the module drives the water splitting and at
the same time causes the ions to migrate to the selectively permeable
membrane where they pass to the next concentrate chamber. Once the ions are
trapped in the concentrate compartment they are carried away by the
concentrate stream.

EDI feedwater must be direct coupled RO product water and prevented against
recontamination!

Open storage tanks, degasifiers, and softeners between RO and EDI are often
responsible for EDI fouling. Each filtration step, between RO and EDI, that might
introduce new particles to the EDI feed water will require an extra 1µ absolute
pre-filtration step directly before the EDI.

Physical Contamination:

• Plastic shavings from piping installation,
• Metal shavings from construction work,
• Dirt, dust, pollen, construction or welding debris,
• Resin beads/fines.

Chemical contamination:

• Oxidants, such as chlorine,
• Polyvalent cations, such as iron and manganese.

Limits for Iontech modules:

Pre-treatment is crucial for the functionality of the EDI process. Direct-coupled
Reverse Osmosis product water quality is required as feedwater for EDI.

EDI feedwater Limitation (RO permeate or better)

You need to know the following parameters for calculation: Conductivity in
µS/cm, CO in ppm, SiO in ppm. FCE = Feedwater Conductivity Equivalent =
Conductivity + CO * 2,66 + SiO * 1,94 = ≤ 40 µS/cm Example calculation FCE:

Conductivity = 15 µS/cm = 15,00
CO = 7,5 ppm * 2,66 = 19,95
SiO = 0,5 ppm * 1,94 = 0,97 +

35,92 µS/cm

TEA = Total Exchangeable Anions = CO * 2 + Conductivity * ⅔ = ≤ 25 ppm as
CaCO Example calculation TEA:

Conductivity = 15 µS/cm * 2/3 = 10,00
CO = 7,5 ppm * 2 = 15,00 +

25,00 ppm as CaC0

Both numbers give you basically the same information. 25 ppm TEA as CaCO
compares to ~38 µS/cm FCE. As silica is not part of the TEA, and 1 ppm SiO is
maximum allowed and compare to 1,94 µS/cm, it comes approx. to the same 40
µS/cm FCE as the maximum allowable load for EDI modules

When the FCE or TEA is too high, you’re loading more ions than the EDI module can extract. When the module is well regenerated, you might not notice
this if it occurs for a short time. However, the resins inside the EDI will slowly become more and more exhausted what will finally result in poor product quality.
A high load of CO will result almost directly in poor product quality.

As long as the high load is formed by non-scaling ions in principle it will no cause damage to the module. Once the feedwater conductivity equivalent is
back within the specification limits the resins will automatically regenerate and the module will restore itself.

Usually EDI systems operate at high recoveries of 90 to 95% what will result in concentration factors of 10 to 20 times the incoming ions.
All positive ions move towards the cathode through the cation-selective membrane and can’t move through the next anion-selective membrane.
Along the Cation membrane, inside the concentrate chambers, the H ions are 2 to 9 times more concentrated and will create low pH spots along the Cation
membrane surface. All negative ions move to the anode through the anion-selective membrane and can’t move through the next cation-selective membrane. At the Anion membrane, inside the concentrate chambers, the OH ions are 2 to 9 times more concentrated and will create high pH spots along the Anion membrane surface. H and OH ions concentrate 2 to 9 times and create low and high pH areas at the surface of the membranes inside the concentrate chamber where it can easily facilitate hardness and/or silica scaling. The maximum allowable recovery depends on the concentration of hardness
and silica in the feedwater.

Total Hardness: (as CaCO ) ≤ 0,2   0,2 – 1,0 ppm as CaCO3

Silica: (SiO ) ≤ 0,5 0,5 – 1,0 ppm as SiO
Allowable Recovery all modules 95% - 90 %

Iron, Manganese, and Sulphide are held tightly by the resins and may oxidize and precipitate in the resin before they can be transferred to the concentrated
waste stream. This might cause internal problems in any EDI module.

The tolerance for free chlorine (Cl ) is very low like below 0,02 ppm. Basically, EDI modules are even more sensitive to chlorine than most RO Thin Film
Composite (TFC) membranes. Free chlorine might cause damage on an EDI stack before you see any signs of it on the downstream RO system.
As for all oxidants, the ideal concentration is as low as zero, not detectable!

The organics that define TOC, Total Organic Carbon, will cause resin and membrane fouling. This will cause inefficient ion transportation and removal.
TOC fouling should be limited as much as possible.

During operation, the feedwater pH should be between 4 and 11.
During a periodic cleaning, a pH of 1 for acid cleaning and a pH 12 for caustic
cleaning is allowed.

The temperature of water feeding an Iontech EDI module should be between 5
℃ and 45 ℃. When the temperature decrease to below 5 ℃ the electrical
resistance of any (C)EDI module or E-Cell stack can increase to a critical point
as it will cause that a higher DC voltage will be needed. Below 5 ℃ you might
come to a point that the power supply voltage limitation will be reached and that
the performance declines.

The maximum inlet pressure is 6,9 bar. 

The Anode plate is made from coated titanium (TiO2 –> TiO2) while the Cathode is made from Stainless Steel.