What is electrodeionisation (EDI)?
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.
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H₂O ↔ 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.
What are the main reasons of malfunctioning EDI systems?
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.
What are the limitations of Iontech EDI modules?
Limits for Iontech modules
| Minimum limits | Nominal | Maximum limits | ||||
| Module | Dilute | Concentrate | Dilute | Dilute | DC power | |
| Type | Flow | Flow | Flow | Flow | Voltage | Current |
| IT-ED04-SI | 0,22 m3/h | 16 l/h | 0,44 m3/h | 0,67 m3/h | 55 VDC | 6 Amp |
| IT-ED10-SI | 0,60 m3/h | 37 l/h | 1,10 m3/h | 1,70 m3/h | 135 VDC | 6 Amp |
| IT-ED18-SI | 1,10 m3/h | 61 l/h | 2,00 m3/h | 3,10 m3/h | 240 VDC | 6 Amp |
| IT-ED24-SI | 1,40 m3/h | 82 l/h | 2,60 m3/h | 4,20 m3/h | 290 VDC | 6 Amp |
| IT-ED30-SI | 1,70 m3/h | 100 l/h | 3,40 m3/h | 5,10 m3/h | 320 VDC | 6 Amp |
| IT-ED45-SI | 2,60 m3/h | 150 l/h | 5,10 m3/h | 7,60 m3/h | 400 VDC | 6 Amp |
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)
| 1 | FCE – Feedwater conductivity equivalent incl. CO₂ and silica | 40 | µS/cm |
| 2 | TEA – Total Exchangeable Anions (as CaCO3) | 25 | ppm |
| 3 | Total Hardness (as CaCO3) | < 1,0 | ppm |
| 4 | Silica (SiO₂) | < 1,0 | ppm |
| 5 | Iron, Manganese, Sulfide | < 0,01 | ppm |
| 6 | Total Chlorine (as Cl₂) | < 0,02 | ppm |
| 7 | Total Organic Carbon (TOC) | < 0,5 | ppm |
| 8 | pH range | 4 – 11 | |
| 9 | Temperature | 5 – 45 | ℃ |
| 10 | Inlet pressure | < 7 | bar |
What is the difference between FCE and TEA and how can i calculate them?
You need to know the following parameters for calculation: Conductivity in µS/cm, CO₂in ppm, SiO₂in ppm. FCE = Feedwater Conductivity Equivalent = Conductivity + CO₂* 2,66 + SiO₂* 1,94 = ≤ 40 µS/cm Example calculation FCE:
| Conductivity = | 15 | µS/cm | = | 15,00 | |
| CO₂ = | 7,5 | ppm * 2,66 | = | 19,95 | |
| SiO₂ = | 0,5 | ppm * 1,94 | = | 0,97 | + |
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| 35,92 µS/cm |
TEA = Total Exchangeable Anions = CO₂ * 2 + Conductivity * ⅔ = ≤ 25 ppm as CaCO3 Example calculation TEA:
| Conductivity = | 15 | µS/cm * 2/3 | = | 10,00 | |
| CO₂ = | 7,5 | ppm * 2 | = | 15,00 | + |
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| 25,00 ppm as CaC03 |
Both numbers give you basically the same information. 25 ppm TEA as CaCO3 compares to ~38 µS/cm FCE. As silica is not part of the TEA, and 1 ppm SiO₂ 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.
What happens if the FCE or TEA is too high?
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 CO₂will result almost directly in poor product quality.
As long as the high load is formed by non-scaling ions in principle it will not 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.
Why is the feedwater limit for Hardness and Silica so low?
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 CaCO3) | ≤ 0,2 | 0,2 – 1,0 | ppm as CaCO3 |
| Silica (SiO₂) | ≤ 0,5 | 0,5 – 1,0 | ppm as SiO₂ |
| Allowable Recovery all modules | 95 | 90 | % |
Why is the feedwater limit for Iron, Manganese and Sulfides so low?
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.
What is the free Chlorine tolerance for EDI modules?
The tolerance for free chlorine (Cl₂) 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!
What is the feedwater limit for TOC?
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.
What are the pH limits for Iontech EDI modules?
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.
What are the temperature limits for Iontech EDI modules?
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.
What is the inlet pressure limitation for an Iontech EDI module?
The maximum inlet pressure is 6,9 bar.
Electrodeionization (EDI) versus Mixed Bed (MB) deionisation?
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Electrodeionisation |
Mix Bed |
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Type of process |
Continuous, self-regenerating |
Batch, Exhausting |
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Hydraulic control |
Quite simple |
Quite complex |
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Operational cost |
Low |
High |
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Maintenance cost |
Low |
High |
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Personal risk operators |
Low |
High, due to use of chemicals |
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Environmental |
Friendly |
Chemical polluting |
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Downtime |
Not applicable |
During regeneration process |
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Physical contamination |
High sensitive |
Low sensitive |
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Footprint |
Relative small |
Relative large |
From which material are the Anode and Cathode made?
The Anode plate is made from coated titanium (TiO2 –> TiO2) while the Cathode is made from Stainless Steel.