Producing high-quality and efficient Continuous Electro Deionisation stacks

EDI is a chemical-free process for producing Ultra-Pure Water by removing ions from Reverse Osmosis product water using an electric potential. 

Electrodionisation is a logical evolution of conventional ion-exchange technology. Equal to conventional ion exchange, ion exchange resins exchange cations and anions from the feedwater for hydrogen and hydroxyl ions, producing high-purity demineralized water. The key operational difference and a main feature of EDI is that resins are continuously regenerated without the use of chemicals. This electrochemical process is achieved by ion-conducting membranes under a DC current. Hydrogen and hydroxyl ions are produced by the water dissociation reaction (water splitting) and continuously regenerate the resins without adding any chemical reagents.

H2O ↔ H+ + OH-

The electric potential at each end of the module drives water splitting and, at the same time, causes ions to migrate to the selectively permeable membrane, where they pass into 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 purely RO product water and must be prevented from 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 into the EDI feed water will require an additional 1 µm absolute pre-filtration step directly before the EDI. Air intake filters with a 0,2 µm rating are required on all open connections of storage tanks feeding the EDI system.  


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:

The minimum required feed pressure depends on the operating choice.

• Counter-Current 4,1 bar

• Co-Current 3,1 bar 

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

E-Cell operating Limitation (RO permeate or better)

Actual feed water quality specifications and performance may vary depending on flow rate through each stack and site conditions. Entries here based on nominal flow. Reference fact sheets and Winflows projection software to verify actual performance.

Stack bolt tightening is not required for E-Cell stacks. Which benefits: less external maintenance required & less potential for stack leakage/weeping. 

E-Cell systems typically operate in a once-through, counter-current flow configuration. Standard Veolia E-Cell systems are designed for once-through, counter- current flow. The Dilute and Concentrate flows in the stack run in opposite directions. The Dilute Inlet is at the bottom of the E-Cell stacks (the Electrode stream is fed internally from the Dilute Inlet). The Dilute Outlet is at the top. The electrode outlet is at the top of the stacks. The Concentrate Inlet is at the top and Concentrate Outlet (Bleed) at the bottom of the stacks. An advantage of counter-current flow is greater tolerance for feed water hardness than co-current flow. Hardness scaling occurs more slowly, so cleaning to remove it is required less often at the same level of feedwater hardness and recovery with counter-current flow than with co-current flow. With counter-current flow, the feed water may contain up to 1 ppm of CaCO3 hardness.  In counter-current flow, the minimum system feed pressure at nominal E-Cell stack flow is 4.1 bar (60 psi). The feed pressure requirement may be higher depending on downstream conditions. Once-through, co-current flow, where Concentrate  Inlet is at the bottom of the  E-Cell stack and Outlet (Bleed) at the top, is also possible, but may be applied only where the feed water contains <0.10 ppm as CaCO3 of hardness. In co-current flow, the minimum system feed pressure at nominal E-Cell stack flow is 3.1 bar (45 psi). The feed pressure requirement may be higher depending on downstream conditions.  The lower typical feed pressure requirement is an advantage of co-current over counter-current flow.  

You need to know the following parameters for calculation:
Conductivity in µS/cm, 
CO2 in ppm,
SiO2 in 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  +
 --------------------
 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
 --------------------
 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 the maximum allowed, compared 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 briefly. However, the resins in the EDI will gradually become exhausted, leading to poor product quality. A high CO2 load will almost directly result in poor product quality.
As long as the high load is caused by non-scaling ions, it will not damage 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%, resulting 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, H+ ions are 2 to 9 times more concentrated, creating low-pH spots along the Cation membrane surface.

All negative ions move to the anode through the anion-selective membrane and cannot 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, creating high-pH spots along the Anion membrane surface.  

H+ and OH- ions concentrate 2 to 9 times, creating low- and high-pH areas at the membrane surface within the concentrate chamber, where they can readily promote hardness and/or silica scaling. 

The maximum allowable recovery depends on the concentration of hardness and silica in the feedwater.

Iron, Manganese, and Sulphide are tightly held by the resins and may oxidize and precipitate within 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 (Cl2) is very low, like below 0,05 ppm. Basically, EDI modules are even more sensitive to chlorine than most RO Thin Film Composite (TFC) membranes. Free chlorine can damage an EDI stack before you see any signs in 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 lead to inefficient ion transport 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 12 for caustic cleaning is allowed.

The water temperature entering an E-Cell EDI stack should be between 5°C and 40 °C. When the temperature drops below 5 °C, the electrical resistance of any (C)EDI module or E-Cell stack can reach a critical point, requiring a higher DC voltage. Below 5 0C, you might reach the power supply voltage limit, and performance will decline.

The maximum inlet pressure is 6,9 bar (100 psi).

The Anode plate is made of coated titanium (IrOx/TiO2 on Ti), while the Cathode is made of Stainless Steel.