US Filter CDI are the Economical, Chemical-Free Way to Remove CO2 from Water

Reverse osmosis (RO) membranes do an excellent job of removing dissolved salts from water, however, they do not remove dissolved gases such as C0₂. In fact, considerable levels of C0₂-in some cases as high as 30 ppm-may be found in RO permeate. To make water with a resistivity greater than 1 Mw-cm, the C0₂ must be removed to a level less than 1 ppm, which generally requires the use of ion exchange resins.

Besides lowering the resistivity (or raising the conductivity) of water, C0₂ in the feed water increases the frequency of resin regeneration. Resins exhaust faster but system "life" is not affected.

C0₂ present in water cannot be detected by conductivity alone. As much as 60 ppm of C0₂ can be present in water with a conductivity of 10 µs/cm (see Table A). In contrast, only 4 ppm of NaCl can be present in 10 µs/cm water.

Table A: Normal conductivity measurement does not always detect C02

Conductivity Measurement    CO₂ Present     NaCI present
    1 µS/cm    0.6 ppm    0.4 ppm
    2.5 µS/cm    4 ppm    1 PPM
    10µ/cm    60 ppm     4 ppm

The thin cells and mixed-bed resin of USFilter CDI modules provide the best CO₂ removal of any continuous electrodeionization (CEDI) device on the market, with the added benefit of removing silica. They do not require caustic or acid for resin regeneration, and are extremely economical to operate.

Adding caustic is not the answer
In some industries, it is common practice to raise the pH upstream of the RO system by adding caustic. This converts CO₂ to bicarbonate alkalinity:

CO₂ (aqueous) + OH– = HC0₃–

The RO membrane can then reject the bicarbonate ions.

There are several drawbacks to this method. First, it requires handling hazardous caustic. Second, it increases the risk of scaling the RO system. Third, the operating cost is high. And finally, it is very difficult to consistently make high-resistivity water using this process alone.

The Proof is in the CDI Product Water
USFilter conducted laboratory tests on both RO and CDI systems. CDI modules were fed with RO permeate, and the load was increased by adding CO₂ to the feed. USFilter operated the CDI modules at various flow rates and feed concentrations, and developed performance curves. In order to achieve 1 Mw-cm (or 1 µS/cm); the product water must contain less than 1 ppm of C0₂ Achieving 1 Mw-cm consistently is difficult with reverse osmosis, even with product-staged (two-pass) RO and with caustic injection. USFilter found that RO performance is also sensitive to variations in feed pH, temperature and RO system recovery. The laboratory studies proved that 1 Mw-cm was consistently achieved with the USFIlter CDI System. Figure 2 illustrates typical performance of a CDI module, as a function of the concentration of COZ in the feed water, when operated at the nominal flow rate. Figure 2 also illustrates that USFilter CDI Systems are much less sensitive to COZ than other CEDI systems on the market today. ("thickcell CEDI").

CDI Systems-the best choice for CO₂ removal
To achieve water quality of 1 Mw-cm or 1 (µ/cm) or better, product water must contain less than 1 ppm of CO₂. Reverse osmosis alone cannot consistently do this. Other technologies, such as conventional ion exchange, pH adjustment and degasification have significant drawbacks.

In addition, some other CEDI systems have difficulty removing CO₂. Using a USFilter CDI System downstream of an RO unit will consistently produce 1 Mw-cm or better quality water. What's more, they are economical and chemical free.

CDI modules, operating in the electro-regeneration mode, remove CO₂ by converting it to bicarbonate and carbonate ions, using the OH– that is created electrochemically. The electric field causes the ions to transfer from the feed to the concentrate compartments inside the module.

The following design features further enhance the ability of CDI modules to remove dissolved CO₂: