Carbon Dioxide and EDI
Carbon dioxide is typically exists in a dissolved state in the feedwater. Measures to reduce the CO2 in the EDI feed should be taken because CO2 is in fact an ionic load on the EDI module. Carbon dioxide must be measured and ADDED to the Ionic load in the RO permeate.
Tip: Each ppm of CO2 as CO2 =2.66 µS/cm OR ppm as CO2 x 2.66 = µS/cm
1) Caustic Injection before the RO
One way to remove dissolved CO2 is to raise the pH prior to the RO to convert the CO2 to bicarbonate. The bicarbonate is then rejected by the RO membrane and sent down the drain. Usually a small pH increase to 8-8.5 is required. In the case of a 2-pass RO, the caustic would be injected between the first and second passes. See the chart below for the relationship of bicarbonate alkalinity and CO2 on pH.
The higher the pH, the higher the bicarbonate and lower free carbon dioxide. This method is used more often due to the minimal cost incurred by adding a chemical injection system. For end users that do not wish to injection chemicals, see below.
The other method of reducing dissolved carbon dioxide is through degasification. There are two main methods, 1) forced draft, and 2) membrane degasification. One manufacturer of membrane degassing modules is Membrana with their Liqui-Cel product.
- Reduce the dissolved CO2 feeding the EDI system as much as possible
Suggested CO2 Test Kit - Hach Model CA-23 (#143633), smallest increment 1.25 mg/l