Silica and Boron are two of the most difficult ions to remove in a feedwater due to their weakly ionized nature. Both are usually are one of the last to be removed and the first ions to pass through an EDI device. In addition, if the feed silica is high, silica scale can form in the EDI module which in turn increases the module resistance and affects the removal efficiency of the electric field. As a result, product quality decreases. Below are some guidelines when silica and boron are an issue.
EDI systems that include ion exchange softening can usually operate at 95% recovery.
EDI systems that DO NOT include ion exchange softening must operate at 90% recovery or below.
NOTE: In some cases the recovery may be limited by the concentration of silica in the feed water, where the maximum recovery can be calculated as follows:
- Maximum recovery, % = 100 – (4 x ppm feed silica)
In the field, the silica concentration in the EDI feed water can be measured using a silica test kit.
Suggested Test Kit:
- Silica – Hach Model SI-7 (#2255000)
- Smallest increment 0.02 ppm
- Optional (for power plant applications)
- Boron - Field samples sent to lab or use Boron analyzer