SNOWPURE, LLC, 2005-2018
VERSION 3.5 (XL+EXL)
–FEBRUARY 2018
PAGE 75
Appendix #9-
B: Silica Fouling and Cleaning in Electropure™ EDI
Silica can foul an EDI system in 2 cases.
1. When the feed silica level is too high
2. When the operating voltage is too low
Best practice is to avoid silica precipitation.
1. In areas with high silica in the raw water, use the best quality RO
membranes, and use RO membranes that are designed to reject silica.
SnowPure recommends
Nitto Denko CPA5-LD
, available from
Hydranautics.
2.
Operate your Electropure™ EDI at 10% higher than standard voltage.
The higher voltage encourages more water splitting in the polishing
section of the EDI module, generating larger amounts of OH-, keeping the
silica in soluble form, and enhancing the transport into the concentrate.
3. Minimize the CO
2
in the feed, it competes with the removal of SiO
2
.
4. Once formed, polymerized silica is very difficult to remove.
Mechanisms of silica precipitation:
1. At the pH of high purity water (7.0) silica is uncharged (pK
1
= 9.8) so will
not diffuse into the concentrate if it is in the water phase. When adsorbed
on a virgin anion resin (pH = 14 surface) it becomes charged (H
3
SiO
4
-
)
2. If the H
3
SiO
4
-
ions are not removed from the resin, they may accumulate
and polymerize to form (SiO
2
)
n
The solubility of silica is about 6 ppm
according to R. Sheikholeslami and S. Tan (1999), hence our practical
EDI feed limit of 0.5 ppm.
3. Silica precipitation is enhanced in RO situations with cations such as
Ca
+2
, but this should not be an issue in EDI in the polishing section.
Symptoms of silica fouling:
1. EDI quality decreases (polishing section is hindered)
2. EDI current decreases (less water splitting)
3. No SiO
2
mass balance, more silica enters the EDI feed than is removed in
the concentrate, this indicates a buildup.
Cleaning procedure for silica precipitation:
The primary way to remove polymerized silica precipitation/fouling from the
resins is via a high-pH treatment. For RO cleaning, NaOH + EDTA + SDS at
high temperature is recommended. NaOH increases the pH which makes
silica more soluble. pH should be greater than 11 for best results. EDTA
may sequester cations if present but is not likely to help in EDI. SDS
(surfactant) helps to remove colloids, and may be useful in EDI.
Two silica
cleaning solutions are recommended for Electropure™ EDI:
1. NaOH in DI water at pH 12-13, temperature 40-50°C.
2.
Avista RoClean™ high-pH cleaner at 2%, temperature 50°C. Powder
form is RoClean™ P111. Liquid form is RoClean™ L211.