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–FEBRUARY 2018
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Appendix #9-A: Silica Removal by RO and Electrodeionization
Silica is one of the more important minerals to remove from water for Power
generation and Semiconductor applications. It is also one of the most difficult. Silica
chemistry is complex. On the most basic level, silica is found in both
colloidal/polymerized and reactive/molecular forms. Silica level in feedwater
depends on the geology of the region and whether the source is surface or well-
water. Silica in raw water will range from under 2 ppm to over 100 ppm.
Physical membrane processes such as ultrafiltration and reverse osmosis effectively
remove colloidal silica, and much of the reactive silica.
The removal of reactive silica by RO or EDI depends on its charge. Silica has little
charge at neutral pH near 7.0 since the pK
1
of silicic acid is 9.8. The lack of charge
makes it difficult to exchange with ion exchange resin, or to remove with RO or EDI;
where raising the pH to above 9.8 helps with the driving force.
Silica scaling is also an issue. The solubility of silica at pH 6-8 is only 120 ppm at
25
o
C. This means that a 30 ppm RO feedstream with 75% recovery will begin to
scale. There are two prevention techniques for silica scaling. One is the use of
silica antiscalant in the RO process, which mechanism is to delay the formation of
solid silica. The other is raising the pH, which increases the solubility limit of silica.
At pH 10.0, silica is soluble up to 310 ppm. Of course, high pH will cause hardness
scaling if the feed is not softened.*
Commercial RO modules will pass silica at about twice the passage as chloride ion**.
Most spiral RO module manufacturers claim 99.7% rejection for individual (200 psi)
high quality elements. 99.0%-99.5% is a reasonable silica rejection rate for a well-
designed RO system with quality elements. Note: Nitto Denko CPA5-LD is reputed
to have the best silica rejection of all brackish water RO membranes.
With a 20-ppm silica feed, with 75% recovery, and 99.0% rejection elements, the RO
permeate (and EDI feed) can be maintained at 0.5 ppm.
For higher levels of silica in the feed, the RO system should be designed with higher
quality RO elements or lower recovery. Using 99.7% rejection elements and 65%
recovery, the RO feed can be up to almost 90 ppm.
It is important to maintain the silica in the EDI feed under 0.5 ppm in order to:
Avoid scaling in the EDI concentrate, and
Minimize silica levels in the high-quality EDI product water.
Strategies for effective silica removal in the EDI:
1. High quality RO system (for removal of feed silica)
a. High quality RO modules
b. RO feed pH raised
c.
HERO™ process
2. CO
2
removal (CO
2
competes with silica in the EDI)
a. CO
2
removal pre-RO using Corosex
®
media
b. CO
2
removal by RO using pH >= 8.3
c. CO
2
removal interstage RO
d. CO
2
removal post RO using Liqui-
Cel™