Silica and silica-based compounds occur naturally in water. Because precipitates of silica can reduce boiler and heat exchanger efficiency and damage costly turbines, water used in the power industry typically requires 10 to 100 parts per billion (ppb) of silica to prevent boiler scale.
Silica is also a concern in the microelectronics industry, as very small amounts of silica left on a semiconductor device by rinse water can cause the device to fail. Depending on the grade of water desired, the American Society for Testing and Materials (ASTM) electronics grade water specifications can require silica concentrations below 5 ppb, with some specification requirements below 0.2 ppb. Reagent-grade water used for analytical purposes can require less than 3 ppb.
In high-purity water treatment, most particulate and colloidal silica is removed by microfiltration, reverse osmosis (RO) and ultrafiltration (UF), so that only dissolved (reactive) silica remains. Dissolved silica is very weakly ionized, which means it has little electrical charge, making it hard to remove and difficult to measure with standard conductivity sensors. Dissolved silica may also exist as different species, depending on the pH of the water. At close to neutral pH, silica exists primarily as silicic acid (H2Si03), which is very weakly ionized. As the pH increases, the acid dissociates into the bisilicate ion (HSi03-), allowing the silica to be removed by deionization (DI). The most common forms of silica in water are shown in Table 1.
Traditional mixed-bed DI will remove dissolved silica; however, the ion exchange sites will exhaust as the silica, carbon dioxide and other dissolved solids are adsorbed onto the anion resin. The resin must be replaced or chemically regenerated with caustic solution. Regeneration can introduce foreign contaminants such as resin extractables, microbes, impurities in regeneration chemicals and rinse water, and particles generated by the fluidization and osmotic shock on the resin beads. A serious problem with mixed bed DI is breakthrough. Because silica is weakly ionized, it will break through the resin, even before the resistivity declines. Once silica breaks through the resin, it will leak into the product water, eventually leading to high-concentration silica "spikes" in the water.
Particulate (granular, suspended) >0.2 µm Microfiltration
Colloidal (unreactive) 0.001 to 0.1 µm2 RO and OF
Dissolved (reactive) Submicron- RO. DI. CEDI
molecular level polishing
USFilter CDI Systems Consistently Remove Dissolved Silica to Extremely Low Levels and No Chemical Regeneration
CDI systems consistently remove feed water silica-in some cases below detection limits-with no leakage and without the need for chemical regeneration. Unlike mixed-bed DI, CDI systems remove silica by providing the necessary hydroxyl ions through water splitting, rather than relying on a limited amount of fixed anion exchange sites. After adsorption, the silica is removed from the CDI system product water and transferred into the concentrating stream under the influence of an electric field. This prevents the silica from precipitating out of solution and polymerizing on the resin beads. Thus, there is no need to dissolve silica from the beads with warm caustic. Because they do not require chemical regeneration, CDI systems cost less to operate and eliminate the hazards associated with storing and handling chemicals.
The Proof is in the On-Site Tests
In tests conducted on CDI systems at several customer sites, including some of the world's largest microelectronics' manufacturers, the product water silica, total organic carbon, microbial and particulate contaminant levels were lower than those from mixedbed DI polishers. The patented design features and operating modes of USFilter CDI systems enable them to remove silica better than competitive CEDI devices. There is no downward pH shift or resin exhaustion that can cause silica breakthrough or spikes into the product water as feed conditions change.
Studies confirm that CDI systems remove a minimum of 95% silica up to a certain maximum feed conductivity equivalent (FCE) value, provided certain feed water parameters are within the ranges in Table 2. The lower the FCE value, the higher the silica removal.
A paper presented in 1994 on a CDI pilot system at the Southern California Edison (SCE) Huntington Beach, CA., Generating Station showed that the system typically removed 98% of the feed water silica, and frequently exceeded 99% over 14 months. The FCE ranged from 6.4 to 62 µS/cm and the feed silica concentration ranged from 0.2 to 4.0 ppm. SCE purchased a full-scale 200-gpm CDI system to replace the pilot unit, and published a paper in 1994 after the full-scale system treated over 25-million gallons of water. The paper reported an average silica removal of 96% (the final product water silica levels were 2-12 ppb) with a feed conductivity of 5 to 9 µS/cm and 7 PPM CO2, yielding an FCE of 23 to 28 µS/cm.
Studies conducted between August and October 1997 on a CDI system at a microelectronics company also showed superior silica removal. Pretreated with two-pass RO, the system was used to remove trace impurities and polish the water to electronics-grade quality. When operated according to specification, the CDI system removed silica to below the detection limit of 0.5 ppb.
Table 2. CDI System Operating Ranges for Optimum Silica Removal
Parameter H-Series Systems P-Series Systems
FCE, µS/cm <_50 ><>
System flow rate range, gpm 2 to 50 40 to >1000
Feed silica, ppm as Si02 <_1 .0 =""><>
USFilter CDI Systems effectively remove dissolved silica to below detection limits-more consistently than either mixedbed polishers or competitive CEDI devices. And, because they do not require chemical regeneration, CDI systems cost less to operate and eliminate the hazards associated with chemical handling and storage.