RO Water Chemistry and Pretreatment: Silica Scale Prevention

RO Water Chemistry and Pretreatment:  Silica Scale Prevention

Silica can be a challenge to manage in an RO systems.  Changes in pH and temperature can have major impacts on its scale forming potential.  So can the presence of aluminum and iron. 

Water Chemistry and Pretreatment Silica Scale Prevention Silica Scale Prevention Dissolved silica (SiO2 ) is naturally present in most feedwaters in the range of 1 – 100 mg/L. The prevailing forms of silica are meta silicic acids as (H2SiO3 )n with low n numbers. Since silicic acid is a weak acid, it is mostly in the undissociated form at or below a neutral pH. Supersaturated silicic acid can further polymerize to form insoluble colloidal silica or silica gel, which can cause membrane scaling. The maximum allowable SiO2 concentration in the concentrate stream is based on the solubility of SiO2 . The scaling potential for the concentrate stream will be quite different from that of the feed solution because of the increase in the concentration of SiO2 and the change in pH. It can be calculated from the feedwater analysis and the RO operating parameters. As the pH exceeds neutral, silicic acid dissociates into the silicate anion (SiO3 2-)n . This can react with calcium, magnesium, iron, manganese or aluminum to form insoluble silicates.

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