Silica in RO Water: Scaling Risk, Testing and Control
Quick answer: why is silica important in RO?
Silica can become a limiting scale-forming constituent in reverse-osmosis systems because its concentration increases in the concentrate as recovery rises. The risk depends on silica concentration, pH, temperature, recovery and the presence of metals or colloidal material.
What forms of silica may be present?
Water can contain dissolved reactive silica as well as colloidal or particulate silica. These forms do not behave identically, so a general TDS test cannot determine silica scaling risk.
Why recovery matters
As permeate leaves an RO system, rejected dissolved substances become more concentrated in the reject stream. Higher recovery therefore raises concentrate silica. If projected concentrate exceeds the applicable solubility or scaling limit, recovery or pretreatment may need adjustment.
What should be checked?
- Feed-water silica concentration
- Feed and concentrate pH
- Water temperature
- RO recovery
- Alkalinity and hardness
- Iron, aluminium and other metals
- Colloidal-silica or turbidity risk where relevant
How silica scaling risk may be controlled
Depending on the water analysis and membrane system, control may involve reducing recovery, improving pretreatment, selecting a suitable antiscalant, controlling pH within design limits, or using a process that reduces silica before the RO. There is no universal antiscalant dose for silica.
Frequently asked questions
Can a TDS meter detect silica?
No. A handheld TDS meter cannot identify silica concentration.
Does increasing RO recovery increase silica risk?
Usually, yes, because rejected silica becomes more concentrated in the reject stream as recovery increases.
Can silica scale be removed by any general RO cleaner?
No. Cleaning chemistry must match the foulant and membrane limits.
Reference
DuPont’s FilmTec RO/NF technical manual describes silica scaling calculations using feed silica, recovery, pH and temperature. See the FilmTec RO/NF Technical Manual.
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