Silica Scaling Explained

Of all the scales that form in RO systems, silica is the one operators dread, because it doesn't play by the usual rules and it doesn't come off easily. If your feed water is high in silica, it often sets the ceiling on how much water you can recover. Here is why, and what to do about it.

What silica scaling is

Silica in water exists mostly as dissolved silicic acid. As the reject stream concentrates, silica can exceed its solubility and come out of solution, but instead of forming neat crystals like carbonate does, it polymerises into a glassy, gel-like layer on the membrane. That layer behaves more like glass than like a mineral deposit, which is exactly what makes it so stubborn.

Why it's hard to control

Two things make silica difficult. First, its solubility is low, around 120–150 mg/L as amorphous silica at neutral pH and room temperature, and it falls as the water cools, so cold feed water tightens the limit further. Second, once it has polymerised, ordinary acid cleaning doesn't dissolve it. Carbonate scale you can lift with a low-pH clean; polymerised silica largely ignores it.

When it forms

Silica scales when the concentration on the reject side pushes past its solubility, which happens at high recovery on a silica-bearing feed. Low temperature makes it worse by lowering solubility. Certain pH ranges and the presence of metals like iron and aluminium accelerate polymerisation. So the risk is highest on cold, high-silica water run at aggressive recovery.

How to prevent it

Because removal is so hard, silica is a prevention game. A silica-specific antiscalant with dispersant chemistry raises the concentration the water tolerates before silica drops out, which is the main tool. Beyond that, you cap recovery to keep reject silica under the limit, keep the feed a little warmer where you can since warmth raises solubility, and control iron and aluminium that would otherwise seed polymerisation. In extreme cases, silica is reduced upstream before the RO.

How to remove it

If silica scale does form, cleaning is difficult and only partly effective. A high-pH alkaline clean can lift some of it, especially if you act before it fully cures. Stronger options exist but involve hazardous chemistry and risk to the membrane, so they're a last resort. The honest position is that once silica has set, some loss of membrane performance is often permanent, which is why the effort goes into stopping it forming.

Summary

Silica scaling is low-solubility, temperature-sensitive and very hard to reverse, so it's managed by keeping silica in solution rather than removing it later. A silica antiscalant, a sensible recovery cap and attention to temperature and metals keep it in check. If silica is limiting your recovery, that's usually a sign it's time to review the antiscalant and the operating point.

Talk to us

Membrania supplies silica-specific antiscalants and can review your feed analysis to find the recovery and dose that keep silica under control. Send us your numbers and we'll help.