RO Antiscalant Dosing: A Practical Guide for Plant Operators

Scale is one of the most common reasons RO plants lose output. As water passes through the membranes, the reject side becomes more concentrated. At some point salts such as calcium carbonate, calcium sulphate and silica cannot stay dissolved and begin to deposit – usually on the last elements first.

Antiscalants are threshold inhibitors. A small amount disrupts crystal growth and keeps many times that amount of scale-forming salts in solution long enough for them to leave with the reject. They also disperse fine particles such as iron and silt.

• After any chlorine removal step (SMBS or carbon filter).
• Before the cartridge filter, so the antiscalant is fully mixed before the high-pressure pump.
• From its own day tank and dosing pump – never mixed with acid, chlorine or coagulant.

The supplier uses your feed water analysis (calcium, magnesium, alkalinity, sulphate, barium, strontium, silica, iron, pH and temperature) and your recovery to predict which salts will exceed saturation in the reject. Most plants end up at a few mg/L of product on the feed flow. Higher recovery or harder water needs more.

• Using one dose for all seasons when borewell water changes.
• Diluting antiscalant in water that contains chlorine or hardness.
• Letting the day tank run dry or the pump lose prime.
• Raising recovery without re-checking the dose.

Raw permeate flow changes with temperature and pressure, so look at normalised data. Clean when normalised permeate flow drops by about 10–15%, salt passage rises noticeably, or the stage pressure drop climbs by around 15%. Cleaning early is much easier than recovering a badly fouled membrane.

GreenFission supplies RO antiscalants and membrane cleaners and can calculate a starting dose from your water analysis.