Reverse osmosis concentrates everything the membrane rejects. Towards the end of the system, salts such as calcium carbonate and calcium sulphate become so concentrated that they start to crystallise on the membrane surface. That scale cuts output, pushes up pressure and power, and eventually destroys membranes. An antiscalant is the small dose of chemical that keeps those salts in solution until they leave in the reject stream.
What scales an RO membrane
- Calcium carbonate: the most common scale in hard, alkaline South Indian groundwater
- Calcium sulphate, barium sulphate and strontium sulphate: hard to remove once formed
- Silica: a problem in many borewell waters and in ZLD plants that push recovery high
- Iron and manganese: foul membranes if not removed in pre-treatment
Which scale you are fighting depends on your water analysis, which is why antiscalant choice should start with a recent feed-water report. See Water Hardness Explained for background.
How an antiscalant works
Antiscalants are threshold inhibitors: at a few parts per million they disturb crystal growth so that salts stay dissolved well beyond their normal solubility. Some also disperse fine particles so they do not settle on the membrane.
They are dosed continuously into the RO feed, usually after the media filter and before the cartridge filter, so they are fully mixed before the water reaches the membranes.
Choosing and dosing
- Match the product to your main scaling risk (carbonate, sulphate or silica) using the feed-water analysis.
- Set the dose from the membrane supplier's or antiscalant supplier's projection for your recovery rate. Doses are typically a few mg/L of feed water.
- Check compatibility with the membrane manufacturer, and do not mix antiscalants from different suppliers in the same dosing tank.
- Watch normalised permeate flow and differential pressure. A steady decline means scaling or fouling is starting.
Antiscalant in ZLD plants
Plants running RO ahead of evaporators for zero liquid discharge push recovery as high as possible, which makes scale control even more important. These systems often need closer monitoring and periodic cleaning in place (CIP) on top of the antiscalant.
Where to get it
We supply antiscalant for RO and boiler systems, along with the pre-treatment coagulants and disinfectants that keep RO feed water clean.