An antiscalant is a chemical dosed into the feed water of a reverse osmosis (RO) plant to stop scale forming on the membranes. As RO pushes pure water through the membrane, the salts left behind become more concentrated. Without an antiscalant, salts such as calcium carbonate, calcium sulphate and silica come out of solution and coat the membrane, cutting output and raising pressure.
Quick answer: dose antiscalant continuously after the cartridge filter and before the high-pressure pump. The right grade and dose depend on your feed water report; send us your water report on WhatsApp and we will suggest both.
How antiscalant works
Antiscalants are threshold inhibitors and dispersants. A small amount, far less than would react with all the salts, disturbs crystal growth so scale cannot build up on the membrane. Dispersant grades also keep fine particles, iron and silica moving so they leave with the reject water.
Which antiscalant do you need?
The grade depends on what is in your water. Our RO antiscalant range covers the common cases:
| Your feed water | Grade to consider |
|---|---|
| Most RO plants, with chlorine present upstream | Zyax ZYPERM 110, low pH broad-spectrum antiscalant |
| High pH and high alkalinity | Zyax ZYPERM 120, high pH antiscalant |
| Borewell water with high silica | Zyax ZYPERM 130, high silica antiscalant |
| Prefer to make up dosing solution on site | Zyax ZYPERM 140, powder antiscalant |
| Moderate hardness, cost-sensitive plants | Zyax ZYPERM 150, economy antiscalant |
The hardness and silica limits for each grade are listed on the grades and specifications section.
Where to dose it
- Pre-treatment: sand or multimedia filter, then carbon filter or dechlorination.
- Cartridge filter.
- Antiscalant dosing point, with enough pipe length or a static mixer so it mixes fully.
- High-pressure pump, then the RO membranes.
Dose it continuously whenever the plant runs. Interlock the dosing pump with the high-pressure pump so it never runs dry or doses into a stopped plant.
How to set the dosing pump
Once you have a recommended dose in ppm (parts per million), the pump setting follows from your feed flow:
- Antiscalant needed (grams per hour) = feed flow (m³ per hour) × dose (ppm).
- Example: a feed flow of 10 m³/h at a recommended 3 ppm needs 30 g of neat antiscalant per hour. If you dilute it 1:9 in the dosing tank, the pump must deliver 300 g (about 300 ml) of solution per hour.
Check tank levels daily against this rate. Running out of antiscalant for even a day can scale a membrane.
Signs your antiscalant is not working
- Permeate output falls while pressure rises.
- Pressure drop across the last membranes in the train increases.
- White, crusty deposits in the reject line or on the membrane end.
If scale has already formed, clean the membranes with a low pH cleaner. See how to clean RO membranes.
Not sure which grade fits your plant? Share your water report and plant capacity; we will suggest the grade and dose.
Send water report on WhatsAppFrequently asked questions
What does an antiscalant do in an RO plant?
It stops dissolved salts such as calcium carbonate, calcium sulphate and silica from forming scale on the RO membranes as the water is concentrated.
How much antiscalant should I dose?
It depends on your feed water analysis, the plant's recovery and the antiscalant grade. The dose is usually worked out from a water report, then set on the dosing pump.
Where is antiscalant dosed?
After the cartridge filter and any dechlorination, and before the high-pressure pump, so it is fully mixed before the water reaches the membranes.
Can I use antiscalant with chlorine in the water?
Some grades tolerate chlorine, but polyamide RO membranes do not. Remove chlorine before the membranes, usually with sodium metabisulphite or a carbon filter.
