RO Antiscalant Selection: Matching Chemistry to Feedwater

Three Grades, Three Chemistries

Grade	Target Scale	Feedwater Condition	Dosage
General Purpose	CaCO₃, CaSO₄, BaSO₄, mild silica	Standard brackish/municipal	2–5 ppm
High Silica	Reactive + colloidal silica	SiO₂ > 120 ppm in concentrate	3–8 ppm
High Hardness	CaCO₃, CaSO₄ at extreme levels	Ca hardness > 500 ppm, LSI > 2.5	3–6 ppm

Decision Tree

1. Calculate LSI of your RO concentrate (not feedwater)
2. Check silica: If concentrate SiO₂ > 120 ppm → High Silica grade
3. Check hardness: If Ca hardness > 500 ppm or LSI > 2.5 → High Hardness grade
4. Otherwise → General Purpose grade

LSI Quick Reference

LSI = pH - pHs (saturation pH)

LSI Value	Meaning	Action
< 0	Undersaturated — corrosive	No scaling risk, but may need corrosion inhibitor
0 to 1.0	Mild scaling tendency	General purpose antiscalant sufficient
1.0 to 2.5	Moderate scaling	General purpose at higher dose, or high-hardness grade
> 2.5	Severe scaling	High-hardness grade required

Dosing Guidelines

• Inject antiscalant before the RO high-pressure pump
• Use a metering pump with pulse dampener for steady dosing
• Typical concentration in feedwater: 2–8 ppm (product-dependent)
• Verify compatibility with your membrane manufacturer's recommendations

Common Mistakes

1. Using general-purpose antiscalant for high-silica water → silica deposits within weeks
2. Overdosing → antiscalant itself can foul membranes
3. Not adjusting dose when recovery rate changes
4. Mixing incompatible antiscalants during product changeover