Reverse Osmosis Antiscalant Dosing Equation
Antiscalants prevent calcium-, barium- and silica-based scaling on RO membranes. They are dosed neat (100%) at 2–5 ppm of feed flow for brackish water RO and 1.5–4 ppm for seawater RO (SWRO). Underdose = membrane fouling; overdose = wasted chemical, possible biological food source.
Where:
- Flow = Main flow rate in m³/hr
- Dose_PPM = Target concentration in mg/L or ppm
- Strength_% = Percentage active ingredient of the stock chemical
- SG = Specific Gravity (density relative to water) of the stock
Related dosing calculators
Other chemical-specific dosing calculators in the same series — same formula, different defaults:
- Chemical Dosing Calculator (generic) — the universal seed page
- Chlorine Dosing Calculator — Drinking Water · Cooling Towers
- Phosphate Dosing Calculator — Boiler · Cooling · Anti-Corrosion
- Polymer Dosing Calculator — Wastewater · Flocculant · Make-down
- All Chemical Engineering Calculators →
Frequently Asked Questions
2–5 ppm of feed flow for brackish-water RO; 1.5–4 ppm for seawater RO. The exact dose comes from the antiscalant supplier's projection software (e.g. Avista Advisor, Genesys Membrane Master) which models the recovery, ionic profile and saturation indices (LSI, S&DSI, calcium sulphate).
Higher TDS = higher saturation index for sparingly soluble salts → higher antiscalant requirement. Brackish water RO at 70% recovery typically needs 3 ppm; SWRO at 45% recovery needs 2 ppm; high-recovery (80–90%) brackish systems may need 5–8 ppm of a phosphonate-blend antiscalant.
Yes — overdosed phosphonate antiscalants can act as a phosphorus food source for biofilm in the lead elements, accelerating biofouling. Polyacrylate antiscalants are less prone to this. Stay within the supplier's recommended max dose (usually 6–8 ppm).
Most antiscalants are stable concentrated but degrade or precipitate when diluted to <20% — especially in hard make-down water. Dose pumps with PVDF or 316L heads inject the neat product into the RO feed line just before the cartridge filter.
Always re-run the projection software with the new product's max-saturation limits before changeover. Some antiscalants are not compatible (e.g. cationic + anionic types), so flush the dosing line for 2–3× residence time when switching. Re-baseline membrane pressure-drop after 2 weeks to confirm performance.