PAC, Ferric Chloride & Generic Coagulant Dosing Equation
Coagulants destabilise colloidal turbidity so it can flocculate and settle. PAC, ferric chloride and ferric sulphate dominate municipal practice — each at different optimum doses and strengths. This calculator handles any liquid coagulant: input the chemical's % active and SG, and the same mass-balance formula gives the L/hr dose.
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
- Alum Dosing Calculator — Coagulation · Jar Test Scale-Up
- Polymer Dosing Calculator — Wastewater · Flocculant · Make-down
- Chlorine Dosing Calculator — Drinking Water · Cooling Towers
- All Chemical Engineering Calculators →
Frequently Asked Questions
PAC (polyaluminium chloride): best for low-alkalinity / cold water; lowest residual aluminium. Alum: cheapest per kg active; needs alkalinity. Ferric chloride: best for high-organic / colour removal; works at lower pH (4.5–5.5); colours water yellow if overdosed. Always confirm with a comparative jar test on the actual raw water.
5–25 mg/L for most surface waters; 25–50 mg/L for tropical rivers in monsoon. PAC is typically dosed at 30–50% the alum-equivalent dose. Sold as 10–18% Al₂O₃ liquid (SG 1.20–1.36).
Optimum ferric coagulation pH is 4.5–5.5 for colour removal and 6.5–7.5 for turbidity removal. Outside these ranges the dose can need to double for the same result. Always couple FeCl₃ dosing with pH adjustment downstream.
Raw water variability (turbidity, alkalinity, organics, temperature) means optimum dose can shift 50–200% within a week, especially after heavy rain. Daily 6-cup jar tests at 5–60 mg/L give the operator the right dose before plant scale-up — saves 20–40% chemical cost vs flat dosing.
Yes — over-coagulation causes restabilisation: floc breaks apart, turbidity passes through filters, residual metals spike. The dose-vs-turbidity curve is U-shaped: too little = no removal, optimum = clear water, too much = turbid carry-over. Always operate at or just above the jar-test optimum.