Chemical Dosing Equation
Calculating the correct dosing pump rate ensures that the proper concentration of a chemical is injected into a moving stream of water or another fluid.
Where:
- Flow = Main water 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 chemical
Frequently Asked Questions
Multiply the main flow rate by the target PPM dose. Divide that result by the density of the chemical and the percentage of active ingredient.
Chemical dosing is the controlled addition of treatment chemicals to a water or process stream at a precise rate to achieve a target concentration or effect. Common applications include: chlorination (disinfection), pH correction (acid/alkali dosing), coagulation (turbidity removal), fluoridation, antiscalant dosing (preventing scale in membranes and heat exchangers), and corrosion inhibitor dosing.
Chemical dose (mg/L) × Flow rate (m³/h) × 1000 = chemical required (g/h). For liquid chemicals: Pump flow rate (L/h) = [Dose (mg/L) × Process flow (m³/h)] / [Chemical concentration (g/L)]. Example: 2 mg/L chlorine dose, 100 m³/h flow, using 12% sodium hypochlorite (120 g/L): Pump rate = (2 × 100 × 1000) / 120,000 = 1.67 L/h.
For dilute aqueous solutions (water-based), 1 ppm (part per million by mass) ≈ 1 mg/L. This equivalence holds because water density ≈ 1 kg/L, so 1 mg per kg ≈ 1 mg per litre. This approximation breaks down for concentrated solutions or non-aqueous fluids. Always use mg/L for precise dosing calculations in water treatment; ppm is acceptable for approximate field work.
Common dosing pump types: Diaphragm metering pumps — most common; accurate ±1%; handle corrosive and viscous chemicals; suitable for 0.1–1000 L/h at up to 10 bar. Peristaltic (tubing) pumps — fluid only contacts the tubing; no seals; ideal for aggressive chemicals; easy maintenance; lower accuracy (±5%). Piston pumps — high pressure (up to 700 bar) and high flow rates; used in reverse osmosis and high-pressure injection. Solenoid-driven pumps — compact and inexpensive for low flow rates.
Key water treatment chemicals and typical doses: Sodium hypochlorite (NaOCl, 12–15%) — disinfection, 1–5 mg/L as Cl₂; Aluminium sulphate (alum) — coagulation, 10–100 mg/L; Ferric chloride — coagulation, 10–80 mg/L; Sodium hydroxide (caustic soda, 50%) — pH raise, dose to target; Sulphuric acid (98%) — pH lower; Fluorosilicic acid (H₂SiF₆, 20%) — fluoridation, target 0.7 mg/L F⁻; Antiscalant (polymer) — RO membrane protection, 2–10 mg/L.