Chlorine Dioxide (ClO₂) Generation & Dosing Equation
Chlorine dioxide is the gold standard for Legionella control in cooling towers and hot-water systems — it does not form trihalomethanes, works across pH 4–10, and penetrates biofilm. ClO₂ is generated on-site from sodium chlorite + acid (or chlorine) and dosed at 0.1–0.8 ppm residual.
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
- Calcium Hypochlorite (HTH) Dosing Calculator — HTH · Pool · Tank Disinfection
- RO Antiscalant Dosing Calculator — RO · Desalination · Membrane Protection
- All Chemical Engineering Calculators →
Frequently Asked Questions
0.3–0.8 mg/L continuous residual in cooling towers per ASHRAE 188 and HSE L8. For potable hot-water systems, 0.2–0.5 mg/L. ClO₂ is 2.6× more potent than chlorine on a mass basis for biofilm penetration, but residual decays in 6–24 hours so continuous dosing is required.
Two-precursor systems: sodium chlorite (NaClO₂, 25%) + hydrochloric acid (HCl, 9%) → ClO₂ in a reactor. Three-precursor systems add sodium hypochlorite for higher yield. Generator outputs are typically 1–500 g/hr ClO₂ as a 0.2–0.5% solution. Bulk ClO₂ cannot be transported (Class 5.1 explosive at >10%).
ClO₂ does not form trihalomethanes (THMs) or haloacetic acids (HAAs), works at high pH where Cl₂ loses potency, penetrates biofilm 100–1000× better, and oxidises iron and manganese without lowering pH. Downside: more expensive, requires on-site generator, must monitor chlorite (ClO₂⁻) by-product (<1.0 mg/L EPA limit).
30–70% of ClO₂ reduces to chlorite ion (ClO₂⁻) in normal water-treatment use — chlorite is a regulated by-product (EPA MCL 1.0 mg/L for drinking water). Apply enough ferric or ferrous iron (Fe²⁺/Fe³⁺) to scavenge excess chlorite if approaching the limit.
Yes — ClO₂ is FDA-approved (21 CFR 173.300) at up to 3 mg/L for food washing and 5 mg/L for poultry chiller water. EU regulation (EC 1935/2004) allows ClO₂ for water in contact with food at up to 0.5 mg/L residual. Lower-than-chlorine taste threshold makes it preferred for bottled water and breweries.