Chlorine (NaOCl, Ca(OCl)₂, Cl₂ gas) Dosing Equation
Chlorine is the workhorse disinfectant of municipal water, swimming pools and cooling towers. Whether you dose 12% sodium hypochlorite, 65% calcium hypochlorite (HTH) granules or gaseous chlorine, the calculation comes down to mass balance — chlorine demand × flow ÷ stock strength.
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
- Calcium Hypochlorite (HTH) Dosing Calculator — HTH · Pool · Tank Disinfection
- Chlorine Dioxide (ClO₂) Dosing Calculator — ClO₂ · Legionella · Cooling
- Caustic Soda Dosing Calculator — pH Raise · Neutralisation
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Frequently Asked Questions
WHO and most national codes target a free-chlorine residual of 0.2–0.5 mg/L at the consumer tap, which usually requires a 1–3 mg/L dose at the treatment plant to overcome chlorine demand from organics, ammonia and pipe biofilm. For highly contaminated raw water or breakpoint chlorination of ammonia, doses of 5–10 mg/L are common.
Shock chlorination targets 50–200 mg/L free Cl₂ for 30–60 minutes. For a 10,000 L tank at 100 mg/L target with 12% NaOCl: dose = (10,000 × 100) ÷ 144,000 = 6.94 L of 12% NaOCl. Always allow contact time before flushing and de-chlorinating.
Open-recirculating cooling towers typically need 1–4 mg/L free chlorine residual to control biofilm and Legionella. Allowing for 1–2 mg/L demand from organics and corrosion inhibitors, the applied dose is usually 2–6 mg/L, often shock-dosed at 5–10 mg/L weekly.
Yes. Calcium hypochlorite (HTH) is sold as 65–70% available chlorine, compared with 10–15% for liquid sodium hypochlorite. 1 kg of 65% HTH releases the same Cl₂ as ~5.4 L of 12% NaOCl. HTH is preferred where storage volume is limited; NaOCl is preferred where dust handling is a hazard.
12% NaOCl loses ~1% strength per week at 25°C, faster in heat or sunlight. Field strength is typically 9–11% even when labelled 12%. For accurate dosing, titrate stock strength monthly or assume 90% of nominal. Refrigerated storage and dark tanks extend life.
Chlorine Dosing for Water Disinfection
Chlorine is the most widely used disinfectant in drinking-water treatment globally, with a track record stretching back to the early 20th century. Its dominance comes from a unique combination of properties: it kills the major waterborne pathogens (cholera, typhoid, dysentery, and most viruses) within the contact times achievable in real treatment plants; it leaves a measurable residual that protects water in transit through the distribution system; it is cheap; and the dose can be controlled accurately with simple instrumentation. The chlorine dosing calculator on this page sets the dose required to satisfy chlorine demand from natural organic matter and ammonia plus a target residual concentration at the point of use.
The Three Forms of Chlorine in Practice
Gaseous chlorine (Cl₂) — cylinder-supplied, the most concentrated form (100% Cl₂ equivalent), used by larger plants. Highly toxic; requires gas-handling infrastructure (vacuum regulators, scrubber for safety, leak detection). Sodium hypochlorite (NaOCl) — supplied as 12–15% liquid (“industrial bleach”), or generated on-site from brine electrolysis. The most common form for medium and small plants because it is easier and safer to handle than gas. Loses strength in storage (~1–2%/week at 25 °C). Calcium hypochlorite (Ca(OCl)₂) — dry granular, typically 65–70% available chlorine. Used for emergency dosing, swimming pools, and very small plants where bulk-liquid storage is impractical. The calcium hypochlorite dosing calculator handles this form directly.
The Dosing Equation and the Residual Concept
The total chlorine dose required = chlorine demand + target residual. Chlorine demand is the amount consumed by reactions with organic matter, iron, manganese, hydrogen sulphide, and ammonia in the raw water; it is determined experimentally by chlorine-demand testing or estimated from raw-water TOC. The target residual is the free chlorine concentration you want available at the far end of the distribution system — typically 0.2–1.0 mg/L per WHO and EPA guidance, with at least 0.2 mg/L always present at the most distant tap. Total dose = demand + residual.
The dosing formula is the standard chemical-dosing equation: Feed rate (kg/day) = Dose (mg/L as Cl₂) × Flow (ML/day) × (100 / available chlorine %). For sodium hypochlorite (12% available chlorine), a 30 ML/day plant with a 2.5 mg/L total dose needs: 2.5 × 30 × (100/12) = 625 kg/day of NaOCl solution = ~565 L/day at SG 1.10. Pump throughput ~ 24 L/h.
Free vs Combined vs Total Chlorine
When chlorine is added to water containing ammonia, it forms chloramines (combined chlorine — monochloramine NH₂Cl, dichloramine NHCl₂, trichloramine NCl₃). Chloramines are weaker disinfectants than free chlorine but persist longer in distribution. The breakpoint chlorination curve shows the transition: below the breakpoint dose, all chlorine forms combined chloramines and residual rises slowly; at breakpoint, chloramines are oxidised to N₂ and residual drops to nearly zero; beyond breakpoint, additional chlorine appears as free chlorine and residual rises steeply. For drinking water with ammonia present (e.g. some surface waters and groundwaters), the dose must exceed breakpoint to deliver a free-chlorine residual. The breakpoint dose is approximately 7.6 mg Cl₂/L per mg NH₃-N/L.
Worked Example: Setting a Plant’s Chlorine Dose
A 50 ML/day groundwater plant has a 24-hour chlorine demand of 1.0 mg/L (mostly iron oxidation) and the distribution system requires a 0.5 mg/L free residual at the most distant tap. Total dose = 1.0 + 0.5 = 1.5 mg/L. With 12% sodium hypochlorite: feed = 1.5 × 50 × (100/12) = 625 kg/day = ~570 L/day = 24 L/h pump output. For 30-day bulk storage (with 20% safety margin for hypochlorite decay): tank volume = 570 × 30 × 1.2 = 20,500 L → specify 22 m³. The calculator on this page produces this calculation in one step and lets you sweep different chlorine forms (gas, NaOCl, Ca(OCl)₂) and concentrations to compare logistics.
Disinfection Byproducts and the THM Trade-Off
Chlorine reacts with natural organic matter (NOM, mostly humic and fulvic acids in surface water) to form trihalomethanes (THMs) and haloacetic acids (HAAs), which have regulatory limits (US EPA: total THMs < 80 μg/L, total HAAs < 60 μg/L). Higher chlorine dose → more byproducts; longer contact time → more byproducts. Modern plants minimise THM formation by removing NOM upstream (enhanced coagulation, GAC adsorption) so that less chlorine demand needs to be satisfied. For source waters with high NOM, consider switching primary disinfection to ozone or UV and using chloramine for distribution residual to slash THMs.
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