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Chlorination and disinfection

Enter flow, demand and residual to see how much hypochlorite or chlorine to feed, whether the contact tank achieves the required CT and whether the residual meets the rules.

Pre-sizing and estimating tool: results do not replace design, calculations or reports signed by a licensed professional, nor verification against the applicable codes and standards.

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Flow and dose
Product

Sodium hypochlorite loses 10% to 20% of its strength in a month at 77 °F; store it cool, dark and for a short time.

Contact time (CT)
—Product use
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CT achieved and CT required versus temperature

CT required by the EPA regression for Giardia with free chlorine at the pH and residual entered. Cold water needs more contact time.

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Hypochlorite use versus target residual

Daily sodium hypochlorite volume with the demand entered. Vertical lines mark the country residual limits.

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Method, assumptions and references

Equations used

Dose = demand + residual; available chlorine (lb/d) = 8.34 · Q (MGD) · dose (mg/L)
Sodium hypochlorite: g/L = wt% · SG · 10 (or trade % · 10) · (1 − loss); volume = available chlorine ÷ g/L; calcium hypochlorite = available chlorine ÷ strength
CT = residual · t10; t10 = baffling factor · V ÷ Q
Required CT, Giardia, free chlorine (EPA; Smith et al., 1995): T < 12.5 °C: 0.353 · I · (12.006 + e^(2.46 − 0.073 T + 0.125 C + 0.389 pH)); T ≥ 12.5 °C: 0.361 · I · (−2.261 + e^(2.69 − 0.065 T + 0.111 C + 0.361 pH))

Assumptions and limits

  • Demand measured in the lab at the same contact time. Residual at the contact tank outlet equals the target residual. I = logs of Giardia inactivation.
  • The EPA regression approximates the Guidance Manual tables (0.5 to 25 °C, pH 6 to 9, residual 0.4 to 3 mg/L) and runs a few percent above them in cold water: confirm with the official CT tables.
  • Hypochlorite strength loss is an entered average; it grows with concentration, temperature and light.

Standards and technical documents

  • EPA. Guidance Manual for Compliance with the Filtration and Disinfection Requirements (Surface Water Treatment Rule), 1991
  • EPA 815-R-99-013: Disinfection Profiling and Benchmarking Guidance Manual (Appendix E, CT regression)
  • AWWA C651 (desinfecção de redes) e AWWA B300: Hypochlorites
  • The Chlorine Institute: Pamphlet 1 (Chlorine Basics) e Pamphlet 96 (Sodium Hypochlorite Manual)
  • WHO. Guidelines for Drinking-water Quality, 4th ed. incorporating the 1st and 2nd addenda, 2022
  • White, G. C. Handbook of Chlorination and Alternative Disinfectants, 5th ed., Wiley, 2010

U.S. regulations

  • EPA 40 CFR 141.65: maximum residual disinfectant level (MRDL) for chlorine 4.0 mg/L as Cl₂
  • EPA 40 CFR 141.72 (Surface Water Treatment Rule): at least 0.2 mg/L entering the distribution system and detectable residual
  • OSHA 29 CFR 1910.1000 (chlorine PEL), 1910.1200 (Hazard Communication) and 1910.119 (PSM, 1,500 lb of chlorine)
  • EPA 40 CFR 68 (Risk Management Program, 2,500 lb of chlorine); DOT 49 CFR 171–180

Frequently asked questions

How do I calculate sodium hypochlorite feed?

Add demand and residual to get dose, then lb/d of chlorine = 8.34 × MGD × mg/L. For 200 gpm (0.288 MGD) at 2 mg/L, that is 4.8 lb/d; with 12.5 trade percent bleach (125 g/L, 1.04 lb/gal) and 10% loss, about 5.1 gal/d.

What chlorine residual does EPA require?

At least 0.2 mg/L entering the distribution system and a detectable residual in the system for surface water systems; the MRDL is 4.0 mg/L as Cl₂, as a running annual average.

How is CT calculated?

CT is residual (mg/L) times t10 contact time (min). A 10,000 gal tank at 200 gpm with average baffling (0.5) gives t10 of 25 min; at 1 mg/L, CT = 25 mg·min/L.

How much calcium hypochlorite replaces liquid bleach?

Granular 65% calcium hypochlorite carries 0.65 lb of available chlorine per lb. For 4.8 lb/d of chlorine, use about 7.4 lb/d.