Breakpoint Chlorination Dose Calculator
Need free chlorine past the ammonia hump? Enter your ammonia-N and background chlorine demand for a theoretical breakpoint dose estimate — then confirm it with a lab breakpoint curve before changing feed rates.
Theoretical Breakpoint Dose
Theoretical estimate only. Real breakpoint varies with pH, temperature, and contact time. Run a lab breakpoint curve (jar test with graded chlorine doses) and confirm the free-chlorine residual before adjusting plant feed.
| Item | Value |
|---|
How the calculation works
Formulas
Breakpoint dose (mg/L as Cl₂) ≈ 7.6 × ammonia-N (mg/L as N) + chlorine demand (mg/L)
Ammonia-driven portion = 7.6 × ammonia-N
Free residual past breakpoint = applied dose − breakpoint dose
Breakpoint dose (mg/L as Cl₂) ≈ 7.6 × ammonia-N (mg/L as N) + chlorine demand (mg/L)
Ammonia-driven portion = 7.6 × ammonia-N
Free residual past breakpoint = applied dose − breakpoint dose
Notes & limitations
The ~7.6:1 Cl₂:NH₃-N breakpoint weight ratio is the theoretical stoichiometry from White’s Handbook of Chlorination and Alternative Disinfectants and AWWA M20. Field breakpoint curves, not this estimate, set the plant dose.
- Some references use 8:1–10:1 as a practical rule of thumb to allow for side reactions — the 7.6:1 used here is the theoretical value. Your jar-test curve is the authority.
- Breakpoint is pH- and temperature-sensitive, and the curve shifts with contact time. Re-run the curve when raw water changes seasonally.
- Overshooting breakpoint wastes chemical and can spike DBPs; undershooting leaves you in the dichloramine taste-and-odor zone. Verify with DPD free/total chlorine testing.
← All water treatment calculators · Related: Chlorine Dose Calculator