Breakpoint Chlorine-to-Ammonia Ratio

Ammonia in the source water hijacks chlorine. The first chlorine you add makes monochloramine, a weak combined residual; keep adding and you make dichloramine, then nitrogen trichloride, and eventually the chlorine oxidizes the ammonia all the way to nitrogen gas. The residual actually falls as you increase the dose over that middle stretch — the famous dip in the breakpoint curve — and only past the low point, the breakpoint, does free chlorine start to accumulate. Stoichiometry puts breakpoint at 7.6 parts chlorine to 1 part ammonia nitrogen by weight (from 3Cl₂ + 2NH₃ → N₂ + 6HCl), so 0.5 mg/L of ammonia nitrogen needs about 3.8 mg/L of chlorine before you see any free residual at all.

Practice runs richer than theory: real waters demand 8:1 to 10:1 because organics and other reduced species compete, and plants commonly target 10:1 to be safely past the hump. The failure mode is stopping halfway. An operator sees the residual dropping as they increase the feed, assumes something is wrong, and backs off — landing squarely in the dichloramine valley where the water tastes and smells worst and the disinfection is weakest. That swampy "swimming pool" odour is chloramines, not excess chlorine; the cure is almost always more chlorine, not less. The mirror image is deliberate chloramination, where a plant holds a 4:1 to 5:1 ratio on purpose to keep a long-lived combined residual in a sprawling distribution system with minimal trihalomethane formation.