The Chlorine Dosage That Stopped Working
A municipal water treatment plant serving 250,000 people draws water from a river whose ammonia concentration has risen from 0.1 mg/L to 1.2 mg/L after upstream fertilizer runoff. Operators keep the free chlorine residual at 0.5 mg/L at the clear well, but consumer complaints of a persistent swimming-pool odour have tripled in a month, and bacteriological samples from two dead-end distribution mains show low-level coliform growth. The plant manager proposes simply doubling the chlorine dose to restore the residual. A junior chemist notes that the odour worsened after the last dose increase. Chlorine demand tests show a breakpoint curve with a pronounced hump before the breakpoint. The plant has no ammonia analyser on site, only a colorimetric DPD free/combined chlorine kit, and the budget for the year is nearly exhausted.
1. Diagnose why raising the chlorine dose made the odour worse, and explain what the breakpoint curve is telling the operators.
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The river ammonia reacts with free chlorine to form chloramines, principally monochloramine, then dichloramine and nitrogen trichloride as the Cl2:NH3-N ratio rises. The odour is not free chlorine but dichloramine and nitrogen trichloride, which are far more volatile and pungent. On the breakpoint curve, the initial rising limb is chloramine formation; the hump is where combined chlorine peaks; the dip is where chloramines are oxidised and destroyed; beyond the breakpoint, free chlorine reappears. Doubling the dose from an already-combined state pushed the system further into the dichloramine/trichloramine region rather than past the breakpoint, so odour intensified. The correct action is to calculate the breakpoint demand from the ammonia concentration and dose past it, then maintain a true free residual, not simply to add more chlorine blindly. At 1.2 mg/L NH3-N, the stoichiometric breakpoint is roughly 7.6 mg/L as Cl2 (mass ratio about 7.6:1), plus the immediate demand of the raw water, so the practical target is well above the current 0.5 mg/L free residual.
2. Given that the plant has only a DPD free/combined kit and no ammonia analyser, design a practical monitoring and control strategy for the next three months.
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The DPD kit can still be used diagnostically: measuring free and total chlorine separately gives combined chlorine by difference, and a large combined fraction signals chloramine dominance. Operators should run a simple breakpoint test on a raw-water sample each shift: split the sample into several portions, dose increasing chlorine concentrations, mix and wait a fixed contact time, then measure free and total residual. Plotting these reveals the hump and the breakpoint empirically without an ammonia analyser. Control rules: never dose into the hump; if combined chlorine exceeds about 0.2 mg/L at the clear well, increase dose until free residual appears, then back off to maintain 0.5-1.0 mg/L free. For the dead-end mains, institute flushing and consider switching to chloramination deliberately for distribution stability once the ammonia is under control at the plant, because a monochloramine residual persists longer than free chlorine. Record all dosages and residuals so the dosing trend can be reviewed weekly against river ammonia reports from the fertilizer plant.
3. A colleague argues the coliform regrowth is unrelated to the chloramine problem because coliforms are killed by any chlorine residual. Evaluate this claim.
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The claim is wrong in two ways. First, monochloramine is a much weaker disinfectant than free chlorine; its CT values for coliform inactivation are far higher, so a 0.5 mg/L combined residual in a dead-end main with long residence time and biofilm may not achieve adequate kill. Second, the distribution mains have biofilm; chloramines penetrate biofilm poorly compared with free chlorine, and coliforms can survive and regrow in the biofilm even when bulk-water residual looks acceptable. The low-level coliform detections are therefore consistent with the chemistry: the plant is producing chloramines, not free chlorine, and the network is under-disinfected. The fix is not more of the same but restoring a free chlorine residual past the breakpoint at the plant, flushing the affected mains, and possibly a temporary hyperchlorination of the dead ends. The junior chemist's observation that odour worsened after the dose increase is the key clue that the system was being driven deeper into chloramine territory.