Chlorine Demand Spike After Filter Backwash
A 2,400 m³ outdoor pool complex serves a community centre. Free chlorine is held at 1.5 mg/L and cyanuric acid at 55 mg/L. On Monday morning the operator backwashes two sand filters in sequence and tops up with 40 m³ of municipal make-up water. By 14:00 the ORP controller reads 610 mV (was 740 mV), combined chlorine has risen from 0.2 to 0.6 mg/L, and several swimmers report stinging eyes. Free chlorine still tests at 1.4 mg/L on the DPD kit. The bather load today is 180 people, higher than the usual 90. The facility must stay open until 20:00 and the health code requires free chlorine ≥1.0 mg/L, combined chlorine ≤0.4 mg/L, and ORP ≥650 mV.
1. Diagnose why free chlorine reads adequate while ORP and combined chlorine indicate a problem, and state what the DPD reading is masking.
Read the worked answer
Free chlorine measured by DPD is a total residual number; it does not distinguish how much of that chlorine is still an active disinfectant from how much has already reacted with bather waste (sweat, urine, sunscreen) to form chloramines. The rising combined chlorine (0.2 → 0.6 mg/L) is direct evidence that chlorine is being consumed faster than it is being dosed. ORP is the better real-time indicator of disinfection potential: it fell from 740 to 610 mV, below the 650 mV code minimum, because chloramines are far weaker oxidisers than free chlorine. The DPD result (1.4 mg/L) is technically 'free' chlorine by the test method but is largely tied up as chloramines or otherwise unavailable. The trigger was the backwash/make-up event plus a doubled bather load: fresh make-up water carries its own chlorine demand, and 180 bathers introduce far more ammonia and organics than 90, so demand outstripped the feed rate. The lesson: never accept a single DPD number as proof of disinfection; cross-check ORP and combined chlorine.
2. The pool must stay open until 20:00. Give an ordered action plan with the specific parameters you would change and the readings you would use to confirm recovery.
Read the worked answer
- Boost free chlorine now. Raise the setpoint so free chlorine climbs to roughly 3–5 mg/L (well above the 1.0 mg/L floor) to burn off chloramines; this is a shock/breakpoint approach. Do not exceed the local maximum free chlorine limit - verify the code ceiling before dosing.
- Stop the source of demand. Reduce bather load if possible (cap admissions, stagger entry) until ORP recovers; 180 bathers on a compromised system will keep feeding ammonia.
- Verify circulation and filtration. Confirm both backwashed filters are actually online and not in rinse/bypass, and that flow rate and turnover are normal - a filter left in backwash wastes chlorine and passes dirty water.
- Check cyanuric acid. At 55 mg/L, CYA locks up a meaningful fraction of free chlorine and blunts ORP response. Do not add more stabiliser; consider partial dilution if CYA is at or above the local ceiling.
- Confirm recovery with three readings, not one: free chlorine in target range, combined chlorine back ≤0.4 mg/L, and ORP ≥650 mV sustained for at least 30–60 minutes. Only then return to normal setpoints.
- Document the event, the readings, and the corrective actions in the log, and notify the duty manager. If ORP cannot be brought above 650 mV by 20:00, close the pool - the code minimum is not negotiable.