Filter 2 Turbidity Breakthrough on the Night Shift

The sequence matters more than any single step.

1. Verify the number before acting on it. Pull a grab sample and check the turbidimeter for a fouled lens, bubble, or drifting zero. A false spike has caused more than one unnecessary filter shutdown.
2. Fix the coagulant, because that is almost certainly the cause. A pump that lost prime means under-dosed or unsettled floc reached all six filters. Re-prime, confirm the stroke setting, and verify actual feed by day-tank drawdown, not by the pump dial.
3. Look at settled-water turbidity. If clarifier effluent turbidity also rose, the filters are being loaded, and the other five will break through in run order.
4. Decide on Filter 2 using the numbers, not the trend. With filter-to-waste unavailable, the practical threshold is 1.0 NTU: two consecutive 15-minute readings at or above that level on an individual filter is a regulatory trigger, not just an operating nuisance. If turbidity keeps climbing after coagulant is restored, close the influent and effluent valves and take the filter out of service.
5. Notify the chief operator and document times, readings, the prime failure, and every action.

Turbidity and CT are linked, and the link is the reason a filter event cannot be treated as a filter-only problem.

CT side. CT credit is the product of chlorine residual, contact time (clearwell volume times a baffling factor, divided by peak hourly flow), and the assumed low turbidity of the water entering contact. Turbidity particles shield Giardia and viruses and exert chlorine demand. If higher-turbidity water reached the clearwell, the residual at the outlet can fall even though the dose did not change. Check free chlorine at the outlet of the contact basin, not at the chlorine injection point, and be cautious about taking full CT credit for a period in which the residual was depressed.

Compliance side. The rule measures CFE, so a 0.14 NTU CFE is not a violation. The individual filter reading is a leading indicator and has its own follow-up requirements if it reaches 1.0 NTU on two consecutive 15-minute readings. A treatment technique violation would arise if CFE exceeded 0.3 NTU in more than 5% of the month's measurements or any CFE reading exceeded 1.0 NTU. That is the line that converts a bad night into a Tier 2 public notice and a report to the primacy agency.

Documentation. Record both the individual and combined values; the record is what a sanitarian will reconstruct the event from.

Losing filter-to-waste removes the plant's ability to ripen a filter without sending it to the clearwell, so the operating strategy has to compensate.

• Tighten the action point. With no filter-to-waste, take the filter out of service earlier rather than riding the turbidity up.
• Protect the backwash sequence. After each backwash, watch the first 15-30 minutes of filter effluent closely; that is when ripening spikes appear and when the missing line would normally be used.
• Shorten run times if needed. Filter 2 was at 62 hours; if breakthrough is recurring, consider a shorter run or lower loading rate rather than letting turbidity set the backwash trigger.
• Watch the clearwell. During any higher-turbidity excursion, check free chlorine at the clearwell outlet and adjust if residual is being consumed.
• Escalate the repair. A tagged-out actuator that disables a treatment barrier is an operational risk item, not routine maintenance; ask for a temporary repair or an emergency work order, and note the limitation in the shift log and in the monthly report.
• Consider a written contingency describing how filters are managed while the line is unavailable, so the decision is not reinvented on each shift.