Filter Effluent Turbidity Rise After Storm Event
A 4.5 MGD conventional surface-water plant treating a reservoir source has just come through a 2.1-inch rain event. Raw water turbidity rose from 3 NTU to 22 NTU and pH fell from 7.8 to 7.2. The plant feeds alum at a manually adjusted 28 mg/L and polymer at 0.3 mg/L; jar tests are run once per shift. Filter run times have shortened to 52–60 hours. Combined filter effluent is 0.28 NTU, but Filter 3 has read 1.2 and 1.3 NTU on the last two consecutive 15-minute turbidimeters. Clearwell free chlorine is 1.1 mg/L at pH 7.4 and 9°C; the operator has a CT table. The shift operator proposes increasing chlorine feed to 1.6 mg/L and continuing the filter run until the next shift. The plant serves 18,000 people and has two finished-water reservoirs.
1. What is the immediate significance of the Filter 3 readings, and what should the operator do before the next shift? Explain the regulatory and treatment reasoning.
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Combined filter effluent at 0.28 NTU is below the 0.30 NTU combined filter effluent limit, but that is not the only test. Under the surface water treatment rules, an individual filter reading above 1.0 NTU for two consecutive 15-minute samples requires follow-up. The operator should not wait for the next shift. Immediate steps: backwash Filter 3 and observe the backwash for media loss or mudballs; check headloss and filter rate; confirm the turbidimeter is calibrated and the sample line is not air-bound; run a jar test at the higher raw turbidity and adjust alum/polymer to improve floc; and consider reducing plant flow or taking Filter 3 out of service. Within 24 hours, perform a filter profile or take documented corrective action. If the problem cannot be corrected, notify MDE as the primacy agency and record the event. Raising chlorine does not fix a filtration barrier failure and can mask the real problem.
2. The operator wants to raise clearwell chlorine to compensate. Evaluate that decision, including CT and disinfection byproduct implications.
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Chlorine is the backstop, not the fix. Raising clearwell chlorine from 1.1 to 1.6 mg/L increases CT, but CT must be calculated from actual clearwell volume, flow, baffling, pH, and temperature. At 9°C and pH 7.4, the CT required for Giardia is relatively high; the operator should verify the existing CT before changing dose. More importantly, turbidity above 1.0 NTU can shield Cryptosporidium and Giardia, so filtration must be corrected first. Additional chlorine also raises THM and HAA5 formation, especially with storm-driven organic precursors. A moderate increase may be justified as a temporary disinfectant residual safeguard if CT is marginal, but only with DBP monitoring and a plan to return to normal after filtration is stable. Document the decision and the CT calculation.
3. What follow-up records and notifications should be made?
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Records should include raw and finished turbidity, filter run times, backwash dates and durations, jar-test results, coagulant doses, filter profile, corrective actions, and turbidimeter calibration. Because the individual filter exceeded 1.0 NTU on two consecutive readings, the plant must document whether a filter profile was performed within 24 hours or whether MDE was notified. The monthly report should show the individual filter data and combined filter effluent percentages. The plant should also review its CT calculation and disinfection records for the event. If the filter is taken out of service or the plant reduces production, record the reason. Notify MDE promptly if the combined filter effluent exceeds 1 NTU, if the plant cannot meet the 95% combined limit, or if the individual filter follow-up cannot be completed.