Filter 3 Turbidity Rise After a Raw Water Event
A 4.0 MGD conventional surface-water plant treats a river supply. After two days of heavy rain, raw turbidity rose from 12 to 140 NTU and raw pH fell from 8.1 to 7.4. The operator increased alum from 18 to 32 mg/L and polymer from 0.3 to 0.5 mg/L. Clarifier effluent turbidity is 2.8 NTU, and settled floc is small and pin-floc. Filtered water turbidity on Filter 3 rose from 0.08 to 0.31 NTU over six hours; Filters 1 and 2 are at 0.12 and 0.15 NTU. Filter 3 run time has fallen from 52 to 18 hours, and headloss is rising quickly. Finished water pH is 7.6, alkalinity is 38 mg/L as CaCO3, and chlorine residual is 1.1 mg/L. The clearwell has about 6 hours of storage at current demand. The lead operator is off shift, and the SCADA trend shows the filter-to-waste valve was closed manually at 06:00.
1. What immediate actions should be taken for Filter 3 and the finished-water supply, and why?
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Take Filter 3 out of service or return it to filter-to-waste immediately. The SCADA trend showing the filter-to-waste valve was manually closed at 06:00 is the most likely cause: the filter was put into service before the media ripened or while carryover was still high. Reopening the valve and wasting the filtrate protects the clearwell. Because clearwell storage is only about 6 hours, also reduce plant or filter loading and verify that combined filter effluent stays below the 0.3 NTU 95th-percentile limit. Avoid repeated backwashing without diagnosis; short runs with rising headloss and turbidity suggest floc carryover or breakthrough, so check coagulant dose, floc strength, filtration rate, and media condition. Monitor Filter 3 turbidity every 15 minutes and do not return it to service until turbidity is stable and below the permit action level. Notify the supervisor and KDHE if a reporting trigger is met.
2. Evaluate whether coagulation is optimized. What specific tests and changes would you make?
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It is not yet demonstrated that coagulation is optimized. The raw water changed sharply to 140 NTU, pH 7.4, and alkalinity 38 mg/L, yet alum was increased by about 78% without a jar test. Alum consumes roughly 0.45 mg/L of alkalinity per mg/L of dose, so 32 mg/L consumes about 14 mg/L as CaCO3, leaving low alkalinity and a pH depression that can produce pin floc. Small, light floc and short filter runs are classic signs of poor coagulation and flocculation, not simply a dirty filter. Run jar tests on the current raw water at several alum doses, for example 20, 26, 32, and 38 mg/L, with and without polymer. Measure settled turbidity, floc size and settleability, and filtered turbidity. Consider pH or alkalinity adjustment with caustic or lime, or a PACl blend with wider pH tolerance. Verify rapid-mix energy, flocculation stages, and filter-aid polymer. Use streaming current or zeta potential if available. Select the dose that produces low settled turbidity and robust floc, not merely the lowest clarifier turbidity.
3. What regulatory and reporting issues arise if Filter 3 effluent exceeds 1.0 NTU for two consecutive 15-minute samples?
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Two consecutive 15-minute individual filter effluent readings above 1.0 NTU is a reportable individual-filter exceedance, even if combined filter effluent remains below 0.3 NTU. The operator should document the readings, notify the supervisor, contact KDHE within the timeframe required by the system permit, commonly 24 hours, and complete the required filter assessment. The combined filter effluent must still meet the 0.3 NTU limit in at least 95% of monthly measurements, and every sample over 0.3 NTU must be recorded. If finished-water turbidity exceeds the permit limit, or if the system cannot maintain disinfection, additional notice may be required. The key reasoning is that individual-filter performance is evaluated separately from combined effluent; one filter can exceed its trigger while blended water still looks acceptable.