Filter 3 Turbidity Rise With Lower Headloss
A 6 MGD conventional surface-water plant treats a reservoir supply: raw turbidity 2.5 NTU, alkalinity 60 mg/L, pH 7.8. After a thunderstorm, raw turbidity rises to 22 NTU and alkalinity drops to 45 mg/L. The operator keeps the alum dose at 28 mg/L and the anionic polymer at 0.15 mg/L. Filters are dual-media, run 30 hours, and were last backwashed 6 hours ago. Filter 3 effluent turbidity rises from 0.07 to 0.55 NTU over 40 minutes, while headloss stays lower than the other filters. Filter-to-waste is available but not in service. The clearwell is at 78% and demand is 5.2 MGD. The lab's morning jar test showed good floc at 32 mg/L alum, but the polymer make-down tank was refilled at 6 a.m.
1. What is the most likely explanation for Filter 3's behavior, and how would you verify it?
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Filter 3 is passing floc while headloss is lower than its peers. That pattern points to a filter problem, not a simple coagulant underdose: an underdose would usually raise turbidity on all filters and often increase headloss by forming weak floc that blinds the surface. Lower headloss with high effluent turbidity suggests media loss, a disturbed media bed, or channeling after backwash. Verify by comparing Filter 3's backwash sequence and air-scour records, probing media depth at several points, checking for a sand boil or mounded media, measuring turbidity at 15-minute intervals during ripening, and confirming the polymer make-down batch strength. The polymer refill at 6 a.m. is a second suspect; a weak or unmixed batch can cause floc carry-through. The jar test used 32 mg/L alum while the plant fed 28 mg/L, so the plant is also slightly underdosed.
2. What immediate actions would you take to protect finished water?
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Place Filter 3 on filter-to-waste immediately if the clearwell can be protected; 0.55 NTU is above the 0.3 NTU individual filter limit and must not reach finished water. If filter-to-waste is not available, take Filter 3 out of service and backwash it, then inspect the media before returning it. Notify the shift supervisor and check clearwell turbidity and finished-water turbidity; if finished water exceeds 0.3 NTU, increase monitoring and consider using the alternate supply. Because demand is 5.2 MGD and the clearwell is at 78%, only about 4.7 MG is available; losing one filter during a 22 NTU raw-water event still leaves capacity if the other filters are stable, but do not return Filter 3 until turbidity is below 0.1 NTU for at least 15 minutes. Increase filter effluent monitoring to every 15 minutes on all filters.
3. What process changes would you make for the next 24 hours?
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For the next 24 hours, run a jar test at the actual plant polymer batch and at 32–35 mg/L alum, because alkalinity dropped to 45 mg/L and alum consumes about 0.5 mg/L alkalinity per mg/L. Check settled pH; keep it above 6.5 to avoid poor floc. Repair or replace the polymer make-down feed and verify dose by drawdown. Backwash filters one at a time and record turbidity, headloss, and backwash flow; a lower-than-normal headloss after backwash justifies a media inspection. Keep Filter 3 on filter-to-waste until the media is verified and turbidity is stable. After the storm, do not lower alum until raw turbidity is below 5 NTU and the filters have run a full cycle. Document the event for the next sanitary survey.