Filter Effluent Turbidity Rise After a Watershed Runoff Event
A 6 MGD conventional surface-water plant (alum coagulation, tapered flocculation, two sedimentation basins, six dual-media filters at 4 gpm/ft2) has run clean all spring: raw turbidity 2 NTU, combined filter effluent 0.07 NTU, runs of 55 hours. Then 1.5 inches of rain falls on the 40-square-mile agricultural watershed. Overnight, raw turbidity climbs to 16 NTU, color rises from 8 to 45 CU, UV254 doubles, alkalinity falls from 95 to 72 mg/L as CaCO3, and raw pH drops from 7.5 to 7.2. Nobody has touched the alum (20 mg/L) or the polymer (0.3 mg/L). By mid-afternoon combined filter effluent turbidity is 0.42 NTU, two filters have ended runs on turbidity at 15-18 hours, and finished pH is 7.5 where it used to be 7.9. Chlorine residual and CT are still in compliance.
1. Two filters terminated on turbidity breakthrough at roughly a third of their normal run length, not on headloss. What does that pattern say about the coagulant condition, and what is the mechanism?
Read the worked answer
Pattern. Turbidity breakthrough at 15-18 hours with headloss still available means particles are leaving the bed rather than loading it. That is the signature of an under-destabilized coagulant, not an overloaded filter. Over-dosing looks the opposite: runs limited by headloss and a matted surface layer.
Mechanism. The storm load is mostly colloidal clay plus dissolved organic carbon (color 8 to 45 CU, UV254 doubled). NOM complexes with alum and consumes it before particles are destabilized, so the dose that worked at 2 NTU and 8 CU leaves this water only partially coagulated. The resulting flocs are small and weak: they lodge in the upper media, then shear off as the bed compacts - short runs, early breakthrough, and pin floc heading for the clearwell.
Corroborating data. The alkalinity drop to 72 mg/L and raw pH 7.2 are consistent with added coagulant demand; note that 20 mg/L of alum consumes only about 9-10 mg/L as CaCO3, so most of the alkalinity loss is storm dilution, not chemical demand. Confirm with settled-water turbidity and individual filter particle counts: all filters rising together points at coagulation, one filter alone points at that filter.
2. Design the jar test program that will actually answer this question, including what you measure and how you would scale it to the plant.
Read the worked answer
Run the test on the water entering the plant that day, at its actual temperature, with a filtered endpoint - settled turbidity alone hides pin floc.
Matrix. Six jars at alum doses of 20, 25, 30, 35, 40 and 45 mg/L, all with the current 0.3 mg/L polymer, plus one jar at the current 20 mg/L as a control. Add a second row with polymer omitted so you learn whether the polymer is helping or masking a coagulant shortfall.
Mixing. 30 seconds at about 300 rpm, then 15 minutes of tapered flocculation at 40-50 rpm, then 20 minutes of settling.
Endpoints. Settled turbidity at 5, 10 and 20 minutes, then filter 200 mL through a 0.45 micron membrane and read filtered turbidity. Target settled below 2 NTU and filtered below 0.10 NTU. Pick the lowest dose that meets both - over-dosing buys you turbidity at the cost of filter run length and alkalinity.
Secondary checks. Record settled sludge volume (a dose that flocs well but blankets the basins is not a win) and confirm post-dose pH so the downstream corrosion-control target is still reachable. Never jump straight to the jar-test dose; step up in 2-3 mg/L increments across a shift and watch each filter's turbidity and run length.
3. List your operating actions for the next four hours and the regulatory triggers you must watch.
Read the worked answer
First four hours.
- Step alum up to the jar-test dose in 2-3 mg/L increments (do not double it), verify polymer make-down strength and feed rate, and confirm rapid-mix energy and flocculator speed are still at design.
- Reduce loading: cut plant flow if raw storage or clearwell volume allows, and take the worst filter out for backwash and a media inspection. Use filter-to-waste after backwash if available.
- Watch the sedimentation basin blankets - the higher dose raises solids loading there, and carryover to the filters defeats the fix.
- Bring finished-water pH back toward the corrosion-control target. Dropping from 7.9 to 7.5 on 72 mg/L alkalinity is a distribution risk, not a treatment win.
Regulatory triggers. Conventional filtration requires combined filter effluent at or below 0.3 NTU in at least 95% of monthly measurements and never above 1.0 NTU; an individual filter above 1.0 NTU, or above 0.5 NTU for two consecutive 15-minute readings, drives reporting and self-assessment requirements. Document every reading, and if you exceed, notify the primacy agency under its public notification rule. Do not blend to dilute - dilution is not a treatment technique and will not survive an inspection.