Turbidity Excursion After a Reservoir Turnover
A 12 MGD conventional surface water plant draws from a stratified 640-acre reservoir. In late September the reservoir destratifies; the intake, which sits 18 ft below full pool, begins pulling water with a raw turbidity that climbs from 3 NTU to 38 NTU over 36 hours, with a corresponding jump in manganese from 0.02 to 0.31 mg/L and dissolved oxygen dropping toward 1 mg/L at the intake depth. Finished water turbidity rises from 0.06 to 0.22 NTU despite the coagulant dose being held at the same 22 mg/L alum setpoint. Filter run times fall from 40 hours to 14 hours, and one filter shows a turbidity breakthrough spike of 0.6 NTU at hour 11 that the operator catches on a grab sample. The plant has an alternate intake 6 ft deeper, which is currently in the anoxic zone. The state has a combined filter effluent limit of 0.3 NTU and individual filter effluent monitoring requirements. The operator on shift has authority to adjust coagulant dose and filter loading but must notify the chief operator before changing intake depth.
1. The operator must decide within the hour whether to change intake depth, adjust coagulant dose, or both. What is the defensible course of action, and what reasoning supports it?
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The deeper intake is in the anoxic zone, where manganese and iron are dissolved and oxygen is depleted. Drawing from 6 ft deeper would likely increase dissolved manganese loading rather than reduce it, so switching depth alone is not the fix and could worsen the taste and manganese problem. The defensible action is to keep the current intake, or if anything raise it toward the oxic epilimnion if the intake structure allows, and to immediately optimize treatment for the changed raw water.
The raw water change is a shift in particle character: higher turbidity plus dissolved manganese that oxidizes slowly. Holding the alum setpoint at 22 mg/L while raw turbidity went from 3 to 38 NTU means the coagulant-to-turbidity ratio collapsed. The operator should run a jar test or bench dose series and raise coagulant dose to restore the ratio, consider an oxidant (permanganate or chlorine) upstream of coagulation for soluble manganese, and verify pH is in the effective alum range. Because the deeper intake is worse, the intake-depth change should be proposed to the chief operator with the water-quality data, not made unilaterally. Document the raw and finished turbidity trend, the dose change, and the filter breakthrough event.
2. A filter shows a 0.6 NTU breakthrough spike at hour 11. What actions and notifications are required, and how should the operator think about the individual filter effluent requirement?
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A single filter effluent above the individual filter limit triggers a response: the filter should be taken out of service or backwashed immediately, and the event documented. Under the individual filter effluent provisions, a filter exceeding 1.0 NTU (or 0.5 NTU depending on the rule tier) after 4 hours of operation, or exceeding 0.5 NTU in two consecutive months, has reporting and possibly assessment consequences. The 0.6 NTU grab reading is above 0.5 NTU, so the operator must treat it as a potential trigger, verify with a second reading and the online turbidimeter, and record it.
Immediate steps: backwash the filter, check for media loss or upset, confirm the coagulant dose change is taking effect, and increase filter surveillance frequency. Notify the chief operator and prepare the documentation. The reasoning is that a single spike may be a transient from the raw water change, but the rule cares about the recorded value and the pattern; the operator should not dismiss it because combined effluent stayed under 0.3 NTU. The combined limit does not excuse the individual filter value.
3. Filter run times dropped from 40 to 14 hours. How should the operator evaluate whether this is a treatment problem or a hydraulic loading problem?
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Shortened runs can come from more solids loading, poorer coagulation producing pin floc that penetrates media, or higher hydraulic loading. The operator should check the mass balance: raw turbidity rose roughly twelvefold, so solids loading to the filters increased substantially even at the same flow. That points first to a coagulation problem, not just hydraulics. A jar test showing whether floc is large and settleable, plus settled water turbidity before the filters, distinguishes the cases. If settled water turbidity is elevated, the fix is upstream in coagulation and sedimentation. If settled water is good but runs are still short, examine filter loading rate, backwash effectiveness, and media condition.
The operator should also confirm backwash is adequate: short runs with poor backwash can indicate media clogging or mudballs. Compare current filter loading rate to design, check for flow surges, and review the backwash sequence. The judgement is to diagnose before reacting: raising coagulant dose without checking settled water may fix the symptom or may just shift the problem. Document settled water turbidity, loading rates, and backwash records so the chief operator can see the reasoning.