Filter Effluent Turbidity Rise After Spring Runoff
A 2.5 MGD conventional surface-water plant treating a prairie river has seen raw-water turbidity jump from 4 NTU to 38 NTU after two days of heavy rain. The alum dose is 22 mg/L, settled-water turbidity is 3.1 NTU, and combined filter effluent is 0.42 NTU and climbing; one filter is at 94 inches of headloss and the other just returned to service after backwash. Clearwell free chlorine is 1.1 mg/L, pH is 7.4, water temperature is 8°C, and the clearwell has about 45 minutes of contact time at current flow. The plant has a jar tester, but the operator on shift has not run one since the turbidity changed. The state turbidity limit is 0.3 NTU for combined filter effluent, and each filter must stay below 1 NTU. The next state report is due in six hours.
1. List the first four actions you would take in the next 30 minutes, and explain why each is ordered where it is.
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Order the actions by public-health risk and by whether they prevent irreversible loss of treatment control.
- Verify the reading before reacting. Confirm 0.42 NTU with a second turbidimeter and check each filter effluent separately. A rising combined value may come from one filter, not both, and the response differs.
- Notify the supervisor and start a jar test immediately. The raw water changed from 4 to 38 NTU, so the coagulant demand almost certainly changed. The jar test gives evidence for a dose change instead of guessing.
- Protect the individual filter limit. If either filter effluent is at or above 1 NTU, remove that filter from service and backwash; if both are below 1 NTU, keep them online while investigating. Do not backwash solely because headloss is 94 inches if the filter is still producing good water; headloss alone is not the permit limit.
- Increase monitoring and check disinfection. Measure filter effluent every 15 minutes, check clearwell chlorine and CT, and document times. High turbidity can shield pathogens, so the operator must know whether disinfection is still adequate before making a flow change.
This order prevents a bad instrument from driving a bad process change, gets evidence quickly, and keeps the plant inside the 1 NTU individual-filter limit while the 0.3 NTU combined limit is evaluated.
2. How would you use jar testing to adjust coagulant dose and filter operations? Give a decision rule.
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Run the jar test at the current 22 mg/L and at stepped doses such as 18, 26, 30, and 34 mg/L, using the plant's rapid-mix, flocculation, and settling times. Measure settled turbidity and observe floc size and settleability. The goal is not the lowest jar turbidity at any cost; it is the lowest dose that reliably keeps settled water low enough that filters can stay below 0.3 NTU combined without short runs.
A practical decision rule: if the best jar dose gives settled turbidity below about 2 NTU and filters remain below 0.3 NTU, change the dose and recheck within one filter run. If settled turbidity cannot be held below about 3 NTU at any reasonable dose, reduce plant flow and consider filter-to-waste if the plant has that capability. If a filter exceeds 1 NTU, take it out of service. Also check coagulant feed calibration, raw pH, alkalinity, and rapid-mix operation, because a failed feed pump can look like a coagulant-demand change. Overdosing can carry floc onto filters and shorten filter runs, so increase dose in steps, not all at once.
3. What are the regulatory and disinfection implications if combined filter effluent exceeds 0.3 NTU for more than 15 minutes?
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Under the Surface Water Treatment Rule, combined filter effluent must be at or below 0.3 NTU in at least 95% of measurements each month, and no individual filter may exceed 1 NTU. A single reading above 0.3 NTU is not automatically a violation, but it is a treatment-performance signal. If an individual filter exceeds 1 NTU, that is a violation and the filter should be removed from service.
The operator should document the event, notify the state if the permit requires it, and evaluate CT. With 1.1 mg/L free chlorine, pH 7.4, 8°C, and about 45 minutes of contact time, the operator must calculate whether the required CT is met. High turbidity matters because particles can shield pathogens from chlorine, so a CT that is adequate on paper may not be adequate in practice. If CT cannot be met, increase chlorine dose carefully or reduce flow, while staying below the maximum residual disinfectant level. The wrong response is to delay reporting or to backwash repeatedly without finding the cause.