Filter Breakthrough After a Storm
A 12 MGD conventional surface-water plant treats a river source with alum and polymer, settling, and six dual-media filters. At 06:00 the raw turbidity is 9 NTU; by 14:00 it is 48 NTU after 1.8 inches of rain, and the finished-water turbidity has risen from 0.08 to 0.37 NTU. Filter runs have fallen from 36 hours to 11 hours, and one filter is passing floc. Jar tests at the current 18 mg/L alum dose show settled turbidity of 6.5 NTU; at 26 mg/L it is 2.1 NTU, but the plant's caustic feed is limited and finished pH is already 7.1 with alkalinity 48 mg/L. The clearwell has 1.1 MG in storage and the distribution system demands 8.5 MGD. The shift operator wants to increase alum, backwash all filters, and keep production at full rate. The primary clarifier sludge blanket is rising. The plant has no automatic filter-to-waste valve and the state primacy agency requires notification of treatment technique violations.
1. What immediate treatment changes would you make, in what order, and what would you monitor to confirm they are working?
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First protect the filters and finished water, not just chase raw turbidity. I would stop or throttle the filter that is passing floc, place it on filter-to-waste if piping allows, and reduce plant rate until settled water is stable. Next, run a confirmatory jar test and raise alum toward 26 mg/L in steps, with polymer optimized, because settled turbidity at 18 mg/L is unacceptable for a 48 NTU raw load. Watch rapid-mix energy and flocculation; overdoing polymer can blind filters. The caustic limit matters: more alum consumes alkalinity and depresses pH, so I would monitor pH and alkalinity every 2 hours and adjust caustic within its capacity. Backwash filters based on headloss and turbidity, not all at once, to avoid washing away media or overwhelming the clearwell. Confirm success with settled turbidity, individual filter effluent turbidity, particle counts if available, filter run length, and finished turbidity below 0.3 NTU. If finished turbidity stays high, increase monitoring and notify the primacy agency under the treatment-technique rule.
2. Should the plant continue at 12 MGD? Explain how storage, filter loading, and clarifier sludge blanket affect your decision.
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Not automatically. The plant may have hydraulic capacity, but effective treatment capacity falls when raw turbidity jumps from 9 to 48 NTU and filter runs drop from 36 to 11 hours. At 12 MGD, six filters are loaded heavily while one is passing floc; that risks finished water above 0.3 NTU and short-run breakthrough. Storage is 1.1 MG against 8.5 MGD demand, roughly a 3-hour buffer, but storage only helps if the plant can refill it later. I would reduce production to about 9 to 10 MGD, match demand, and use storage cautiously while filters recover. The rising clarifier sludge blanket means solids are carrying over; I would waste sludge, check blanket depth, and avoid shocking the clarifier with a high rate. If filter-to-waste is manual, I would assign an operator to manage it. Return to full rate only after settled turbidity, filter effluent, and run length are stable for several hours.
3. What public and regulatory notifications would you consider if turbidity remains above 0.3 NTU for more than a few hours?
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I would distinguish a brief upset from a treatment-technique violation. Finished water above 0.3 NTU is a red flag, but the rule is based on the percentage of monthly samples and individual filter effluent limits; a few hours is not automatically a violation. I would notify the state primacy agency promptly because the plant has a treatment problem and the agency may require increased monitoring or an assessment. I would document times, values, filter runs, doses, and corrective actions. I would not issue a boil-water notice for turbidity alone unless the agency directs it or there is evidence of pathogen breakthrough, because turbidity is an indicator, not a direct acute contaminant. I would prepare internal talking points for customers and coordinate with the agency before any public notice. If high turbidity persists or the agency declares a violation, I would follow its public-notice tier and timing.