Storm-Driven Turbidity Spike With Two Filters Down
A 4.0 MGD conventional surface-water plant draws from a river that rose six feet after two days of rain. Raw turbidity climbed from 5-8 NTU to 95-115 NTU; raw pH 7.5, alkalinity 88 mg/L as CaCO3. The alum dose is still 26 mg/L with a polymer filter aid. Settled turbidity is now 3.5 NTU, up from 0.8 NTU. Two of the four 350-ft2 filters are out of service for media replacement, so the two on line are running near 3.7 gpm/ft2 and filter runs have collapsed from 55 hours to 14. Combined filter effluent turbidity was 0.12 NTU yesterday and reads 0.31 NTU this hour. Filter 2's individual effluent just came back at 1.4 NTU on a grab sample. Clearwell free chlorine is 1.0 mg/L with 25 minutes of contact time ahead of the first customer, and the plant is producing 3.7 MGD. The lab can run one more jar test today.
1. State, in priority order, what you do in the next two hours and defend the ordering.
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Pathogen protection outranks production, so the filter result comes first. Filter 2 at 1.4 NTU exceeds the 1.0 NTU individual filter effluent limit and must come off line, with its water wasted or recycled to the head of the plant, not sent to the clearwell. That leaves one filter, so you cannot hold 3.7 MGD: cut plant flow to what a single filter can carry at a loading rate you can defend, and make up the difference from clearwell storage or an interconnect to protect distribution residual.
Next, fix the chemistry. Settled turbidity of 3.5 NTU against 95-115 NTU raw means the coagulant dose is far too low for the particle load; run the remaining jar test across an alum dose range and a pH range before the next shift, and check that the polymer is not blinding the remaining filter. Then recheck CT, because CT is residual times contact time and the required value rises as water gets colder and pH rises; if you cannot hold credited CT at reduced flow and 1.0 mg/L, lower flow further rather than trusting the residual. Document every 15-minute turbidity reading and every decision, because the record is what shows whether the filter was taken off line promptly.
2. The jar test shows 60 mg/L alum gives 0.6 NTU settled water but drops pH to 6.4. Do you take that dose? What else must you adjust?
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The 60 mg/L dose is the right direction but not a finished answer. Going from 26 to 60 mg/L alum consumes roughly 34 mg/L of alkalinity (about 0.45 mg/L as CaCO3 per mg/L of alum), which would take 88 mg/L down to the low 50s and push pH toward 6.4. That is a corrosion-control problem downstream even though the settled water looks good. Confirm the jar test used the actual raw water and the actual polymer dose, then plan a caustic or lime feed to hold finished pH in the range your corrosion-control plan specifies.
Also weigh the trade-offs the dose creates: better sweep floc and lower settled turbidity, but more solids to the remaining filters, shorter runs, and more backwash water you may not have capacity to treat. Check that the higher dose does not carry alum post-precipitation into the clearwell at low pH. If the plant cannot hold pH and turbidity at once, blending or reducing production is safer than chasing a jar-test number. Record the jar-test conditions so the next shift can repeat them, and verify the dose change with 15-minute settled and filtered turbidity, not a single grab.
3. At 10:00 the remaining filter's effluent reaches 1.1 NTU and the clearwell is drawn down 4 feet. What are your options, and what triggers notification?
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You are now out of filter capacity, so the honest choices are: reduce flow until the last filter holds under 1.0 NTU, take finished water from storage or an interconnect, or shut down and buy water. Producing water you cannot filter is not on the list. If the last filter must come off line, the plant stops sending water to the clearwell and the system runs on storage while you restore capacity.
Finished-water turbidity above 1 NTU, or a filter that cannot be returned to compliance, is a treatment-technique failure and a public-notification event; call your compliance contact and Ohio EPA the same day and follow the Tier 1 notice requirements in the rule rather than waiting for the monthly report. Document the 15-minute readings that show when the filter crossed the limit, the time it was removed from service, and the volume of water produced between the exceedance and the shutdown. Notify the distribution side and any wholesale customers so they can manage storage and pressure.