Filter 3 Turbidity Climb After a Storm

Diagnosis. A young run (11 hours) with low headloss (2.1 ft) but rapidly rising effluent turbidity points to particle pass-through, not a loaded filter. The storm drove raw turbidity up 5.5x and dropped pH; that changes both coagulant demand and floc character, while the alum stroke stayed fixed. The most probable cause is coagulant underfeed producing small, weak floc that shears in the filters and penetrates the media. Media loss or a turbidimeter fault are the secondary hypotheses and are quick to check.

Actions, in order:

1. Verify the coagulant is actually being fed - rotameter, drawdown test, and pump stroke versus the setpoint.
2. Pull and verify the Filter 3 turbidimeter: check the calibration/standard and cross-check with a second instrument. Do not act on a number you do not trust.
3. Run a jar test immediately on the current raw water; adjust alum dose and, if needed, pH correction to rebuild floc.
4. Take Filter 3 out of service rather than let it keep rising - a filter passing floc is worse than one filter down.
5. Notify the supervisor and record the time, readings, and actions; check whether Filter 4 or the others are trending the same way.
6. When the coagulant is corrected and turbidity is stable, return the filter using filter-to-waste until effluent is below the plant's target.

With four filters: 9.4 MGD ÷ 4 = 2.35 MGD per filter. 2.35 MGD × 694.4 gpm/MGD = 1,632 gpm; 1,632 gpm ÷ 700 ft² = 2.33 gpm/ft².

With three filters: 9.4 MGD ÷ 3 = 3.13 MGD per filter. 3.13 × 694.4 = 2,176 gpm; 2,176 ÷ 700 = 3.11 gpm/ft².

So removing Filter 3 raises loading about 33%, from 2.33 to 3.11 gpm/ft². That is still inside the typical 2–4 gpm/ft² range for a conventional plant, but it is a real stress when the floc is already poor.

Judgment: yes, remove it, but treat it as a temporary, monitored condition:

• Confirm no other filter is within an hour of backwashing, because losing a second filter pushes the remaining two to ~4.7 gpm/ft², which is unacceptable.
• Watch headloss rate and effluent turbidity on the three remaining filters at 15-minute intervals; if any of them starts climbing, cut plant flow in coordination with distribution rather than risk a second pass-through event.
• Account for backwash return flow and any hydraulic surge when the plant returns to four filters.

No violation yet, but the margin is thin.

Limits that apply to a conventional plant:

• Combined filter effluent (CFE): must be ≤ 0.3 NTU in at least 95% of measurements each month and may never exceed 1.0 NTU. At 0.24 NTU the CFE is compliant, but it is the number that binds, and it has been drifting up.
• Individual filter effluent (IFE): the regulatory trigger is not 0.3 NTU. It is a sustained excursion - commonly two consecutive 15-minute readings above 1.0 NTU - which requires a filter self-assessment. Filter 3 at 0.45 NTU has not reached that trigger.
• Monthly 95% calculation: CFE readings are taken every 15 minutes, roughly 2,880 per month. The 5% allowance is a cushion for excursions, not an operating target.

Judgment: compliance is not the question at 13:20; the trend is. A filter that climbs from 0.07 to 0.45 NTU in 25 minutes and is still rising will likely break 1.0 NTU within the hour if the coagulant is not fixed. Act on the operational trend now, and confirm with the primacy agency whether a disinfectant CT credit is at risk if turbidity climbs further - CT credit depends on filtered water quality, and that is a compliance exposure independent of the turbidity limit itself.