Filter Run Shortened by Coagulant Change
A conventional surface-water plant treats a river source whose turbidity swings between 4 and 60 NTU after storms. The operator switched from alum to a pre-hydrolyzed PACl product to reduce alkalinity consumption. Design flow is 2.4 MGD; the four dual-media filters normally run 36–48 hours between backwashes. Within a week of the switch, filter runs fell to 14–16 hours, finished turbidity rose from 0.08 to 0.22 NTU, and the clearwell chlorine demand increased. Jar tests at the bench show good floc at the current dose, but full-scale filter influent turbidity is 1.5–2.0 NTU versus 0.6 NTU before. The plant has no online particle counter, only grab turbidimeters at filter effluent. The superintendent wants to increase backwash frequency to restore production and defer any chemical changes until after the summer demand peak.
1. Identify the most likely cause of the shortened filter runs and explain why increasing backwash frequency alone is the wrong first response.
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The pattern points to floc that is small, weak, or poorly filtered rather than to a solids-loading problem: bench jar tests look good, but full-scale filter influent turbidity nearly tripled and effluent turbidity rose. PACl is sensitive to dose and mixing energy. If the dose was carried over from the alum program, or if rapid-mix/flocculation energy was not re-optimized, the result is a floc that passes into the filters and blinds the media surface quickly.
Shortened runs plus higher effluent turbidity means the filters are being loaded with material they cannot hold or capture, not that they are simply getting dirtier faster. Increasing backwash frequency treats the symptom: it consumes more treated water, stresses media (especially if bed expansion is marginal), and does nothing about the floc reaching the filters. Worse, it masks the underlying coagulation problem and allows higher-turbidity water to reach the clearwell, which explains the rising chlorine demand. The correct first step is to confirm and correct the coagulation chemistry (dose, pH, mixing) before touching backwash schedules.
2. List the specific data you would collect in the next 24 hours to confirm the diagnosis before making changes.
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Collect and trend, side by side with the pre-switch baseline:
- Raw and settled turbidity at the same times each day, plus raw pH, alkalinity, and temperature.
- Coagulant dose actually applied (lb/day and mg/L) and feed-pump calibration, to rule out a dosing error.
- Settled-water turbidity, not just raw and filtered; this separates coagulation failure from filter failure.
- Filter influent turbidity and headloss at the start and end of each run, and time-to-turbidity-breakthrough versus time-to-headloss-limit.
- Backwash data: rate, bed expansion, duration, and post-backwash initial turbidity (filter-to-waste or slow start).
- Rapid-mix and flocculation energy or velocity gradient, if measurable, and any change in mixing since the switch.
- Chlorine demand across the clearwell and filtered-water pH. The key discriminator is settled-water turbidity: if it is elevated, the problem is upstream coagulation; if it is normal while filter influent is high, look at filter hydraulics or media.
3. Draft the recommendation you would give the superintendent, including a defensible interim operating strategy.
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Recommend re-optimizing coagulation now rather than deferring, and frame it as a compliance and capacity issue, not just a production one. Interim strategy:
- Run jar tests and a short full-scale step-dose trial of PACl to find the dose that returns settled turbidity to baseline; adjust rapid-mix energy if the product calls for it.
- Hold filter loading down by reducing filtration rate if possible and by keeping filters online only while effluent turbidity is acceptable.
- Increase monitoring: grab filter effluent turbidity at least every 2 hours and after each backwash, and add settled-water turbidity to the daily log.
- Do not simply raise backwash frequency; if you must run more backwashes to keep production, document it and watch bed expansion and media loss.
- Notify the primacy agency if finished turbidity approaches the regulatory limit, and document the corrective action timeline. The message to the superintendent: deferring the chemistry fix risks exceeding the turbidity limit during the peak, when the plant can least afford it.