Anthracite in the Wash Troughs and a Sudden Turbidity Breakthrough
Clear Creek Water Plant (6.0 MGD design) filters through four dual-media filters, each 480 ft² with 20 inches of anthracite over 12 inches of sand, rated at 5 gpm/ft². Filters run to 8 feet of headloss or 72 hours, whichever comes first. Two days ago, during a 90°F afternoon at peak demand of 5.6 MGD, the operator backwashed Filter 4 at 20 gpm/ft² for 12 minutes and saw anthracite floating over the wash trough weirs. The plant stayed at 5.6 MGD. This morning, Filter 2's effluent turbidity climbed from 0.06 NTU to 1.1 NTU in about 25 minutes while its headloss gauge read only 4.2 feet. The water level in the filter box is 8 inches above the media surface. Clearwell turbidity is 0.14 NTU. The operator wants to let Filter 2 keep running because headloss is still low and demand is high.
1. What is the most likely explanation for Filter 2's behavior, and what evidence supports and contradicts it?
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Most likely: lost or redistributed media, not air binding.
Evidence for media loss/redistribution:
- Anthracite was observed floating over the troughs two days ago, which is direct evidence the backwash expanded the bed far beyond design. Water at 90°F is much less viscous than at 40°F, so the same 20 gpm/ft² produces far more expansion in summer; 20 gpm/ft² is at the high end even for cold water.
- Headloss is only 4.2 feet but effluent turbidity broke through in 25 minutes. A shallow bed (or a bed with sand pulled up into the anthracite and anthracite pushed to one side) has little depth to trap particles: it fouls little, shows low headloss, and passes turbidity almost immediately.
- The failure is sudden, not gradual, which fits a disturbed bed rather than simple loading.
Evidence against air binding:
- Air binding produces high headloss and a negative pressure (vacuum) in the bed near the end of a run as the driving head exceeds the water depth above the media, then a sudden turbidity release. Here headloss is low, and the filter box is 8 inches above the media, so a deep vacuum is unlikely at 4.2 feet of headloss.
Confirm by: measuring media depth with a core sampler across the bed at several points on all four filters; probing for sand boils, mudballs, and cracks; checking backwash rate and expansion records; and reviewing whether the flow was pushed above design during the peak.
2. What do you do with Filter 2 right now? Walk through the operational and compliance steps.
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Immediately:
- Take Filter 2 out of service, or place it on filter-to-waste if the plant is piped for it. Never deliberately send 1.1 NTU water to the clearwell, even at peak demand.
- Start the standby capacity or reduce plant output to stay within the rated filtration rate of the remaining filters. Running three filters at 5.6 MGD means about 6.5 gpm/ft², well above the design 5 gpm/ft² - that is how you turn one problem into four.
- Pull a grab sample to verify the turbidimeter; check the instrument calibration and the sample line.
- Check the other three filters for the same pattern (low headloss plus rising effluent turbidity), and increase effluent monitoring while the raw water and the beds are unsettled.
- Confirm clearwell turbidity and chlorine residual; the 0.14 NTU clearwell reading suggests dilution and that the excursion was brief, but verify.
Compliance and records:
- A filter effluent reading above 1 NTU is an MCL exceedance; document the reading, the time, the corrective action, and the clearwell turbidity at the time.
- Notify the Kentucky Division of Water per your permit.
- Anticipate a Tier 2 public notice (generally due within 30 days) if the exceedance is confirmed; verify the tier with the state.
- Return Filter 2 to service only after turbidity is stable and clearly below the 0.3 NTU combined-effluent target, and after the bed has been inspected and rebuilt if needed.
3. What follow-up work do you schedule in the next two weeks, and what happens if you do not fix the backwash?
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Backwash practice.
- Reduce the rate to produce 20–25% bed expansion, measured directly with a clear expansion tube or by marking the media line on the wall, not by watching for media in the troughs. Warm water needs a lower rate than cold water for the same expansion - write a temperature-based rate table and post it.
- Consider air scour plus low-rate water wash, and always finish with a proper rinse before returning to service.
- Stop backwashing during peak demand; the temptation to rush the wash leads to shortcuts.
Inspection and records.
- Core-sample media depth at multiple points on all four filters. If anthracite is below 20 inches, or sand has migrated into the anthracite layer, plan media replacement or restratification.
- Inspect underdrains and nozzles for damage and check for media in the wash-water troughs, which indicates the gravel or support layer is also compromised.
- Review turbidity records for the trigger: repeated filter effluent above 1 NTU in consecutive months can require a filter assessment or a comprehensive performance evaluation.
If you do not act: each backwash removes more anthracite, all four filters drift toward shallow beds, runs get shorter, more water is used for washing, and turbidity breakthrough becomes routine. Pathogen passage through a compromised filter is the outcome that matters, and it will not announce itself with high headloss.