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ABC Class II Water Treatment study materials
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Filter 2 Losing Media After the Water Warms Up

ABC Class II Water Treatment · Put your knowledge to work in context.

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Filter 2 Losing Media After the Water Warms Up

Filter 2 at a 2.4 MGD plant is 320 ft2, designed for 24 in of dual media and 55-hour runs at about 5 gpm/ft2. Over three weeks its runs have fallen to 21 hours, and effluent turbidity breaks through at 0.30 NTU by hour 18 while headloss is only 4.5 ft against an 8-ft terminal setpoint. Backwash is set at 15 gpm/ft2 for 8 minutes, the same setting used all winter. Influent water temperature is now 24 C; it was 6 C in February. The operator measured only 18 in of media remaining and saw media in the wash troughs during the last backwash. Filters 1, 3, and 4, which share the same backwash header, are running 48 to 52 hours.

1. What is the most likely cause of Filter 2's decline, and which observations support it versus rule out competing explanations?

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Most likely cause: the backwash rate is over-expanding the bed now that the water is warm, and media is being washed into the troughs.

Supporting evidence:

  • Backwash flow = 15 gpm/ft2 x 320 ft2 = 4,800 gpm. That rate was tuned for 6 C water. Cold water is more viscous and expands less; at 24 C the same rate expands the bed far more, and 40-50% expansion will carry media over the trough lip. Media in the troughs is direct confirmation, not inference.
  • Media depth of 18 in against 24 in design means roughly 25% of the bed is gone.
  • Temperature is the only thing that changed on a schedule matching the symptom onset.

What the data rule out:

  • Coagulation failure would hit all four filters, since they receive the same settled water. Filters 1, 3, and 4 are running 48-52 hours, so the coagulant is fine.
  • Channeling or mudballs can shorten runs, but they do not put media in the troughs.

The signature is a bed that is too thin to do its job.

2. Why does this filter show turbidity breakthrough with headloss only at 4.5 ft, when a normal run ends on terminal headloss?

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Headloss and turbidity breakthrough are two different failure modes, and this filter is failing on the wrong one.

In a healthy deep bed, particles are captured in the top few inches and form a cake. Headloss builds across that cake and reaches the 8-ft terminal value before particles can travel all the way through the bed - that is the design intent, and it is why you terminate the run on headloss.

With 6 in of media missing:

  1. There is less depth for the remaining solids to travel through, so the penetration front reaches the bottom of the bed while there is still plenty of unused headloss capacity above it.
  2. The surface cake is thinner and forms more slowly, so the 8-ft terminal value is never approached.
  3. Media loss also disturbs the support gravel and underdrain interface, which can create localized higher-velocity paths that let floc straight through.

So 4.5 ft of headloss with 0.30 NTU effluent is not "the filter is still clean" - it is a short-circuit. Treat turbidity breakthrough as a stop-run condition regardless of headloss. This filter should be on filter-to-waste or out of service.

3. List your immediate and long-term corrective actions, including how you would set the backwash rate going forward.

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Immediate (this week):

  1. Take Filter 2 out of service, or run it to waste if supply requires it. Do not put 0.30 NTU water into the clearwell.
  2. Run a bed expansion test on a good filter: start low and increase backwash flow until the bed is 20-25% expanded, measured with a probe against the media surface, and record the rate and the water temperature. Do this at several temperatures to build a temperature-adjusted setpoint. At 24 C expect it to land well below 15 gpm/ft2 - roughly 10-12 gpm/ft2 - but the expansion measurement is the authority, not the number.
  3. Re-set all four filters to the temperature-adjusted rate and verify no media is reaching the troughs on the next backwash.
  4. Inspect Filter 2 for disturbed gravel, mudballs, and underdrain damage before returning it to service.

Long-term:

  1. Replace media to the 24-in design depth and re-check after placement.
  2. Add a backwash flow totalizer and a written expansion-check procedure in the SOP, keyed to water temperature.
  3. Trend run length and backwash water use per filter; a sudden increase in backwash volume is an early media-loss indicator.
  4. Never restart a filter without filter-to-waste, and confirm turbidimeter calibration.