Filter Run Collapse on a Rising Head
A conventional rapid sand filter has been running 38 hours. Head loss has climbed from 2.1 ft to 7.4 ft, and effluent turbidity rose from 0.08 NTU to 0.42 NTU over the last four hours. The filter-to-waste valve is available but the clearwell is at 71% and demand is high. The operator's run-termination criteria: backwash at 8 ft head loss, 1.0 NTU effluent, or 100 hours, whichever comes first. The plant superintendent wants to push to the end of the shift, about six more hours, because the other filters are also near their limits and backwashing now means one filter out of service during peak demand.
1. What is happening in this filter, and what should the operator do?
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The filter is in filter breakthrough - turbidity is rising while head loss is also still climbing. That combination is the dangerous one. A filter that reaches terminal head loss without turbidity rise is just loaded up. A filter whose effluent turbidity climbs is passing particles, and those particles are exactly where pathogens hide.
The operator should take the filter out of production now, not in six hours:
- Route to waste if the clearwell can spare it, or shut the filter and backwash.
- Do not wait for the 1.0 NTU criterion - the 0.42 NTU reading is a trend, and breakthrough accelerates. By the time it hits 1.0 the filter has been passing material for hours.
- Backwash per procedure and confirm the backwash is effective (proper bed expansion, adequate rinse). A filter that breaks through at 38 hours may be under-backwashed, media-bound, or short-circuiting.
- After backwash, return to service and watch the first-hour turbidity closely; a filter that repeats this pattern needs media inspection.
The superintendent's concern about clearwell level is real, but the answer is to manage the clearwell with the other filters and storage - not to keep a passing filter online. Finished-water turbidity is a health-based limit, and the exam wants you to prioritize it over production pressure.
2. The clearwell is at 71% and dropping. How do you balance production against the filter problem?
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Treat the clearwell as a time budget. At 71% with known demand, calculate hours of supply remaining at current draw. If you have, say, 10 hours of storage and a backwash plus rinse takes 45 minutes, you can afford to take the filter down now - the arithmetic favors it.
Levers, in order of preference:
- Backwash and return the filter quickly rather than idling it. A well-run backwash plus filter-to-waste rinse is under an hour.
- Stagger the other filters. If they are near their limits, schedule backwashes so only one is down at a time.
- Maximize the remaining filters within their own limits - but do not run any of them past breakthrough.
- Reduce demand-side draw if the system allows (coordinate with distribution on storage use).
- Call for help or notify if storage cannot be maintained.
The wrong move is to let the clearwell number override the turbidity number. Low storage is an inconvenience that recovers; a finished-water turbidity exceedance is a compliance and public-health event. The exam is testing whether you can do the storage math instead of panicking.