Anthracite Loss and Turbidity Spike After Cold-Water Backwash
A 6-MGD conventional surface-water plant has six dual-media filters, each with 18 in of anthracite over 12 in of sand. Backwash is water-only at 18 gpm/sf for 10 minutes, followed by a 5-minute filter-to-waste. Raw water is 6°C in January. After a backwash, Filter 3 effluent turbidity rises to 0.9 NTU and slowly falls to 0.12 NTU over 40 minutes; the other filters are at 0.06–0.08 NTU. The operator sees anthracite particles in the wash-water trough and a small mound of media near the trough lip. Filter 3 headloss is lower than normal after the backwash, and its run length has dropped from 30 hours to 18 hours. Backwash flow is set by a fixed-speed pump and has not changed in two years. The filter has not been opened for media inspection in 14 months. Finished-water turbidity remains below 0.3 NTU, but the state requires individual filter effluent turbidity and reporting of media loss.
1. What is the most likely cause of the media loss and turbidity spike, and what data would confirm it?
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Most likely cause: excessive backwash bed expansion and anthracite carryover. At 6°C water is more viscous, so the same 18 gpm/sf produces greater expansion than at warm temperature; anthracite is lighter and can be washed into the troughs. The media mound at the trough lip, anthracite in the wash water, lower headloss, and shorter runs after backwash support this. Confirm by measuring bed expansion during backwash (target about 15–20% for anthracite), checking backwash rate with a calibrated flow meter, measuring media depths and freeboard, and inspecting for a stuck-open valve or a backwash sequence timer fault. Also compare expansion on a warm day; if expansion exceeds the trough elevation, media loss is expected.
2. What immediate changes would you make to backwash and filter operation?
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Immediate: reduce the backwash rate in steps until expansion is 15–20%, not a fixed 18 gpm/sf; use a temperature-compensated backwash curve or lower the rate in cold water. Verify the rate with a pitot or magnetic meter, not the pump curve. Check that the backwash valve closes fully and that the sequence does not overrun. Keep Filter 3 on filter-to-waste until effluent turbidity is below the utility's target (e.g., 0.1 NTU) and stable. Do not return it to service at 0.9 NTU. Inspect the bed before restarting; if anthracite depth is below design, add clean anthracite and check the support gravel and nozzles. Notify the state of media loss as required.
3. What follow-up inspection and reporting are required?
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Follow-up: open the filter, measure anthracite and sand depths at grid points, check for gravel disturbance, nozzle damage, and trough elevation; repair and record. Recalibrate the backwash flow meter and update the SOP to include temperature correction and expansion measurement. Add a backwash turbidity/flow trend alarm. Continue individual filter effluent turbidity monitoring and report the media loss and corrective action to the state. Recheck run length and headloss after media replacement; if runs remain short, investigate coagulation and filter loading.