Rising Sludge Blanket After a Production Surge
A confectionery plant discharges to a conventional activated sludge plant rated at 1.8 MGD with an 8-hour hydraulic retention time and a 2.5-day sludge age target. Over a long weekend, a syrup spill pushes influent BOD from a normal 900 mg/L to 2,400 mg/L and the flow climbs from 1.1 to 1.6 MGD. By Tuesday the secondary clarifier shows a rising sludge blanket, the SVI has moved from 120 to 260 mL/g, and the mixed liquor dissolved oxygen at the end of the aeration basin is 0.4 mg/L despite both blowers running. Return activated sludge rate is 60 percent of influent flow. Effluent suspended solids are 85 mg/L and rising, and the plant permit limits TSS to 30 mg/L. The operator has one spare blower available.
1. Diagnose the most likely cause of the rising blanket and SVI increase, and state what evidence supports your diagnosis over alternatives.
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The pattern points to an organic overload combined with oxygen limitation, producing both a young sludge age and filamentous bulking.
Evidence for organic overload / low DO:
- Food-to-microorganism ratio has jumped: BOD mass loading roughly doubled to tripled while the biomass inventory is fixed, driving a low sludge age. A young sludge floc is poorly flocculated and compacts slowly.
- End-of-basin DO of 0.4 mg/L with both blowers running is a direct oxygen-transfer shortfall. Low DO selects for filamentous organisms that can outcompete floc-formers at low dissolved oxygen.
- SVI rising from 120 to 260 mL/g is consistent with filamentous bulking, not with a sudden pin floc or denitrification problem.
Why not the alternatives:
- Denitrification in the clarifier would show gas bubbles and a blanket that rises and then settles, often with a clear supernatant; here effluent TSS is climbing steadily and SVI is elevated, which is a settleability problem, not gas entrapment.
- A hydraulic surge alone would dilute rather than concentrate mixed liquor; the BOD spike is the dominant driver.
Conclusion: Organic overload plus low DO is selecting filamentous bacteria and producing a young, poorly settling sludge. Address oxygen transfer and biomass inventory first, then confirm with a filamentous stain and SVI trend.
2. List the immediate operating actions you would take in priority order, and explain the tradeoff in each.
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1. Restore dissolved oxygen. Start the spare blower and verify DO at the end of the basin. Tradeoff: more air costs energy and can raise the blanket if mixing is inadequate, but DO below 1 mg/L will keep selecting filaments. Target 2 mg/L at the basin outlet.
2. Reduce the organic load entering the aeration basin. Coordinate with the industrial discharger to equalize or hold the high-strength flow, and consider flow equalization or a controlled bleed to the headworks. Tradeoff: holding flow risks septic conditions and odors upstream; a controlled dilution may be needed if storage is limited.
3. Increase sludge wasting to rebuild sludge age, but cautiously. The sludge age is too low; however, wasting aggressively during an overload removes biomass you cannot replace quickly. Tradeoff: waste enough to control SVI and maintain the target sludge age, but avoid dropping mixed liquor suspended solids below the level needed to treat the load.
4. Optimize the return activated sludge rate. At 60 percent, the return may be too low to keep the blanket down; a controlled increase can help, but excessive return shortens detention and can worsen the blanket by recycling filaments. Tradeoff: raise return in steps and watch the blanket depth.
5. Protect the effluent. If TSS continues to climb, consider a temporary polymer feed to the clarifier and increased monitoring frequency. Tradeoff: polymer adds cost and can mask the underlying problem, so use it as a bridge while the biological fix takes effect.
3. How would you confirm the diagnosis and verify recovery over the next two weeks?
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Confirmation within 24-48 hours:
- Run a filamentous bacteria stain on mixed liquor and note filament abundance and type. High filament counts with low DO support the diagnosis.
- Check the sludge volume index and settled sludge volume daily, plus a 30-minute settleability test to see whether the blanket is improving.
- Measure mixed liquor suspended solids and volatile suspended solids to compute the actual sludge age and food-to-microorganism ratio.
- Profile DO and ammonia along the basin to confirm oxygen demand is being met and nitrification is not collapsing.
Recovery indicators over two weeks:
- SVI trending down toward 150 mL/g or lower.
- Blanket depth stable and effluent TSS returning below the 30 mg/L limit.
- DO holding at 2 mg/L at the basin outlet with normal blower operation.
- Sludge age back to the 2.5-day target and a stable mixed liquor concentration.
Verification of the fix:
- Recheck the influent BOD and flow trend to confirm the spill has passed.
- Confirm the discharger has re-established equalization or pretreatment so the surge does not recur.
- Document the event, the actions taken, and the results for the permit file and for the next upset response.