Filamentous Bulking at a Low-F/M Activated Sludge Plant
A 2.0-MGD extended-aeration activated-sludge plant receives 1.8 MGD of municipal wastewater with influent BOD5 210 mg/L and TSS 220 mg/L. The aeration basin volume is 1.4 MG, MLSS is 4,500 mg/L, and dissolved oxygen in the aeration basin is 1.2 mg/L. The final clarifier has a rising sludge blanket, and effluent TSS is 38 mg/L against a permit limit of 30 mg/L. SVI is 260 mL/g, and microscopy shows abundant filamentous bacteria (filament index 4 of 5). RAS flow is 0.7 times influent flow, and waste activated sludge is 8,000 gal/day at 1.0% solids. The plant has no primary clarifiers, influent pH is 7.1, and effluent ammonia is 1.5 mg/L. The operator wants to add polymer to the clarifier to stop the carryover. The collection system has no known industrial dischargers, but septic conditions occur in a long force main during low flow.
1. Calculate the approximate F/M and interpret the bulking data. What is the likely cause?
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Approximate F/M: BOD load = 1.8 MGD × 210 mg/L × 8.34 = 3,150 lb/day. MLSS inventory = 4,500 mg/L × 8.34 × 1.4 MG = 52,500 lb. F/M ≈ 0.06 lb BOD/lb MLSS-day, which is very low. The SVI of 260 mL/g and filament index 4/5 confirm filamentous bulking. Low F/M favors filamentous bacteria that can scavenge low substrate. The long force main and septic conditions may also select filaments such as Type 021N, and I would check for nutrient deficiency (N/P) and low DO. The proposed polymer would only improve settling temporarily and could increase sludge production; it does not correct the cause.
2. Evaluate the proposed polymer addition and recommend operational changes.
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I would not rely on polymer as the primary fix. Increase wasting to raise F/M and lower MLSS, but do it gradually so the ammonia limit is protected. For example, increasing WAS from 8,000 gal/day at 1% will reduce SRT and MLSS; calculate the target SRT (e.g., 10–15 days) and waste accordingly. Maintain DO at 2 mg/L in the aeration basin, check RAS rate to return solids without overloading the clarifier, and prevent septic conditions in the force main (aeration, sulfide control, or flushing). Check influent N and P; if deficient, add nutrients. Polymer can be used briefly as a bridge, but monitor effluent TSS and not as a substitute for SRT control.
3. What monitoring plan would you use to confirm improvement and protect the ammonia limit?
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Monitor daily: MLSS, SVI, DO, effluent TSS, ammonia, and clarifier blanket; twice weekly: microscopic filament index and F/M; weekly: influent BOD/TSS, N/P, and force-main sulfide. Set triggers: if SVI remains >150 after SRT correction, investigate filaments; if ammonia rises above permit, reduce wasting. Confirm improvement by a falling SVI, a stable blanket, and effluent TSS <30 mg/L. Keep the state informed if there is a permit exceedance.