Rising Sludge Blanket and Filamentous Bulking

Leading candidate: low-F/M filamentous bulking driven by septic, high-sulfide influent, compounded by low DO

Evidence for:

• F/M 0.06 is well into the low range, which favors filamentous organisms that can scavenge substrate at low concentrations over floc-formers.
• DO 0.8–1.2 mg/L in the basin gives filaments a further competitive edge; many filamentous types (e.g., Sphaerotilus, Type 1701, Microthrix) tolerate or exploit low DO better than floc-formers.
• Sulfide at the headworks is the smoking gun for septicity. Long force mains generate sulfide and readily biodegradable short-chain fatty acids; those favor filaments such as Type 021N and Thiothrix. The new rendering discharger is a plausible additional source of high-strength, easily degradable, nutrient-poor material.
• SVI 290 with a rising blanket and rising effluent TSS is the classic slow-settling signature.

Evidence against other causes:

• No indication of a sudden toxic slug (that would show a drop in MLSS or oxygen uptake, not bulking).
• Nothing in the data points to a simple hydraulic overload of the clarifier - the blanket rose while flow presumably held steady.

Still open: nutrient deficiency (rendering waste can be high in BOD with limited N and P) and a possible low pH in the aeration basin.

• Filament staining and identification on mixed liquor: confirming which filament is present separates low-DO/low-F/M bulking from nutrient-deficiency bulking. The remedy differs.
• Influent and aeration basin N and P with the soluble BOD: if the rendering waste is pushing the BOD:N:P ratio well above roughly 100:5:1, nutrient deficiency is in play.
• Sulfide/sulfate and pH at the headworks and at the rendering discharge point, plus a sampling of that discharge for BOD, COD, pH and nutrients.
• DO profile across the basin - measure at several points and depths, not just one probe; a single reading can hide a low-DO zone.
• Sludge quality trends: SVI, settled sludge volume, RAS and WAS rates, and a check that the F/M calculation itself is right (MLSS basis, correct influent BOD and flow).
• Clarifier hydraulics: blanket depth at several points, solids loading rate, and whether the return rate is adequate to keep up with the blanket.

Immediate

• Raise basin DO to 2.0 mg/L and hold it; this is the cheapest lever and helps immediately.
• Control the sulfide problem upstream: dose nitrate or oxidant in the force mains, or pre-aerate/oxidize at the headworks to knock down sulfide and septicity before it reaches the biology.
• Adjust wasting to raise F/M out of the 0.06 range - do not simply cut RAS to lower the blanket; that pushes solids over the weirs.
• If nutrient deficiency is confirmed, add N and P to the aeration basin.
• If the blanket threatens the permit, temporarily reduce flow or use polymer/ballasting, and increase effluent monitoring.

Structural

• Write enforceable discharge limits and monitoring for the rendering discharger (BOD, sulfide, pH, nutrients, slug control), with a right to sample and to refuse loads.
• Address collection-system septicity permanently - force main cleaning, reduced detention, or continuous odor/sulfide control.
• Review the aeration capacity against the actual oxygen demand including the new industrial load; 0.8–1.2 mg/L may simply mean the blowers no longer match the load.
• Re-baseline the F/M and SRT control strategy so the plant is operated on measured solids, not on habit.