A conventional activated sludge plant is experiencing poor secondary clarifier settling with a rising sludge blanket. Settleometer tests show a fluffy, slow-settling sludge, and the SVI is 180 mL/g. Microscopy of the aeration basin mixed liquor shows abundant filamentous organisms extending from floc. Aeration basin DO is steady at 2.0 mg/L and influent nutrients appear adequate (ammonia-N 25 mg/L, ortho-P 6 mg/L). Which operator action is the most appropriate short-term control response for this condition while you continue to investigate the underlying cause?

Question

Accepted Answer

A) Begin controlled chlorination of the RAS (RAS chlorination) to selectively knock back filaments, and evaluate/adjust F:M (often by increasing wasting to reduce sludge age) to discourage filament dominance Correct

An SVI above 150 mL/g plus poor settling and microscopy showing filaments is classic filamentous bulking. RAS chlorination is a common short-term control because it targets filaments in the return stream and can quickly improve settleability if carefully monitored to avoid harming floc-formers. Adjusting operating conditions that drive filaments—often by correcting a low F:M condition through wasting more (lowering sludge age) or otherwise shifting F:M—helps prevent the problem from recurring.

Answer

B) Feed supplemental nitrogen and phosphorus (for example urea and phosphoric acid) to correct nutrient deficiency and improve settling

Nutrient addition is aimed at true nutrient deficiency (low N or P relative to BOD), which typically shows weak, dispersed floc without a clear filament dominance. In this scenario, nutrients look adequate and the microscope points to filamentous bulking, so blindly adding nutrients is unlikely to correct settling and can create downstream issues (extra ammonia or phosphorus).

Answer

C) Assume a toxic shock is killing biomass; stop returning RAS and route influent to equalization/storage until the upset passes

A toxicity event more commonly produces signs like sudden loss of activity, depressed DO uptake, pin floc/dispersed growth, and deteriorating effluent with stressed or damaged organisms rather than abundant, intact filaments dominating floc structure. Stopping RAS return is an extreme action that can destabilize the process and does not directly address filamentous bulking indicated by the high SVI and microscopy.

Answer

D) Reduce WAS (waste less) to increase MCRT so the sludge has more time to form denser floc and settle better

Wasting less increases MCRT and usually decreases F:M, which can favor filamentous organisms and make bulking worse when filaments are already dominant. While changing F:M can be part of the solution, the typical corrective direction for filament bulking tied to low F:M is the opposite: waste more (or otherwise raise F:M) and/or use targeted RAS chlorination.

SVI Interpretation and Filamentous Bulking Control

Answer Choices

Learn the Formula

Practice More Activated Sludge Questions