Which of the following is a common cause of bulking in activated sludge basins?

• A. Filamentous bacteria and imbalanced F/M ratio with low shear
• B. Rapid settling
• C. Excessive mixing
• D. Low SVI and clear effluent

Answer: (A)

Bulking happens when filamentous bacteria grow too much in the sludge, creating a loose, fluffy structure that doesn’t settle well. When the mixing energy is low, those filaments aren’t broken up as they should be, so they elongate and form networks that trap water and resist compaction. An imbalanced food-to-microorganism ratio tends to favor these filamentous organisms over the normal, floc-forming populations, so the sludge becomes difficult to settle. The combination of filamentous growth and low shear leads to a high sludge volume index and poor effluent clarity, which is the hallmark of bulking.

In contrast, rapid settling would indicate dense, well-formed flocs; excessive mixing increases shear and tends to break apart filaments, not encourage bulking; and a low SVI with a clear effluent signals good settling, not bulking.