Which of the following describes common indicators of bulking and a typical cause?

• A. Clear effluent with rapid settling
• B. Poor settling with high SVI and cloudy effluent; commonly caused by filamentous bacteria or imbalanced food-to-microorganism ratio and low shear
• C. No solids present in the effluent
• D. Reduced aeration

Answer: (B)

Bulking shows up when the sludge blanket settles poorly, so you get a cloudy effluent and a high Sludge Volume Index. The Sludge Volume Index measures how much settled sludge occupies a given volume after settling for a set time; a high value means the sludge is bulky and doesn’t compact well, which is exactly what happens during bulking.

The typical cause is an overgrowth of filamentous bacteria that extend through the sludge and prevent dense packing of the solids. Conditions that favor this include an imbalanced food-to-microorganism ratio and low shear in the aeration basin. Too much substrate relative to biomass or insufficient shear allows these filaments to dominate, leading to poor settling and cloudy effluent.

Other scenarios described by the alternatives aren’t characteristic of bulking: clear effluent with rapid settling indicates good settling performance; no solids present in the effluent isn’t a bulking pattern, and reduced aeration can worsen treatment but isn’t the standard indicator of bulking itself.