A predominance of filaments in an activated sludge plant indicates what condition?

A predominance of filaments in an activated sludge plant is often associated with poor settling of the sludge. When filamentous bacteria become dominant in the biomass, they can create a network of long strands that interferes with the normal settling properties of the mixed liquor. This condition can lead to issues such as scum formation, increased solids in the effluent, and difficulties during the sedimentation process in secondary clarifiers.

Filamentous growth typically indicates imbalances in the treatment process, such as low dissolved oxygen levels, nutrient deficiencies, or environmental stressors. While excessive aeration or high nutrient levels may contribute to filamentous growth under certain circumstances, it is fundamentally the mechanical settling issues that characterize the challenges presented by filamentous organisms.

Therefore, the presence of a high number of filaments suggests that the activated sludge system is likely experiencing problems related to the settling of the biomass, indicating the correct choice is related to poor settling.