Tennessee Wastewater Operator Practice Exam

If the MLSS concentration gets too low, what condition may result?

1. Bulking sludge

2. Straggler floc

3. Floating scum

4. Foaming at the surface

The correct answer is: Straggler floc

When the Mixed Liquor Suspended Solids (MLSS) concentration is too low, straggler floc may occur. Straggler floc consists of small, weakly bonded particles that do not settle effectively during the clarifying phase of wastewater treatment. The reduction in MLSS can lead to a poor environment for effective floc formation and aggregation, which results in these smaller, less cohesive flocs that are prone to remaining in suspension. This situation complicates the separation process in the clarifier, as these particles may be carried over with the effluent instead of being settled out. The other conditions mentioned, such as bulking sludge, floating scum, and foaming at the surface, are typically associated with imbalances in microbial populations or undesirable operating conditions but are not direct consequences of low MLSS. Bulking sludge usually arises from an excess of filamentous organisms that prevent proper settling. Floating scum is often linked to the presence of oils or fats that cause solids to rise, while foaming is associated with certain types of bacteria that produce excess surfactants. Thus, the unique implication of insufficient MLSS specifically results in the creation of straggler floc.