Tennessee Wastewater Operator Practice Exam

After turning off the air for 12 hours in a digester with only 1.5% TS, what is the most likely cause for the lack of settling?

1. Low temperature

2. Filamentous bacteria are present

3. Chemical interference

4. Poor mixing

The correct answer is: Filamentous bacteria are present

The presence of filamentous bacteria is often associated with issues in the settling of sludge in a digester. Filamentous bacteria can cause bulking, which prevents proper compaction and settling of the solids. When air is turned off for an extended period, such as 12 hours, anaerobic conditions may promote the growth of these organisms, especially in a digester with a low total solids (TS) concentration like 1.5%. The imbalance created by filamentous growth leads to an increase in the volume of the sludge and impairs the settling characteristics. This phenomenon can be particularly pronounced in systems where filamentous bacteria thrive, as they tend to incorporate water and can create a network that traps more liquid, further exacerbating the lack of settling. In essence, the growth of filamentous bacteria can dominate the microbial community under anaerobic conditions, leading to poor settling characteristics and reduced overall efficiency of the digester. Other potential causes such as low temperature, chemical interference, or poor mixing, while they can affect settling, are not as directly linked to the specific context of anaerobic conditions and the growth dynamics of filamentous organisms in this scenario. Thus, filamentous bacteria are the most likely culprits behind the lack of settling after the air has been