Tennessee Wastewater Operator Practice Exam

What will happen if a secondary clarifier is already overloaded with solids and the RAS flowrate is turned up?

1. Decrease the blanket depth

2. Increase the blanket depth

3. Strain the RAS line

4. Reduce the settling time

The correct answer is: Increase the blanket depth

Increasing the RAS (Return Activated Sludge) flowrate when a secondary clarifier is already overloaded with solids will result in an increase in blanket depth. The blanket depth refers to the layer of settled solids at the bottom of the clarifier. If the RAS flowrate is increased under these conditions, more settled sludge is returned to the aeration basin, which may not have enough time to settle properly in the clarifier. This situation causes the solids to become disturbed and resuspended, leading to a deeper blanket as the settled solids are unable to effectively consolidate due to the increased hydraulic loading. The clarifier's capacity is overwhelmed, which exacerbates the problem of solids removal and can lead to poor effluent quality. Thus, the emphasis on blanket depth being increased highlights how operational changes, such as adjusting RAS flow, impact the dynamics within the secondary clarifier, particularly when it is already handling an excessive load of solids.