Florida Wastewater Practice Test

What may happen if solids are washing out of the secondary clarifier in an activated sludge plant?

1. Increased Return Activated Sludge flow rate will solve the problem

2. Decreased Return Activated Sludge flow rate will solve the problem

3. Increasing RAS decreases detention time in the clarifier

4. Higher sludge volume index indicates a healthy aeration tank

The correct answer is: Increasing RAS decreases detention time in the clarifier

Increasing the Return Activated Sludge (RAS) flow rate decreases detention time in the clarifier, which is a critical factor in the performance of the secondary clarifier in an activated sludge system. In these systems, the RAS is a portion of the settled sludge that is returned to the aeration tank to maintain a healthy population of microorganisms to treat incoming wastewater. When the flow rate of RAS is increased, the volume of thickened sludge that is returned to the aeration tank rises, thereby reducing the time that the remaining sludge has to settle in the clarifier. This reduction in detention time can lead to insufficient separation of the solids from the treated effluent, which may cause solids to wash out of the clarifier. It indicates that while increasing RAS can initially seem like a solution to ensure adequate biomass in the aeration tank, if not managed properly, it can inadvertently contribute to problems such as carryover of solids in the effluent. This understanding highlights the importance of balancing RAS flow to optimize solid separation in the clarifier while ensuring that the biological treatment process remains effective.