Florida Wastewater Practice Test 2026 – Complete Exam Prep

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.