What operation concentrates aerobic bacteria by moving sludge from the bottom of the secondary clarifier to the aeration tank inlet?

• A. Wasting
• B. Returning
• C. Mixing
• D. Aeration

Answer: (B)

The main idea is recycling biomass back to the aeration basin to keep enough active bacteria in the reactor. Moving settled sludge from the bottom of the secondary clarifier to the aeration tank inlet is called returning (return activated sludge). This returns a portion of the settled biomass to the aeration tank so the aerobically active organisms stay concentrated where they can treat the incoming wastewater, maintaining the proper solids concentration and food-to-microorganism balance for efficient treatment.

Wasting would remove excess sludge to control sludge age and solids retention time, not concentrate bacteria in the aeration basin. Mixing is just blending solids and liquids, not transferring settled sludge. Aeration is the act of supplying air to sustain aerobic conditions, not repositioning sludge.