In an activated sludge system, settled sludge is typically what?

• A. It is disposed of as waste only
• B. It is returned to the aeration tank as recycle to maintain microbial population
• C. It is stirred back into the wastewater
• D. It is burned

Answer: (B)

The main idea is to keep enough microorganisms in the aeration basin to do the treatment work. After solids settle in the clarifier, most of that settled sludge is pumped back to the aeration tank as return activated sludge. This recycling maintains a high biomass concentration and an active microbial population, ensuring continued degradation of the incoming wastewater. A portion of settled sludge is commonly diverted as waste activated sludge to control sludge age and solids loading, rather than sending all settled sludge back or burning it. Returning settled sludge directly to the wastewater or burning it isn’t how the system maintains treatment efficiency.