How is sludge returned to the aeration basin in the activated sludge process?

In the activated sludge process, the return of sludge to the aeration basin is accomplished through a return activated sludge (RAS) system. This system is specifically designed to recycle a portion of the mixed liquor that has undergone sedimentation in the clarifier back into the aeration tank. The main purpose of returning this sludge is to maintain the necessary concentration of microorganisms that facilitate effective aeration and removal of organic matter from the wastewater.

The RAS system typically consists of pumps that transport the settled sludge from the bottom of the secondary clarifier back to the aeration basin. This process ensures that there is a consistent population of microorganisms available to enhance the treatment of incoming wastewater. Maintaining an adequate biomass concentration is crucial for the efficiency of the activated sludge process, as it directly impacts the system's ability to degrade pollutants effectively.

In contrast, the other methods mentioned are not standard practices for returning sludge in the activated sludge process. The direct discharge pump generally refers to pumping effluent rather than returning sludge, gravity transfer from the primary tank is not designed for the specific requirements of RAS, and pneumatic tube systems are not typically associated with sludge management in wastewater treatment.