In the activated sludge process, what is added back to the aeration basin as part of the return sludge line?

In the activated sludge process, a portion of the sludge that is settled out in the secondary clarifier is returned to the aeration basin to maintain a healthy concentration of microorganisms essential for effective treatment. This returned sludge is referred to as return activated sludge (RAS). The correct understanding of this process highlights that typically a fraction of the settled sludge is returned rather than the entirety.

The selected response indicates that half of the return sludge is added back, aligning with the operation's intent to balance the biomass within the aeration basin efficiently. This allows for the continuous processing of wastewater while ensuring that there are enough microorganisms to facilitate the breakdown of organic material during aeration. This recycling of biomass is crucial in maintaining the desired microbial population, which is pivotal for effective aeration and treatment.

Other possible options do not accurately represent the process. For instance, primary sludge refers to solids removed from primary treatment processes and is not used in the aeration basin as RAS. All effluent is not returned since it typically flows out for further treatment or disposal. Secondary effluent is the treated liquid that exits the secondary clarifier and is separate from the biomass returned to the aeration process. Thus, understanding the role of the return sludge is key to grasping the mechanistic