ANS:B - aeration is done with an admixture of previously aerated sludge

1. Aeration is continued till stability: This statement is accurate. In the activated sludge process, aeration is a crucial step where air is continuously pumped into the aeration tank to provide oxygen for the aerobic microorganisms. This promotes the biological degradation of organic matter in the wastewater. Aeration is typically continued until the desired level of treatment and stability is achieved in the wastewater, indicated by factors such as the reduction of organic pollutants and the settling characteristics of the sludge.
2. Aeration is done with an admixture of previously aerated sludge: This statement is not accurate. Aeration in the activated sludge process involves the introduction of air into the wastewater to create aerobic conditions necessary for microbial activity. The process does not involve mixing previously aerated sludge with incoming wastewater during aeration. Instead, the activated sludge from the secondary clarifier is returned to the aeration tank to maintain a population of microorganisms capable of treating the wastewater.
3. Sludge is activated by constant stirring: This statement is not entirely accurate. In the activated sludge process, sludge is not activated by constant stirring. Instead, sludge is activated by providing favorable conditions for the growth and activity of aerobic microorganisms, such as adequate oxygen supply, appropriate nutrient levels, and sufficient mixing to ensure contact between microorganisms and organic matter in the wastewater. While mixing does occur in the aeration tank to ensure uniform distribution of oxygen and microorganisms, it is not the primary mechanism for sludge activation.
4. Water is removed by centrifugal action: This statement is partially accurate. In the activated sludge process, water is separated from the treated sludge through a process called clarification, which typically involves sedimentation in a secondary clarifier. While centrifugal action may be used in other wastewater treatment processes such as sludge dewatering, it is not the primary method for water removal in the activated sludge process.