Answer:
The correct answer is- secondary treatment
Step-by-step explanation:
Wastewater treatment is divided into three main stages which are primary, secondary and tertiary stages. In the primary treatment, the wastewater is held in the settling tank for some time and the solid waste settles with time and the remaining liquid is passed for secondary treatment.
In the secondary treatment, the organic compound present in the liquid is degraded with the help of aerobic bacteria. Aeration tank is there for providing the oxygen to bacteria and wastewater is stirred so that oxygen can be distributed evenly and bacteria can do their work properly.