In sewage treatment, formation of activated sludge takes place...

In sewage treatment, the primary effluent is passed into large aeration tanks during secondary treatment. Here, it is constantly agitated mechanically and air is pumped into it, allowing vigorous growth of useful aerobic microbes into flocs. Once the biochemical oxygen demand (BOD) of the sewage is reduced significantly, the effluent is passed into a settling tank where the bacterial flocs are allowed to sediment. This sediment is called activated sludge.

Therefore, the formation of activated sludge takes place during secondary treatment.

Answer: secondary