Why is some amount of activsted sludge pumped back into aeration tank in secondary treatment of sewage ?

Please find below the solution to the asked query

A small part of the activated sludge is pumped back into the aeration tank to serve as the inoculum (containing bacteria and fungi - Floc)  for the next cycle of sewage treatment. 

Hope this information will clear your doubts about the activated sludge. 
Regards