Which of the following is put into Anaerobic sludge digester for further sewage treatment ?

In sewage treatment plant, the activated sludge is put into an anaerobic sludge digester for further digestion. During this process biogas is produced.

Primary treatment steps involve physical removal of particles – large and small – from the sewage through filtration and sedimentation. Initially, floating debris is removed by sequential filtration. Then the grit (soil and small pebbles) are removed by sedimentation. All solids that settle form the primary sludge, and the supernatant forms the effluent. The effluent from the primary settling tank is taken for secondary treatment.