Which of the following is put into Anaerobic sludge digester for...

A small part of the activated sludge is pumped back into the aeration tanks to serve as inoculums. The remaining major part of the sludge is pumped into large tanks called anaerobic sludge digesters.

Anaerobic sludge digesters: Anaerobic methanogenic bacteria, digest the waste present in sludge. During this digestion, bacteria produce a mixture of gases such as methane, hydrogen sulphide, and carbon dioxide. These gases form biogas and can be used as a source of energy as it is inflammable.