Why do we prefer to call secondary waste water treatment as biological treatment ?

Secondary sewage treatment is mainly a:

Assertion : Secondary treatement of sewage is also called biological treatment while primary treatment is called physical treatment. Reason : Primary sewage treatment depends only upon sedimentation properties of materials present in sewage and filtration.

The secondary treatment of sewage includes

Study the following statements and select the incorrect ones. (i) Physical removal of large and small particles through filtration and sedimentation is called primary sewage treatment. (ii) Secondary sewage treatment is mainly a mechanical process. (iii) Activated sludge sediment in a sewage treatment plant is a rich source of aeroic bacteria. (iv) Biogas, commonly called as gobar gas, is pure methane.