What is the key difference between primary and secondary sewage treatment?

Primary sewage treatment:-
1. It is a mechanical process involving the removal of coarse solid materials.
2. It is inexpensive and relatively less complicated.
Secondary sewage treatment:-
1. It is a biological process involving the action of microbes.
2. It is a very expensive and complicated process.

For more answers visit to website:
https://www.tiwariacademy.com/ncert-solutions/class-12/biology/chapter-10/