Explain the steps involved in preliminary treatment of sewage.

Answer: Preliminary treatment removes large floating and heavy suspended solids from sewage by physical methods:

1. Screening: sewage is passed through screens (bar/mesh) which remove large floating objects such as rags, plastics, paper, wood pieces and debris.
2. Grit removal (grit chambers): the sewage flows slowly through grit chambers where heavy inorganic particles like sand, gravel, grit and pebbles settle down.
3. Removal of oil and grease (skimming), when present.

The sewage then passes to primary treatment (sedimentation tanks) where suspended solids settle as primary sludge.