During preliminary treatment of a sewage

ANS:D - All the above.

During the preliminary treatment of sewage, all of the mentioned processes typically occur:

1. Oil and grease are removed from skimming tanks: Skimming tanks are used to remove oil and grease, which float on the surface of the sewage due to their lower density. These substances can be skimmed off the surface of the water and collected for proper disposal.
2. Floating materials are removed by screening: Screening involves the use of screens or grates to remove large floating debris, such as plastics, sticks, and other solid materials, from the sewage. This process prevents these materials from entering subsequent treatment processes, where they could cause clogging or damage equipment.
3. Grit and sand are removed by grit chambers: Grit chambers are designed to remove heavier inorganic solids, such as sand, gravel, and small stones, from the sewage. These materials settle to the bottom of the grit chamber due to their higher density, where they can be removed before the sewage proceeds to further treatment stages.