The correct answer is D. All of the above.
During preliminary treatment of sewage, oil and grease are removed from skimming tanks, floating materials are removed by screening, and grit and sand are removed by grit chambers.
Skimming tanks are used to remove oil and grease from sewage. The sewage is passed through a tank where the oil and grease float to the surface and are skimmed off.
Screening is used to remove floating materials from sewage. The sewage is passed through a screen that removes large objects, such as paper, rags, and sticks.
Grit chambers are used to remove grit and sand from sewage. The sewage is passed through a chamber where the grit and sand settle to the bottom and are removed.
Preliminary treatment is the first step in sewage treatment. It removes large objects and materials that can interfere with the later stages of treatment.