Preliminary Treatment:-

It consist of solely in separating the material like dead animal, tree branches, paper, wood, etc and also heavy settle-able inorganic solids.

A) Screening:-

• Screening is the very first operation carried out of a sewage treatment plant and consists of passing the sewage through different types of screens as to trap.
• Screen should preferably be placed before the grit chambers. However, if the quality of 'grit' is not of much importance, as in the case of land fillings.
• Depending upon the size of openings, screens may be classified as coarse screens, medium screens, fine screens.

B) Grit chamber:-

• The sewage has an inorganic material which is called as grit . The grit includes sand, silt, day, glass pieces, etc. Naturally the grit varies as the source vary.
• On the average the size of the grit is small i.e about 0.2 mm diameter. The specific gravity of the grit is between 2.0 and 2.6.
• The grit has more settling velocity than any organic matter and it is no putrescible.

C) Skimming Tank:-

• Skimming Tools are sometimes employed for removing oils and grease from used where sewage and placed before the sedimentation tanks.
• They are used where sewage contains too much of grease or oils, which includes fats, waxes, soaps, fatty acids, etc.
• If such greasy and oily matter is not removed from the sewage before it enters further treatment units, it may form unsightly and odours on the surface of the settling tanks.
• The skimming tank consist of a long trough shaped structure divided into two or three lateral compartments.