Question

Explain the steps involved in preliminary treatment of sewage.

Hint:

Think of preliminary treatment as a 3-step physical clean-up: 1. \textbf{Screen} out the big trash. 2. \textbf{Settle} out the heavy sand/grit. 3. \textbf{Settle} out the lighter organic solids. No chemistry or biology involved yet!

Answer 1

The preliminary (or primary) treatment of sewage is a physical process designed to remove large solid materials. It involves the following steps:

Screening: Raw sewage first passes through screens or bar racks. These are meshes of varying sizes that physically block and remove large floating objects like plastic bags, cans, sticks, and rags.
Grit Removal: After screening, the sewage enters a grit chamber. Here, the speed of the sewage flow is slowed down significantly. This allows heavy, inorganic particles like sand, grit, and small stones to settle at the bottom through sedimentation, while the lighter organic solids remain suspended.
Primary Sedimentation: The sewage then flows into a large tank called a primary settling tank or clarifier. In this tank, the flow is very slow, and it is held for a period of several hours. This allows most of the suspended organic solids to settle to the bottom by gravity, forming a mass called primary sludge. Lighter materials like grease and oil float to the top, forming a layer called scum, which is skimmed off. The remaining liquid, now called the primary effluent, is then sent for secondary treatment.