The following units are included in a sewage treatment plant.

1. Screening

2. Grit removal

3. Secondary sedimentation

4. Aeration

5. Primary sedimentation

The correct sequence of these units is:

Explanation:

Sewage treatment plants consists of the following procedures:

1. Screening

• These are placed before grit chambers so as to trap and remove floating matter, such as pieces of cloth, paper, wood cork, hair, fiber, kitchen refuse, fecal solids, etc in the sewage.
• Those floating material if not removed will choke the pipes or adversely affect the working of the pumps.

2. Grit removal basins

• Grit chambers or detritus tanks are the sedimentation basins placed in front of the wastewater treatment plant to remove inorganic particles such as sand, gravel, grits, shells, bones, and other non-putrescible material.
• They are usually placed after the fine screens to remove particles of size up to 0.2 mm or so.

3. Primary sedimentation

• The primary sedimentation tanks are thus designed to remove a part of the organic matter from the sewage effluent coming from the grit chambers.
• These are designed for effecting the settlement of particles by reducing the flow velocity or by detaining the sewage in them.

4. Aeration

• From the primary sedimentation tank, the sewage flows to the aeration tank and is mixed with activated sludge.
• These are rectangular tanks in which air is continuously introduced into these tanks.

5. Secondary sedimentation

• From the aeration tank, the sewage flows to the final secondary sedimentation tank.
• The secondary sedimentation tank removes bioflocculated solids.
• The produce an effluent sufficiently classified to meet discharge standards and must concentrate the biological solids to minimize the quantity of sludge to handle.