Aerotion tank,Grit and sand removal tank, Second sedimentation tank, Bar screen, First sedimentation tank
which step gives most of the sludge which is used for obtaining biogas fuel?

Answer

To determine which step in the sewage treatment process gives the most sludge used for obtaining biogas fuel, we can analyze each step mentioned in the question:

1. **Bar Screen**: This is the first step where large objects such as plastic packets and tin cans are removed from the sewage. This step does not produce sludge; it primarily filters out large debris.

2. **Grit and Sand Removal Tank**: In this tank, grit and sand settle down and are removed. While some solid waste is collected here, it is primarily composed of inorganic materials (grit and sand) and does not contribute significantly to sludge formation.

3. **First Sedimentation Tank**: This is a crucial step in the sewage treatment process. In this tank, solid waste settles down and forms sludge. This sludge is rich in organic matter and is sent to the digester tank for further processing. This is where the majority of sludge is generated, which is later used to produce biogas.

4. **Aeration Tank**: In this tank, compressed air is bubbled through the wastewater to activate aerobic bacteria that digest organic matter. While some sludge is formed here due to the breakdown of organic material, it is not the primary source of sludge.

5. **Second Sedimentation Tank**: This tank follows the aeration process, where additional settling occurs. Some sludge is formed here as well, but it is less than what is produced in the first sedimentation tank.

Based on the analysis, the step that gives the most sludge, which is used for obtaining biogas fuel, is the **First Sedimentation Tank**.

### Final Answer:
The step that gives the most sludge used for obtaining biogas fuel is the **First Sedimentation Tank**.