Which is correct sequence for purification process of STPs ?

Options

A. Primary Sludge - Effluent - Flocs- Active Sludge - Biogas .
B. Flocs - Primary Sludge - Effluent - Active sludge - Biogas
C. Primary Sludge - Effluent - Active Sludge - Flocs - Biogas
D. Effluent - Flocs - Primary Sludge -Active Sludge - Biogas .

Answer

### Step-by-Step Solution for the Purification Process of Sewage Treatment Plants (STPs)

1. **Primary Treatment**: The first step in the sewage treatment process is primary treatment. This involves two main processes: sedimentation and filtration.
- **Sedimentation**: Heavy substances in the sewage settle down to form a layer of sludge at the bottom of the tank.
- **Filtration**: The liquid that remains above the sludge is called the supernatant or effluent.

2. **Formation of Primary Sludge and Effluent**: After sedimentation and filtration, we have two outputs:
- **Primary Sludge**: The solid waste that has settled at the bottom.
- **Effluent**: The liquid waste that is left after the solids have been removed.

3. **Secondary Treatment**: The effluent from the primary treatment is then subjected to secondary treatment.
- **Aeration Tank**: The effluent is passed through an aeration tank where aerobic microbes are introduced. These microbes help in breaking down organic matter.
- **Formation of Flocs**: The microbes grow and form a mesh-like structure known as flocs, which helps in further purification.

4. **Reduction of BOD**: The Biological Oxygen Demand (BOD) of the effluent is measured and reduced during this stage. A lower BOD indicates cleaner water.

5. **Settling Tank**: After aeration, the effluent containing flocs is passed into a settling tank.
- **Sedimentation of Flocs**: In the settling tank, the flocs settle down, forming a layer of activated sludge.

6. **Anaerobic Digestion**: The activated sludge is then treated in an anaerobic sludge digester.
- **Biogas Production**: Anaerobic microbes digest the sludge, producing gases such as methane, carbon dioxide, and hydrogen sulfide. This mixture of gases is known as biogas.

### Correct Sequence of the Purification Process:
1. **Primary Sludge**
2. **Effluent**
3. **Flocs**
4. **Activated Sludge**
5. **Biogas**

### Conclusion:
The correct sequence for the purification process of STPs is:
- Primary Sludge → Effluent → Flocs → Activated Sludge → Biogas