Secondary treatment or biological treatment of waste or sewage is done by the :-

Options

A. Autotrophic microbes naturally present in the sewage.
B. Heterotrophic microbes naturally present in the sewage
C. Photoautotrophic bacteria
D. Chemoautotrophic bacteria

Answer

To solve the question regarding the secondary treatment or biological treatment of waste or sewage, we can follow these steps:

### Step-by-Step Solution:

1. **Understand the Types of Treatment**:
- Wastewater treatment typically involves three stages: primary (physical), secondary (biological), and tertiary treatment.
- The secondary treatment is specifically focused on biological processes to degrade organic matter.

2. **Identify the Role of Microbes**:
- Secondary treatment utilizes microbes to break down organic waste.
- There are different types of microbes, and their roles in sewage treatment vary.

3. **Evaluate the Options**:
- **Option A: Autotrophic microbes naturally present in the sewage**:
- Autotrophic microbes can produce their own food and are not effective in degrading waste material in sewage. Hence, this option is incorrect.
- **Option B: Heterotrophic microbes naturally present in the sewage**:
- Heterotrophic microbes feed on organic compounds and are responsible for breaking down the waste in sewage. This option is correct.
- **Option C: Photoautotrophic bacteria**:
- These bacteria require sunlight to produce their food, which is not feasible in the sewage treatment process. Thus, this option is incorrect.
- **Option D: Chemoautotrophic bacteria**:
- These bacteria also require specific chemicals to produce their food and are not used in biological treatment of sewage. Therefore, this option is incorrect.

4. **Conclusion**:
- After evaluating all the options, the correct answer is **B: Heterotrophic microbes naturally present in the sewage**. These microbes play a crucial role in the secondary treatment of sewage by digesting organic waste.