Secondary sewage treatment is mainly a

Options

A. Physical process
B. Mechanical process
C. Chemical process
D. Biological process

Answer

### Step-by-Step Solution:

1. **Understanding Secondary Sewage Treatment**:
- Secondary sewage treatment is a crucial step in the wastewater treatment process. It primarily focuses on the removal of organic matter from sewage.

2. **Role of Microorganisms**:
- In this stage, microorganisms play a vital role. They decompose organic matter present in the sewage. This decomposition is essential for reducing the pollution load in the wastewater.

3. **Methods of Secondary Treatment**:
- There are several methods used in secondary sewage treatment, including:
- **Trickling Filter Method**: In this method, sewage is passed over a bed of stones or other materials that support the growth of microorganisms. The microorganisms break down the organic matter as the sewage trickles over them.
- **Activated Sludge Method**: This method involves aerating the sewage to promote the growth of microorganisms that consume the organic matter. The mixture is then settled, separating the treated water from the sludge.

4. **Identifying the Type of Process**:
- Since the primary mechanism of secondary sewage treatment involves the action of microorganisms (like bacteria and fungi) to decompose organic matter, it is classified as a biological process.

5. **Conclusion**:
- Based on the information gathered, we can conclude that secondary sewage treatment is mainly a biological process. Therefore, the correct answer is option D.

### Final Answer:
**Secondary sewage treatment is mainly a biological process (Option D).**

---