Secondary sewage treatment is mainly

Options

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

Answer

**Step-by-Step Solution:**

1. **Understanding Sewage Treatment**: Sewage treatment involves the process of removing contaminants from wastewater, which includes domestic, industrial, and municipal waste.

2. **Stages of Sewage Treatment**: The sewage treatment process is generally divided into three main stages: primary, secondary, and tertiary treatment.

3. **Primary Treatment**: This stage involves physical processes such as shredding, churning, flotation, screening, and sedimentation. The goal is to remove large solids and settleable organic matter.

4. **Secondary Treatment**: This stage focuses on biological processes. It primarily involves the breakdown of organic matter by microorganisms. The main goal is to reduce the biochemical oxygen demand (BOD) and remove dissolved organic matter.

5. **Methods of Secondary Treatment**: Common methods include activated sludge processes, trickling filters, and bio-towers, where bacteria and other microorganisms digest the organic pollutants.

6. **Importance of Secondary Treatment**: This stage is crucial as it significantly reduces the organic load in sewage, making it safer for discharge into the environment or for further treatment.

7. **Conclusion**: Therefore, secondary sewage treatment is mainly a biological process aimed at degrading organic matter in wastewater.

---