Secondary sewage treatment

Options

A. Utilizes aerobic bacteria
B. Removes virtually all radioisotopes and toxic metals
C. Is too expensive to be used in most plants
D. Is a mechanical process

Answer

### Step-by-Step Solution for Secondary Sewage Treatment

1. **Understanding Sewage Treatment**:
- Sewage treatment is a process used to remove contaminants from wastewater or sewage. It typically involves multiple stages, including primary, secondary, and sometimes tertiary treatment.

2. **Primary Treatment**:
- In the first stage, known as primary treatment, sewage is treated using anaerobic bacteria. These bacteria thrive in environments without oxygen and help break down inorganic waste into organic waste.

3. **Transition to Secondary Treatment**:
- After primary treatment, the organic waste produced is then sent to the secondary treatment stage. This is where the main focus of secondary sewage treatment lies.

4. **Role of Aerobic Bacteria**:
- In secondary treatment, aerobic bacteria are utilized. These bacteria require oxygen to function and are responsible for further breaking down the organic waste. They help in the dissolution of modified organic waste, making it easier to manage.

5. **Nutrient Recycling**:
- The byproduct of this treatment process is a slurry, which is rich in nutrients. This slurry can be used as a fertilizer and is often sent back to the soil, promoting nutrient recycling in the ecosystem.

6. **Removal of Toxic Substances**:
- While secondary treatment is effective in removing organic waste, it does not specifically target radiotropes and toxic metals. For these contaminants, additional processes may be required, such as using UV rays or other advanced techniques.

7. **Cost Efficiency**:
- Contrary to some beliefs, secondary sewage treatment is generally cost-efficient and is primarily a biological process due to the involvement of microbes.

8. **Conclusion**:
- The correct option regarding secondary sewage treatment emphasizes the use of aerobic bacteria for organic waste breakdown, nutrient recycling, and the biological nature of the process.