The purpose of biological treatment of waste water is to

Options

A. reduce BOD
B. increase BOD
C. reduce sedimentation
D. increase sedimentation

Explanation

### Step-by-Step Solution:

1. **Understanding the Question**: The question asks about the purpose of biological treatment of wastewater. We need to identify what this treatment aims to achieve.

2. **Defining BOD**: BOD stands for Biological Oxygen Demand. It is a measure of the amount of oxygen that microorganisms will consume while decomposing organic matter in water. A high BOD indicates a high level of organic pollution.

3. **Identifying Biological Treatment**: Biological treatment of wastewater is often referred to as secondary treatment. This process involves the use of microorganisms to break down organic matter in the sewage.

4. **Purpose of Biological Treatment**: The primary purpose of biological treatment is to reduce the BOD of wastewater. This is achieved by the microorganisms consuming the organic matter, which in turn decreases the oxygen demand of the water.

5. **Effectiveness of Biological Treatment**: During biological treatment, it is noted that 90 to 95% of the BOD can be reduced. This significant reduction indicates the effectiveness of the process.

6. **Additional Benefits**: Besides reducing BOD, biological treatment also helps in removing pathogens from the wastewater, making it safer for discharge into the environment.

7. **Conclusion**: Based on the information gathered, the main purpose of biological treatment of wastewater is to reduce BOD.

### Final Answer:
The purpose of biological treatment of wastewater is to reduce BOD.

---