Biochemical oxygeb demand (BOD) in a river water

Options

A. has no relatiship with concentration of oxygen in the water
B. gives a measure of Salmonella in the water
C. increases when sewage gets mixed with river water
D. remains unchanged when algal bloom occurs.

Answer

### Step-by-Step Solution:

1. **Understanding BOD**: Biological Oxygen Demand (BOD) is a measure of the amount of oxygen that microorganisms will consume while decomposing organic matter in water. It indicates the level of organic pollution in water bodies.

2. **Role of Microorganisms**: Microorganisms, particularly aerobic bacteria, play a crucial role in breaking down organic materials found in sewage. They require oxygen to carry out this decomposition process.

3. **Impact of Sewage on BOD**: When sewage is introduced into a river, the concentration of organic matter increases. As a result, the demand for oxygen by these microorganisms also increases. This means that the BOD will rise with the addition of more sewage.

4. **Direct Proportionality**: The relationship between the amount of sewage and BOD is directly proportional. This means that as the quantity of sewage increases, the BOD also increases because more oxygen is required for the decomposition of the additional organic matter.

5. **Conclusion**: Therefore, if sewage is mixed with river water, the BOD will increase due to the higher demand for oxygen by the microorganisms that are breaking down the organic material in the sewage.