Biochemical Oxygen Demand (BOD) in a river water

Options

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

Answer

**Step-by-Step Solution:**

1. **Understanding Biochemical Oxygen Demand (BOD):**
- Biochemical Oxygen Demand (BOD) refers to the amount of dissolved oxygen that aerobic bacteria will consume while decomposing organic matter in water. It is measured in milligrams of oxygen per liter of water sample over a specific period and at a certain temperature.

2. **Role of Aerobic Bacteria:**
- Aerobic bacteria utilize dissolved oxygen to break down organic matter. This process is essential for their growth and energy production. As they consume oxygen, the levels of dissolved oxygen in the water decrease.

3. **Impact of Organic Matter:**
- When organic matter, such as sewage, enters a water body, it provides food for aerobic bacteria. The more organic matter present, the more oxygen is consumed by these bacteria, leading to an increase in BOD.

4. **Algal Bloom and Its Effects:**
- Algal blooms occur when excess nutrients (like fertilizers) enter the water, leading to rapid algae growth. While this increases photosynthetic activity and oxygen levels initially, the subsequent decomposition of dead algae by bacteria can lead to a significant decrease in dissolved oxygen, thereby increasing BOD.

5. **Analyzing the Options:**
- **Option 1:** Incorrect, as BOD does not remain unchanged during algal blooms; it typically increases.
- **Option 2:** Incorrect, as BOD is directly related to the concentration of dissolved oxygen in water.
- **Option 3:** Incorrect, as BOD does not measure Salmonella; it measures the oxygen demand of bacteria.
- **Option 4:** Correct, as BOD increases when sewage, which contains high levels of organic matter, is mixed with river water. This leads to increased oxygen consumption by aerobic bacteria.

6. **Conclusion:**
- The correct answer is **Option 4**: Biochemical Oxygen Demand in river water increases when sewage gets mixed with river water.

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