Polluted water can be purified by using

Options

A. Micro-organisms
B. Algae
C. Pesticides
D. Fishes

Explanation

**Step-by-Step Solution:**

1. **Understanding Polluted Water**: Polluted water contains various impurities, including organic matter, pathogens, and chemicals that can harm the environment and human health.

2. **Role of Micro-organisms**: Micro-organisms play a crucial role in the purification of polluted water. They are used in biological treatment processes to break down organic matter.

3. **Biochemical Oxygen Demand (BOD)**: BOD is a measure of the amount of oxygen that micro-organisms will consume while decomposing organic matter in water. A high BOD indicates a high level of pollution, while a low BOD indicates cleaner water.

4. **Sewage Treatment**: Micro-organisms are integrated into sewage treatment systems. They consume the organic pollutants present in the sewage, thereby reducing the BOD and purifying the water.

5. **Treatment Stages**: The purification of polluted water typically involves three stages:
- **Primary Treatment**: Physical removal of large solids and sediments.
- **Secondary Treatment**: Biological treatment where micro-organisms decompose organic matter.
- **Tertiary Treatment**: Advanced treatment to remove remaining impurities and pathogens.

6. **Conclusion**: Polluted water can effectively be purified by using micro-organisms through various treatment processes, primarily focusing on the reduction of organic matter and BOD.

---