When domestic sewage mixes with river water
(a) small animals like rats will die after drinking river water
(b) the increased microbial activity releases micronutrients such as iron
(c) the increased microbial activity uses up dissolved oxygen
(d) the river water is still suitable for drinking as impurities are only about 0.1 %

Options

A. small animals like rats will die after drinking river water
B. the increased microbial activity releases micronutrients such as iron
C. the increased microbial activity uses up dissolved oxygen
D. the river water is still suitable for drinking as impurities are only about 0.1 %

Answer

To solve the question regarding the effects of domestic sewage mixing with river water, we will analyze each option step by step.

### Step-by-Step Solution:

1. **Understanding the Impact of Sewage on Water**:
- When domestic sewage mixes with river water, it introduces a large amount of organic matter into the water. This organic matter serves as a food source for microorganisms.

**Hint**: Think about what happens when organic matter is added to a water body.

2. **Microbial Activity**:
- The microorganisms present in the sewage will begin to decompose the organic matter. This process leads to increased microbial activity in the water.

**Hint**: Consider what happens to the population of microorganisms when they have more food available.

3. **Biochemical Oxygen Demand (BOD)**:
- As microbial activity increases, the Biochemical Oxygen Demand (BOD) of the water also increases. BOD is a measure of the amount of oxygen that microorganisms will consume while decomposing organic matter.

**Hint**: Remember that more microorganisms mean more oxygen is needed for their metabolic processes.

4. **Dissolved Oxygen (DO) Levels**:
- With the increase in BOD, the dissolved oxygen (DO) levels in the water decrease. This is because the microorganisms consume the oxygen present in the water to break down the organic matter.

**Hint**: Think about the relationship between microbial activity and oxygen levels in water.

5. **Effect on Aquatic Life**:
- The decrease in dissolved oxygen can lead to the death of aquatic organisms, including small animals like fish and invertebrates. However, rats drinking the river water may not be directly affected in the same way, as they are terrestrial animals.

**Hint**: Consider the types of organisms that are most affected by low oxygen levels in water.

6. **Analyzing the Options**:
- (a) Small animals like rats will die after drinking river water: This is not necessarily true, as rats are not aquatic animals and may not be directly affected.
- (b) Increased microbial activity releases micronutrients such as iron: This is misleading; while some nutrients may be released, the primary concern is oxygen depletion.
- (c) The increased microbial activity uses up dissolved oxygen: This is true and is the correct answer.
- (d) The river water is still suitable for drinking as impurities are only about 0.1%: This is incorrect, as the presence of sewage makes the water unsafe for drinking regardless of the percentage of impurities.

**Final Answer**: The correct option is (c) the increased microbial activity uses up dissolved oxygen.