Reverse osmosis is done in treatment `"……………."` stage of waste water.

Options

A. primary
B. secondary
C. tertiary
D. `NO_(2)`

Answer

To solve the question regarding the stage of wastewater treatment where reverse osmosis is performed, we can follow these steps:

### Step-by-Step Solution:

1. **Understand Reverse Osmosis**:
- Reverse osmosis is a water treatment process that uses pressure to force water through a semi-permeable membrane, effectively removing contaminants and impurities from water.

2. **Identify the Stages of Wastewater Treatment**:
- Wastewater treatment typically occurs in three main stages:
- **Primary Treatment**: Involves the physical removal of solids from wastewater.
- **Secondary Treatment**: Involves biological processes where aerobic bacteria break down organic matter.
- **Tertiary Treatment**: This is the advanced treatment stage that further purifies the water, often using methods like reverse osmosis.

3. **Determine the Correct Stage for Reverse Osmosis**:
- Reverse osmosis is specifically used in the tertiary treatment stage. This stage focuses on removing remaining contaminants and nutrients that could harm the ecosystem.

4. **Conclusion**:
- Therefore, the correct answer to the question is that reverse osmosis is done in the **tertiary** stage of wastewater treatment.

### Final Answer:
Reverse osmosis is done in the treatment **tertiary** stage of wastewater.

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