During which stage of sewage treatment microbes are not used?

Options

A. Primary treatment
B. Secondary treatment
C. Tertiary treatment
D. Both a and c

Answer

To solve the question "During which stage of sewage treatment are microbes not used?", we can follow these steps:

### Step 1: Understand the Stages of Sewage Treatment
Sewage treatment consists of three main stages:
1. Primary Treatment
2. Secondary Treatment
3. Tertiary Treatment

### Step 2: Analyze Each Stage
- **Primary Treatment**: This stage involves the physical removal of solid particles from sewage through processes like filtration and sedimentation. No microbes are involved in this process, as it focuses on the physical separation of solids from liquids.

- **Secondary Treatment**: This stage is also known as biological treatment. It involves the use of microbes to break down organic matter in the sewage. The primary effluent is aerated, allowing aerobic microbes to grow and consume organic pollutants, thus reducing Biochemical Oxygen Demand (BOD).

- **Tertiary Treatment**: This stage is often referred to as chemical treatment. It is aimed at further improving the quality of water after primary and secondary treatments. In this stage, chemicals are used for disinfection and removal of remaining impurities, and microbes are generally not used.

### Step 3: Identify the Correct Answer
From the analysis:
- Microbes are **not used** in **Primary Treatment** and **Tertiary Treatment**.
- Microbes **are used** in **Secondary Treatment**.

Thus, the correct answer to the question is **both A (Primary Treatment) and C (Tertiary Treatment)**.

### Final Answer
The correct option is: **Both A and C** (Primary Treatment and Tertiary Treatment).

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