During the primary treatment of sewage, solid paricles that settle down are called

Options

A. flocs
B. primary sludge
C. activated sludge
D. anaerobic sludge

Explanation

**Step-by-Step Solution:**

1. **Understanding Primary Treatment of Sewage:**
- The primary treatment of sewage is the initial step in the sewage treatment process. It primarily focuses on the removal of solid particles from the sewage.

2. **Mechanisms Involved:**
- There are two main mechanisms involved in the primary treatment: sedimentation and sequential filtration.
- **Sedimentation:** This process allows solid particles, both large and small, to settle down due to gravity. These particles are generally insoluble in water.
- **Sequential Filtration:** After sedimentation, any remaining floating and suspended solids are removed through filtration.

3. **Products of Primary Treatment:**
- After the primary treatment, two main products are obtained:
- **Supernatant (Effluent):** This is the liquid portion that remains after the solids have settled and is typically sent for secondary treatment.
- **Primary Sludge:** This is the solid material that settles at the bottom during the sedimentation process.

4. **Characteristics of Primary Sludge:**
- Primary sludge is rich in nutrients and contains a variety of microbes. It is not used in the secondary treatment process but can be repurposed as manure or fertilizer due to its nutrient content.

5. **Identifying the Correct Answer:**
- The question asks for the term used to describe the solid particles that settle down during the primary treatment of sewage. Based on the explanation, the correct term is **primary sludge**.

**Final Answer:**
The solid particles that settle down during the primary treatment of sewage are called **primary sludge**.

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