What is activated sludge ?

Explanation

### Step-by-Step Solution

1. **Definition of Activated Sludge**: Activated sludge is a term used to describe a mixture of microorganisms, primarily bacteria and fungi, that are used in the treatment of sewage during the secondary treatment process.

2. **Sewage Treatment Overview**: The sewage treatment process consists of three main stages:
- **Primary Treatment**: This involves physical processes such as screening, sedimentation, and filtration to remove large solids and debris from sewage, resulting in primary effluent.
- **Secondary Treatment**: This stage involves biological processes where the primary effluent is treated with microorganisms to degrade organic matter.
- **Tertiary Treatment**: This stage includes chemical processes to further purify the effluent.
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