Why is secondary treatment of water in sewage treatment plant called biological treatment?

Explanation

**Step-by-Step Solution:**

1. **Understanding Sewage Treatment Process**: The sewage treatment process consists of three main steps: primary treatment, secondary treatment, and tertiary treatment. Each step has a specific function in the overall treatment of wastewater.

2. **Primary Treatment**: In the primary treatment phase, the sewage undergoes sedimentation. This process uses gravity to allow solid waste to settle at the bottom of a tank, separating it from the liquid waste. The liquid that is extracted after this step is relatively clearer but still contains dissolved and suspended organic matter.

3. **Secondary Treatment**: The liquid waste from the primary treatment is then transferred to the secondary treatment stage. This is where the biological treatment occurs. In this step, microorganisms (living cells) are introduced to the wastewater.

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