At a sewage treatment plant , what type of bacteria work:
in the aeration tank.

Explanation

**Step-by-Step Solution:**

1. **Understanding the Aeration Tank**: The aeration tank is a crucial component of a sewage treatment plant where wastewater is treated. It is designed to introduce air into the wastewater to promote the growth of certain bacteria.

2. **Role of Air in the Aeration Tank**: The term "aeration" refers to the process of adding air (specifically oxygen) to the water. This is essential because it creates an environment where specific types of bacteria can thrive.

3. **Identifying the Type of Bacteria**: The bacteria that thrive in the presence of oxygen are known as aerobic bacteria. These bacteria require oxygen to metabolize organic matter present in the sewage.

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