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

Explanation

### Step-by-Step Solution:

1. **Understanding the Digester Tank**: The digester tank is a crucial part of a sewage treatment plant. Its primary function is to break down the sludge that accumulates during the sewage treatment process.

2. **Role of Bacteria**: In the digester tank, specific types of bacteria are employed to facilitate the breakdown of sludge. These bacteria help in decomposing organic matter present in the sludge.

3. **Identifying the Type of Bacteria**: The bacteria that thrive in the digester tank are known as anaerobic bacteria. These bacteria do not require oxygen to survive and are capable of breaking down organic materials in an oxygen-free environment.

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