**Activated sludge** is put into the anaerobic sludge digester for further sewage treatment.

Anaerobic sludge digestion is a process in which organic matter in the sludge is decomposed by bacteria in the absence of oxygen. This process helps in reducing the volume of sludge, stabilizing its organic content, and producing biogas as a byproduct.

Here is a detailed explanation of why activated sludge is put into the anaerobic sludge digester:

**1. What is activated sludge?**
Activated sludge is a mixture of microorganisms (bacteria, fungi, protozoa) that are used to treat wastewater. It is obtained from the secondary treatment process, where the wastewater is mixed with a microbial culture in an aeration tank. The microorganisms in the activated sludge consume and break down the organic matter present in the wastewater.

**2. Purpose of anaerobic sludge digestion:**
Anaerobic sludge digestion is an important step in sewage treatment. It helps in reducing the volume of sludge, stabilizing its organic content, and producing biogas. During anaerobic digestion, the organic matter in the sludge is broken down by bacteria in the absence of oxygen, resulting in the release of methane and carbon dioxide gases.

**3. Role of activated sludge in anaerobic digestion:**
Activated sludge is added to the anaerobic sludge digester to provide a source of microorganisms that are capable of breaking down the organic matter in the sludge. These microorganisms, which are already adapted to the wastewater environment, can efficiently decompose the organic matter anaerobically.

**4. Decomposition of organic matter in anaerobic sludge digestion:**
In the anaerobic sludge digester, the microorganisms in the activated sludge metabolize the organic matter in the sludge through a series of complex biochemical reactions. As a result, the organic matter is converted into simpler compounds such as methane, carbon dioxide, water, and small amounts of residual organic matter.

**5. Biogas production:**
One of the major benefits of anaerobic sludge digestion is the production of biogas. Methane, the primary component of biogas, can be collected and used as a renewable energy source. This helps in reducing the environmental impact of sewage treatment facilities and provides an alternative energy option.

In conclusion, activated sludge is put into the anaerobic sludge digester for further sewage treatment because it provides a source of microorganisms capable of breaking down the organic matter in the sludge. This process helps in reducing the volume of sludge, stabilizing its organic content, and producing biogas.