The correct answer is option 'D' - Biological filter.

Biological filters are commonly used in the anaerobic treatment of sewage. This type of filter utilizes microorganisms to break down organic matter in the absence of oxygen. The anaerobic treatment process is an effective method for treating sewage and wastewater, as it removes pollutants and reduces the biochemical oxygen demand (BOD) of the water.

Here are the details of why the biological filter is used in the anaerobic treatment of sewage:

1. Anaerobic Treatment:
- Anaerobic treatment refers to the process of treating wastewater or sewage in the absence of oxygen. In this process, the organic matter in the wastewater is broken down by anaerobic bacteria, which convert it into biogas (mainly methane and carbon dioxide) and stabilize the wastewater.
- Anaerobic treatment is typically used for sewage with a high organic content, such as domestic wastewater or industrial effluents.

2. Biological Filters:
- Biological filters, also known as anaerobic filters or anaerobic digesters, are one of the main components used in anaerobic treatment systems.
- These filters consist of a bed of porous media, such as gravel, stones, or plastic media, which provide a surface for the attachment and growth of anaerobic bacteria.
- The wastewater is distributed over the filter bed, and as it percolates through the media, the organic matter is digested by the bacteria, producing biogas and treated effluent.

3. Microbial Activity:
- Anaerobic bacteria play a crucial role in the biological filter by breaking down the organic matter present in the sewage.
- These bacteria convert complex organic compounds into simpler compounds, such as organic acids, alcohols, and gases, through a series of biochemical reactions.
- The metabolic activities of the bacteria result in the decomposition of organic matter, reducing the BOD of the sewage and generating biogas as a byproduct.

4. Advantages of Biological Filters:
- Biological filters have several advantages in the anaerobic treatment of sewage.
- They are relatively simple and cost-effective to operate and maintain compared to other treatment methods.
- They require less energy input compared to aerobic treatment processes.
- They can handle high organic loads and are effective in removing organic pollutants from wastewater.
- The biogas produced during the anaerobic treatment process can be captured and used as an energy source.

In conclusion, the biological filter is used in the anaerobic treatment of sewage due to its ability to support the growth of anaerobic bacteria and facilitate the breakdown of organic matter. This process helps in the removal of pollutants and reduction of the BOD of the sewage, making it an effective method for wastewater treatment.