Overview:
Anaerobic digestion is a biological process that breaks down organic matter in the absence of oxygen. It is commonly used in the sludge digestion process to treat wastewater sludge and produce biogas. In this process, anaerobic bacteria convert organic compounds into methane and carbon dioxide through a series of complex reactions.

Explanation:
The correct answer is option 'B': Anaerobic digestion is a low energy process. Here's an explanation of why this statement is correct:

1. Definition of Anaerobic Digestion:
Anaerobic digestion is a biological process that occurs in the absence of oxygen. It involves the breakdown of organic matter by a group of microorganisms called anaerobes. These microorganisms convert complex organic compounds into simpler compounds, such as methane and carbon dioxide, through a series of biochemical reactions.

2. Energy Production:
Anaerobic digestion is known for its ability to produce biogas, which consists mainly of methane (CH4) and carbon dioxide (CO2). Methane is a valuable energy source and can be used for various applications, including electricity generation and heating. The production of biogas makes anaerobic digestion a source of renewable energy.

3. Low Energy Input:
One of the advantages of anaerobic digestion is that it requires relatively low energy input compared to other treatment processes. The main energy requirement is for mixing and temperature control within the digester. Once the process is initiated, the microorganisms involved in anaerobic digestion generate their own energy by breaking down organic matter.

4. Efficiency and Cost:
Anaerobic digestion is considered a cost-effective method for treating wastewater sludge. The low energy input, combined with the production of biogas, makes it an attractive option for wastewater treatment plants. The generated biogas can be used to offset energy costs or even sold to the grid, further enhancing the economic viability of anaerobic digestion.

5. Other Benefits:
Apart from biogas production, anaerobic digestion offers additional benefits. It helps reduce the volume of sludge, making it easier to handle and dispose of. It also reduces the concentration of pathogens present in the sludge, improving its safety for further use or disposal. Additionally, anaerobic digestion can be an environmentally friendly option as it reduces greenhouse gas emissions compared to other treatment processes.

In conclusion, anaerobic digestion is a low energy process that generates valuable biogas from organic matter. Its efficiency, cost-effectiveness, and potential for energy recovery make it a widely used method in the sludge digestion process.