Sludge digestion tanks, also known as anaerobic digesters, are an essential component of wastewater treatment plants. They play a crucial role in the biological decomposition of sludge, a byproduct of the wastewater treatment process. The correct answer to the given question is option 'A', which states that sludge digestion involves aerobic suspended culture.

Here is a detailed explanation of why aerobic suspended culture is the correct answer:

Aerobic suspended culture:
- In aerobic digestion, the sludge is treated in the presence of oxygen.
- The sludge is placed in a tank where it is mixed and agitated to ensure proper oxygenation.
- Oxygen is supplied either through mechanical aeration or by introducing air into the tank.
- The aerobic conditions promote the growth of aerobic microorganisms, such as bacteria, fungi, and protozoa, which consume the organic matter present in the sludge.
- These microorganisms break down the complex organic compounds into simpler forms, such as carbon dioxide and water.
- The process is exothermic, meaning it generates heat, which helps in maintaining the optimum temperature range for microbial activity.
- The digestion tank is designed to ensure proper mixing and aeration to enhance the digestion process.
- The duration of aerobic digestion can vary depending on the specific conditions and the desired degree of sludge stabilization.
- After the digestion process is complete, the sludge is further treated to remove any remaining pathogens or unwanted substances before it can be safely disposed of or used as a fertilizer.

Advantages of aerobic digestion:
- Efficient breakdown of organic matter: Aerobic digestion allows for the complete breakdown of complex organic compounds, resulting in a more stable and less odorous sludge.
- Pathogen reduction: The presence of oxygen helps in the destruction of pathogenic organisms present in the sludge, reducing the risk of disease transmission.
- Reduced odor: The aerobic conditions promote the growth of odor-reducing bacteria, minimizing the unpleasant odor associated with anaerobic digestion.
- Shorter retention time: Compared to anaerobic digestion, aerobic digestion typically requires a shorter retention time, allowing for faster sludge treatment.

In conclusion, sludge digestion tanks involve aerobic suspended culture, where the sludge is treated in the presence of oxygen to promote the growth of aerobic microorganisms and facilitate the breakdown of organic matter. This process has several advantages, including efficient organic matter breakdown, pathogen reduction, reduced odor, and shorter retention time.