What is the process called when sludge digestion occurs without the presence of free oxygen?

The process of sludge digestion that occurs without the presence of free oxygen is known as anaerobic decomposition. In anaerobic conditions, microorganisms break down organic material in the absence of oxygen, which is crucial for treating sewage effectively. This process not only helps in managing waste by reducing the volume of sludge but also produces biogas, which can be harnessed as a renewable energy source.

Anaerobic digestion is a key component of many sewage treatment systems, as it contributes to the stabilization of organic matter and minimizes the environmental impact of waste disposal. The efficiency of anaerobic digestion can be influenced by factors such as temperature, pH, and the composition of the sludge.

Other processes mentioned, like wet oxidation and heat drying, do not involve anaerobic conditions and serve different purposes in sewage treatment. Wet oxidation uses oxygen under high pressure and temperature to oxidize organic material, while heat drying typically involves removing moisture from sludge through thermal means. Detergent compounds do not represent a process of digestion but rather substances that can interfere with treatment operations. Therefore, anaerobic decomposition is the correct answer as it accurately describes the fundamental process occurring in the absence of oxygen during sludge digestion.