Which statement best describes anaerobic digestion in sludge treatment?

• A. It requires oxygen and increases sludge volume.
• B. It produces biogas mainly methane and reduces sludge volume.
• C. It produces primarily carbon dioxide with no energy recovery.
• D. It eliminates all biodegradable waste instantly.

Answer: (B)

Anaerobic digestion is a process that happens without oxygen, carried out by specialized microorganisms that decompose organic matter in sludge. In this environment, organic material is converted through a sequence of steps into methane and carbon dioxide, releasing energy stored in the bonds of the organics. The key outcome is not only stabilization of the sludge and decrease in volatile solids, but also the generation of biogas, which is rich in methane and can be used for energy. This means you get a net reduction in sludge volume because a portion of the solids is transformed into gas rather than remaining as solid matter. The methane-rich biogas is the valuable energy product of the process, rather than CO2 alone or no energy recovery. The idea that it eliminates all biodegradable waste instantly is impossible in practice, as digestion requires time and proper conditions to proceed to completion.