What is sludge digestion, and what are the two main products of anaerobic digestion?

• A. Biological Process That Stabilizes Sludge
• B. Physical Process; Produces Solids
• C. Biological Process that Stabilizes Sludge; Produces Stabilized Biosolids and Biogas
• D. Thermal Process; Produces Steam and Sludge Cake

Answer: (C)

Sludge digestion is a biological process that stabilizes sludge by breaking down organic matter in the absence of oxygen. In anaerobic conditions, microorganisms convert the organics into a stable digestate and biogas. The two main products are stabilized biosolids, also called digestate, which have reduced odor, mass, and pathogens and can be disposed of or reused more safely; and biogas, a methane-rich fuel that can be captured for energy recovery. This combination highlights why the process is not just a physical or thermal treatment—it relies on microbes to transform the sludge into usable energy and safer, stable solids.