Explain anaerobic sludge digestion and its benefits.

Anaerobic digestion stabilizes organic sludge in absence of oxygen through acid-forming and methane-forming bacteria in two stages. Primary digestion occurs at 30-35C (mesophilic) or 50-55C (thermophilic) for 15-30 days, reducing volatile solids by 40-60%. Benefits include: pathogen destruction, volume reduction, odor control, and biogas production (60-70% methane) for energy recovery. Digested sludge is easier to dewater and suitable for land application.