This chapter offers a comprehensive overview of sludge digestion equipment utilized in water and wastewater treatment processes. It begins with the definition and classification of sludge digestion, along with development trends in China and other countries. Anaerobic digestion of sludge is discussed in detail, including the process flow, types and design of digestion tanks, and supporting ancillary equipment. The chapter further covers aerobic sludge digestion, outlining basic principles and different process types, as well as high-temperature aerobic fermentation, including both process steps and specialized equipment. Lime stabilization is also presented, detailing the underlying principles, stabilization procedures, and control parameters crucial for successful operation. Safety operating procedures specific to anaerobic digestion systems are included, encompassing system startup, inlet/outlet control, temperature, alkalinity, volatile acids, pH management, and overall safety. The chapter concludes with common faults and troubleshooting techniques for anaerobic digestion systems, supported by a summary table for practical reference.