This chapter discusses aerobic digestion as a separate treatment after solids-liquid separation is complete. Aerobic digestion is a process in which microorganisms obtain energy by endogenous or auto-oxidation of their cellular protoplasm. The biologically degradable constituents of cellular material are slowly oxidized to carbon dioxide, water, and ammonia, with ammonia being further converted to nitrates during the process. The mechanism by which wastewater sludges is aero-bically stabilized depends on the type of sludge being treated for primary sludge, aerobic digestion follows a series of steps similar to anaerobic digestion. In the aerobic digestion of primary sludge, the primary sludge acts as a food supply for microorganisms. The aerobic digestion of excess secondary sludge is a continuation of the activated-sludge process. Aerobic digestion is usually applied to extended-aeration or contact-stabilization, activated-sludge plants. Aerobic digesters are similar to conventional activated-sludge tanks in that they are not covered or insulated.