Aerobic digestion is typically used in an activated sludge treatment plant. Waste activated sludge and primary sludge are combined, where appropriate, and passed to a thickener where the solids content is increased.

This substantially reduces the volume that is required to be treated in the digester. The process is usually run as a batch process with more than one digester tank in operation at any one time. Air Blowers are used to provide air through the tank and the contents are stirred to keep the contents fully mixed. Carbon dioxide, waste air and small quantities of other gases including hydrogen sulfide are given off.