text: 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 is pumped 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. These waste gases require treatment to reduce odours in works close to housing or capable of generating public nuisance. The digestion is continued until the percentage of degradable solids is reduced to between 20% and 10% depending on local conditions. 
  Where non-sewage waste is being processed, organic waste such as food, cardboard and horticultural waste can be significantly reduced in volume leaving an output that can be used as soil improver or biomass fuel.