Sludge from clarifiers is sent to a digester where the organic matter is digested by microorganisms.  This reduces the amount of sludge which must be sent to waste.  

Digesters can be aerobic or anaerobic.  

The anaerobic digester is a closed tank in which anaerobic organisms slowly digest the organic matter in sludge.  These organisms produce methane, hydrogen sulfide, and carbon dioxide in the process.  The solids are removed from the tank and sent to a drying bed for eventual land application while the supernatant is drawn off and sent back into the wastewater treatment system.  

Aerobic digesters differ from the anaerobic digester described above by being aerated, with air blown through the sludge. The aeration allows aerobic bacteria to digest the sludge.  
Once the solids in the aerobic digester reach about 4%, the aerators are turned off and the sludge is allowed to settle.  Then the sludge can be pumped to the drying beds for eventual land application.