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
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.