Anaeorbic digesters at the Throop plant are used to break down organic material in the primary and waste activated sludge. "Anaerobic" means that there is no oxygen in the treatment tank. The bacteria in this treatment process generate combustible methane gas.
Some of the methane gas produced in the digester is burned as fuel and used to heat the sludge.
The Archbald plant uses aerobic digesters. Air is bubbled through the tank to provide oxygen to the microorganisms that break down the sludge.