The contact stabilization activated sludge process
is characterized by a two-step aeration system.
Aeration of short duration (1/2–2 h) is provided
in the contact tank where raw or primarysettled
in a final settling tank. The settled activated
sludge to be recycled from the final clarifier
is drawn to a separate re-aeration in a stabilization
basin for 3–8 h of aeration time