Figure 2-24. Contact stabilization.
Digesters for two-stage digestion.
(Primary digester [left] has fixed cover; secondary digester [right] has gas-holder cover.)
a. General. Sedimentation or settling following biochemical processes is an
essential part of secondary treatment of wastewater although it is primarily a physical
process. The settling tanks and operating mechanisms are of the same or similar
design as those used in primary sedimentation (see para 2-5c). The retention period is
shorter than for primary sedimentation and the quantity of solids or sludge removed is
usually much less. The sludge is usually transferred to digestion tanks along with
sludge from primary sedimentation units (see para 2-6, see Figure 2-16).
b. Sludge Removal. Secondary sedimentation units do not require as much
attention as primary units, but a regular cleaning and frequent removal of sludge is