TM 5-814-3/AFM 88-11, Volume III
16-6. Sludge digestion.
a. Aerobic sludge digestion. The major function of sludge digestion (and its principal advantage) is the
stabilization of the sludge in terms of volatile content and biological activity. Aerobic digestion accomplishes
this through biological oxidation of cell matter which is done without the production of volatile solids or high
biochemical oxygen demand liquor associated with anaerobic digestion.
(1) Modes of operation. Aerobic digesters can be either continuous or intermittent batch operations.
With batch operation, waste sludge feed will be discontinued at a specified time before digested sludge
withdrawal. In continuous operation, supernatant is constantly withdrawn. This mode of operation is used
when phosphorus is a problem and low phosphorus levels are required in the effluent because batch operation
produces high phosphorus concentrations in the supernatant.
(2) Design factors. A summary of design factors is given in table 16-13. The tank is open, which can
be a problem in cold climates with mechanical aeration; no heating is required although some increase in
volatile solids reduction can be obtained with increased temperature. Tank design is similar to aeration basin
design with the addition of a sludge thickening apparatus. A major disadvantage of aerobic digestion is the
high energy requirement.