Handbook of Civil Engineering Calculations

Calculation Procedure:

1. Find the daily volume of sludge for disposal
   Factors that must be considered in designing aerobic digesters include temperature, solids
   reduction, tank volume (hydraulic retention time), oxygen requirements and energy re-
   quirements for mixing.
   Because the majority of aerobic digesters are open tanks, digester liquid temperatures
   are dependent upon weather conditions and can fluctuate extensively. As with all biologi-
   cal systems, lower temperatures retard the process, whereas higher temperatures acceler-
   ate it. The design of the aerobic digester should provide the necessary degree of sludge
   stabilization at the lowest liquid operating temperature and should supply the maximum
   oxygen requirements at the maximum liquid operating temperature.
   A major objective of aerobic digestion is to reduce the mass of the solids for disposal.
   This reduction is assumed to take place only with the biodegradable content (VSS) of the
   sludge, although there may be some destruction of the inorganics as well. Typical reduc-
   tion in VSS ranges from 40 to 50 percent. Solids destruction is primarily a direct function
   of both basin liquid temperature and sludge age, as indicated in Fig. 3. The plot relates
   VSS reduction to degree-days (temperature x sludge age).
   To ensure proper operation, the contents of the aerobic digester should be well mixed.
   In general, because of the large amount of air that must be supplied to meet the oxygen re-
   quirement, adequate mixing is usually achieved. However, mixing power requirements
   should always be checked.
   The aerobic digester will operate 7 days per week, unlike the thickening facilities
   which operate intermittently due to larger operator attention requirements. The thickened
   sludge is input to the digester at 12,823 gal/day (48.5 L/d), 5 days per week. However, the
   volume of the sludge to be disposed of daily by the digester will be lower due to its oper-
   ation 7 days per week (the "bugs" do not take the weekends off). Therefore the volume of
   sludge to be disposed of daily (0 is:

Qt = (12,823 gal/day)(5/7) = 9,159 gal/day = 1,224 ftVd (34.6 m

3

/d)

Temperature

0

C x sludge age, days

FIGURE 3. VSS reduction in aerobic digester vs. liquid tempera-

ture x sludge age. (Metcalf & Eddy, Wastewater Engineering:

Treatment, Disposal, and Reuse, 3rd Ed., McGraw-Hill

Volatile

solids reduction,

%