378 CHAPTER THIRTEEN
In the anaerobic digestion process, the organic material is converted biologi-
cally, under anaerobic conditions, to a variety of end products including
methane (CH 4 ) and carbon dioxide (CO 2 ). The process is carried out in an air-
tight reactor. Sludge, introduced continuously or intermittently, is retained in
the reactor for varying periods of time. The stabilized sludge, withdrawn con-
tinuously or intermittently from the reactor, is reduced in organic and pathogen
content and is nonputrescible.
In the high rate digestion process, as shown in Fig. 13.7, the contents of the
digester are heated and completely mixed. For a complete-mix flow through
digester, the mean cell residence time ( (^) c) is the same as the hydraulic retention
time ( ).
The digester volume Vrequired is
V cQ (13.69)
whereVrequired volume, ft^3 (m^3 )
(^) crequired hydraulic retention time, min
Qinfluent sludge flow rate, gal/day (m^3 /day)
Then, the
Volumetric loading (BOD/day)/V (13.70)
The volumetric loading is expressed in lb/ft^3 ( daykg/m^3 day).
For high rate digesters, loading range from 0.10 to 0.35 lb/ft^3 d (1.6 to 5.61
kg/m^3 d).
The quantity of volatile solids produced each day is
Px (13.71)
Y[(BODin, lb/d)(BODout, lb/d)]
1 kd (^) c
Fixed cover
Sludge
outlets
Sludge
inlets
Sludge heater
Gas storage
Methane + Carbon dioxide
Mixer
FIGURE 13.7 High-rate single-stage complete-mix anaerobic digester. (Adapted from
Metcalf & Eddy, Wastewater Engineering: Treatment, Disposal, and Reuse, 3rd ed.,
McGraw-Hill.)