Calculation Procedure:- Determine the required digester volume and loading
Anaerobic digestion is one of the oldest processes used for the stabilization of sludge. It
involves the decomposition of organic and inorganic matter in the absence of molecular
oxygen. The major applications of this process ar,e in the stabilization of concentrated
sludges produced from the treatment of wastewater.
In the anaerobic digestion process, the organic material is converted biologically, un-
der anaerobic conditions, to a variety of end products including methane (CH 4 ) and car-
bon dioxide (CO 2 ). The process is carried out in an airtight reactor. Sludge, introduced
continuously or intermittently, is retained in the reactor for varying periods of time. The
stabilized sludge, withdrawn continuously or intermittently from the reactor, is reduced in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, the contents of the digester are
heated and completely mixed. For a complete-mix flow through digester, the mean cell
residence time (0C) is the same as the hydraulic retention time (B).
In the United States, the use and disposal of sewage sludge is regulated under 40 CFR
Part 503 promulgated February 1993. The new regulation replaces 40 CFR Part 257—the
original regulation governing the use and disposal of sewage sludge, in effect since 1979.
The new regulations state that "for anaerobic digestion, the values for the mean-cell-
residence time and temperature shall be between 15 days at 350 C (95^0 F) to 550 C (131^0 F)
and 60 days at 2O^0 C (68^0 F)."
Therefore, for an operating temperature of 350 C (95^0 F), a mean cell residence time of
15 days will be used. The influent sludge flow rate (0 is 60,000 gal/day = 8,021 fVVd
(226.9 m^3 /d). The digester volume Vrequired is computed using
V = (^0) CQ V= (15 </)(8,021 ftVd) = 120,315 ft^3 (3,404.9 m^3 )
Methane + Carbon Dioxide
Sludge
Outlets
FIGURE 13. High-rate single-stage complete-mix anaerobic digester. (Adapted from
Metcalf & Eddy, Wastewater Engineering: Treatment, Disposal, and Reuse, 3rd Ed.,
McGraw-Hill.)
Sludge Heater
Sludge
Inlets
Fixed Cover
GasStorage