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Biological Reactors 743

Gas collection

Floating cover
Circulation
Multiple
sludge inlets
Gas collection

Multiple sludge outlets
Compressed gas
Diffusers

FIGURE 23.10 Complete-mixing (‘‘high-rate’’) anaerobic digester.

kinetics. First, however, the reactions must be delineated (see ‘‘first stage fermentation,’’ or the ‘‘acid production phase.’’
also Section 22.2.3.4). The reaction in simplified form is depicted,

23.4.3.1 Reactions organic molecule þ enzymes (acid formers)
Anaerobic digestion and anaerobic treatment are both fermen- ! enzymes (acid formers) þ cells (acid formers)
tation reactions. Fermentation is by definition a reaction in þ organic acids þ CO 2 þ other products (23:34)
which the reactant organic molecule acts as both the electron
donor and electron receiver (in an aerobic reaction oxygen is The organic acids are predominately those listed in Table
the electron receiver). The organic molecules ordinarily found 22.6, i.e., formic, acetic, propionic, butyric, valeric, isovaleric,
in wastewater sludge and high-strength wastes include fats, and caproic (McCarty, 1964a). These acids, in turn go to
starches, sugars, proteins, etc. To convert these molecules to methane. The product fractions along the different paths
fermentation products requires enzymes. A good many bac- from the complex organics to organic acids, and in turn
teria have the necessary enzymes. Many in fact are facultative from the organic acids to methane are shown in Figure
and need only a period of acclimatization. These bacteria 23.11. This figure, published originally in 1964 in Public
break down organic molecules along complex biochemical Works magazine in Part 1 of a four-part series by McCarty
pathways. For example, Escherichia coli, in fermenting (1964a,b,c,d) has been probably the most frequently cited
100 m moles of glucose will yield at pH 6.2: 1 and 2 macro- figure and article series on the topic of the anaerobic process.
moles glycerol, 423.4.8 ethanol, 2.43 formic acid, 36.5 acetic Figure 23.12 is a schematic adaptation from the original
acid, 723.4.5 lactic acid, 10.7 succinic acid, 88.0 carbon figure, which gives the numerical values.
dioxide, and 75.0 hydrogen (Andrews et al., 1964). Other Methane fermentation is the second stage in the overall
bacteria will yield a different mix of these products and process. It is characterized by low cell yield, a relatively slow
perhaps different compounds. The compounds formed are reaction velocity, and rather fragile ecological requirements
the final end products and represent what is usually called (i.e., pH should be about neutral, temperature should be about

Complex organics
1.0 fraction COD
0.15 0.20 0.65

Propionic acid 0.15 Other intermediates
0.17 0.35
0.13 0.15
Acetic acid
0.72

Methane

FIGURE 23.11 Pathways in methane fermentation. (Adapted from McCarty, P.L., Anaerobic waste treatment fundamentals, Part one—
Chemistry and microbiology, Public Works, September 1964a, p. 111.)