Environmental Engineering FOURTH EDITION

Sludge Treatment and Disposal 21 1

Extracellular

enzymes

Soluble organics

e

Acid-producing

bacteria

I“

products

Methane-producing

bacteria

Figure 10-3. Generalized biochemical reactions in anaerobic sludge digestion.

formers have been inhibited in some way. The acid formers keep chugging away,
making more organic acids, thus further lowering the pH and making conditions even
worse for the methane formers. Curing a sick digester requires suspension of feeding
and, often, massive doses of lime or other antacids.
Most treatment plants have both a primary and a secondary digester (Fig. 10-4).
The primary digester is covered, heated, and mixed to increase the reaction rate. The
temperature of the sludge is usually about 35°C (95°F). Secondary digesters are not
mixed or heated, and are used for storage of gas produced during the digestion process
(e.g., methane) and for concentrating the sludge by settling. As the solids settle, the
liquid supernatant is pumped back to the main plant for further treatment. The cover
of the secondary digester often floats up and down, depending on the amount of gas
that has accumulated. The gas produced during anaerobic digestion contains enough in
methane to be used as a fuel, and is frequently used to heat the primary digester. Large
wastewater treatment facilities may produce enough methane to sell to local utility
companies as natural gas. The city of Portland, OR is using an innovative approach to
convert methane from anaerobic digestion into hydrogen for fuel cells. Unfortunately,
many wastewater treatment facilities still “waste” (burn off) the gas from anaerobic
digestion. As the availability of fossil fuel natural gas decreases, and prices increase,
biologically produced methane may gain in economic value.
Anaerobic digesters are commonly designed on the basis of solids loading.
Experience has shown that domestic wastewaters contain about 120 g (0.27 lb) of