WASTEWATER MICROBIOLOGY 22-15
E q uations 22-16 and 22-19 are a fundam ental part of the development of the design equa-
tions for wastewater treatment processes.
22 -6 DECOMPOSITION OF WASTE
The type of electron acceptor available for catabolism determines the type of dec omposition
(i.e., aerobic, anoxic, or anaerobic) used by a mix ed culture of microorganisms. Each type
of decom position has particular characteristics that affect its use in wastewater treatment.
In the following paragraphs
a general discussion of the three types of decomposition is
presented.
Aerobic Decomposition
Molecular oxygen (O
2
) must be present as the terminal electron acceptor for decomposition to
proceed by aerobic oxidation. The oxygen is measured as dissolved oxygen (DO). When oxygen
is present, it is the only terminal electron acceptor used. The chemical end products of decompo-
sition are pri
marily carbon dioxide, water, and new cell material. Odiferous gaseous end products
are kept to a minimum. In healthy natural water systems, aerobic decomposition is the principal
means of self purification.
A wider spectrum of organic material can be oxidized aerobically
than by any other type of
decomposition. This fact, coupled with the fact that the final end products are oxidized to a very
low energy level, results in a more stable end product (i.e., one that can be disposed of without
damage to the environment and without creating a nuisance condition) than can be achieved by
the other oxidation systems.
Because of the large amount of energy released in aerobic oxidation, most aerobic organisms
are capable of high growth rates. Consequently, there is a relatively large production of new cells
in comparison with the other oxidation systems. This means that more biological sludge is
gener-
ated in aerobic oxidation than in the other oxidation systems.
Aerobic decomposition is the method of choice for large quantities of dilute wastewater
(BOD
5
less than 500 mg/L) because decomposition is rapid, efficient, and has a low odor
potential. Typically, aerobic decomposition is not suitable for high strength wastewater (BOD
5
is greater than 1,000 mg/L) because of the difficulty in supplying enough oxygen and because of
the large amount of biological sludge produced. However, in small communities and in special
industrial applications where aerated lagoons are used, wastewater with BOD
5
up to 3,000 mg/L
may be treated satisfactorily by aerobic decomposition. This is because the daily influent
volume of wastewater is small, the detention time is long, and the lagoon acts as a complete mix
reactor.
Anoxic Decomposition
Some microorganisms can use nitrate (NO
3
as the terminal electron acceptor in the absence of
molecular oxygen. Oxidation by this route is called denitrification.
The end products from denitrification are nitrogen gas, carbon dioxid e, water, and new c ell
material. The amount of energy made available to the cell during denitrification is about the
same
as that made available during aerobic decomposition. As a consequence, the rate of production of
new cells, although not as high as in aerobic decomposition, is relatively high.
Denitrification is of importance in wastewater treatment where nitrogen must be removed
to protect the receiving body. Another important aspect of denitrification is
in relation to final