Aerobic Digestion of Solids
D. E. DREIER, Director of Engineering and Research,
and Vice President
Walker Process Equipment, Inc.
Aurora, Illinois
If micro-organisms are to synthesize new cells, or merely to remain alive,
they must oxidize organic matter either aerobicaUy or anaerobically in order to
obtain enough energy Tor these purposes.
Aerobic biological processes can be described by the basic equation:
Organic Matter + 02 + NH3 = Sludge CeUs + CO2 + H2O
In the early stages of an aerobic process, such as activated sludge with an acclimatized sludge established, if there is an unlimited food supply with proper nutritional balance, the micro-organisms are in the log growth stage and the bacterial growth is limited only to the micro-organisms' ability to reproduce. The
oxygen uptake rate is increasing due to absorption of the organic load and the synthesis of new sludge ceUs. As the oxidation of the organic load proceeds, a declining growth phase is reached due to limitation in the available food, and the
oxygen uptake rate also declines. When there is just enough available food to keep
the micro-organisms alive, a so-called endogenous metabolism exists. When the
substrate is unable to supply sufficient organic matter for synthesis and energy, and
the rate of destruction exceeds the rate of growth, the micro-organisms obtain energy by autodigestion of the ceUs protoplasm and the biologically degradable organic matter in the sludge ceUs is oxidized to carbon dioxide, water, and ammonia. This latter phase is the aerobic digestion with which this paper is
concerned, and its position relative to the activated sludge system is illustrated
schematically in Figure 1.
As aerobic digestion proceeds, the ammonia is converted to nitrates. The
oxygen uptake rate levels off, and mineralization increases due to destruction of
volatile solids. The residue consists of fixed solids and of volatile solids resistant
to further bio-destruction.
RESEARCH STUDIES
Hood (1), in work at Ridgewood, New Jersey, showed from correlation studies
of ecological classifications with oxidation-reduction potential that a well-
environed aerobic digester is in the oligosaprobic zone characterized by reducing
numbers of bacteria, high mineralization, low organic nitrogen, and a high steady
positive oxidation-reduction potential of 700 to 800 millivolts with a flat slope
with respect to time.
Eckenfelder (2) studied aerobic digestion by aerating four-liter samples of
waste activated sludge from a conventional activated sludge plant for a period of
seven days with the temperature maintained at 25 C. His data showed a reduction
in COD of 48. 5 per cent, a reduction in suspended volatile solids of 38. 2per cent,
a decrease in volatile soUds content from 76. 5 to 63. 5 per cent and an increase
in soluble nitrogen from 46 to 94 ppm in the seven days of aeration.
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