A Study of Aerobic Sludge Digestion
Comparing Pure Oxygen and Air
ROBERT M. STEIN, Field Director
C. E. ADAMS, President
Associated Water and Air Resources Engineers, Inc.
Nashville, Tennessee
WILLIAM J. JEWELL, Assistant Professor
Department of Civil Engineering
University of Vermont
Burlington. Vermont
W. WESLEY ECKENFELDER, Professor
Department of Environmental and Water Resources Engineering
Vanderbilt University
Nashville, Tennessee
INTRODUCTION
Sludge dewatering and disposal is one of the most pressing problems in waste
treatment facilities. Inorganic suspended solids can be handled by conventional techniques
with little difficulty, however increased technology is required for solids separation and
disposal of organic solids.
Aerobic digestion is a process designed to convert the biodegradable portion of waste
activated sludge and primary sludge into carbon dioxide and water. The conversion is
accomplished by microbial metabolism and extended periods of aeration. The ideal
product would be a stable humus material that is easily dewatered.
There have been a number of recent technological developments which have increased
the feasibility of pure oxygen as a factor in the biological waste treatment field. As a
conclusion of the Batavia report, Albertsson. et al {I) indicated that it would be desirable to
evaluate oxygen aerated aerobic digestion of waste activated sludge.
The objective of this investigation was to compare air and pure oxygen aeration in
aerobic digestion of a waste activated sludge from a brewery waste treatment system.
Aerobic digestion of activated sludge differs from that of primary sludge in that
activated sludge is a product of microbial metabolism, and auto-oxidation of the bacteria
results in the degradation of the biological sludge. The auto-oxidation of the bacterial cells
allows: 1) oxidation of organic carbon in the waste system so that some will be lost from the
system as carbon dioxide; and 2) recycling of the nutrients contained in the bacterial protein
for use in synthesis. The basic formula for which aerobic digestion takes place is:
microorganisms   rn  + w n
Volatile solids + 02 •=: ^-°2    H2°
nutrients
A number of factors which control the aerobic digestion process, the major ones
appear to be: I) concentration of biodegradable volatile solids; 2) temperature; 3) mixing: 4)
oxygen requirements; 5) characteristics of the solids; 6) nutrient concentration; 7)
microorganism concentration; 8) type of microorganisms present; 9) physiological
conditions of the microorganisms; and 10) detention time.
It should be noted that, there is a distinction between biodegradable volatile solids and
volatile solids. It has been found by Kountz (2), that the non-biodegradable portion is
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