AEROBIC DIGESTION OF AN INDUSTRIAL
WASTE ACTIVATED SLUDGE
Philip S. Pagoria, Assistant Professor
William A. Drewry, Professor
Department of Civil Engineering
Old Dominion University
Norfolk, Virginia 23508
INTRODUCTION
Aerobic digestion of waste activated sludge is a widely applied stabilization technique
in small wastewater treatment systems, both municipal and industrial.   Application of
the process is founded largely on its simplicity of operation, an important criterion at
small scale facilities which cannot justify large capital expenditures nor highly trained
wastewater management personnel.   However, successful operation of the aerobic digestion process to produce a sludge with a desired degree of stabilization may be complicated by variability in characteristics of the sludge being fed to the system.   This
concept was supported by Hegg and Rakness [ 1 ]  when they recommended minimum
solids retention time (SRT) values for various sludges which included both the SRT
during digestion plus the SRT of the sludge in its source process (e.g., SRT of the
activated sludge process).   The purpose of this paper is to examine the variability of
one industrial waste activated sludge and demonstrate how this variation can complicate
the interpretation of aerobic digestion performance.
The source of the sludge under investigation was a complete-mix activated sludge
system of approximately 1.0 mgd (0.044 cu m/sec) capacity, the activated sludge
system being preceded by primary sedimentation.    In this study aerobic digestion of
only waste activated sludge, and not primary sludge was considered.   The sole source
of wastewater for the treatment system was a book cloth mill engaged in the production of bookbinding fabrics produced from starch sized cotton goods.    The wastewater
contained both production wastes and sanitary wastes from the mill employees.
PROCESS DESIGN CRITERIA
Common design criteria for the aerobic digestion process are listed in Table I [2,3].
Of particular importance to this investigation is the selection of a measurement technique for microbial mass concentration and the metabolic activity level of this biomass.
This measurement represents the critical operational control parameter which is utilized
to quantify both the degree of sludge stabilization achieved and, in the case of laboratory or pilot scale studies, to quantify stabilization rate kinetics for process design.
The focus of the present study is on this criterion and its relationship to aerobic digestion process control.
MEASURES OF MICROBIAL MASS CONCENTRATION AND ACTIVITY
Table II presents the parameters which were monitored during the experimental
phase of this investigation, to be described later, as measures of microbial mass concentration and metabolic activity.   The limitations of volatile suspended solids (VSS) as an
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