Application of Aerobic Digestion to
Industrial Waste Treatment
EDWIN L. BARNHART, Research Associate
Manhattan College
Englewood Cliffs, New Jersey
The principle of auto-oxidation of activated sludges has long been
accepted in sanitary engineering practice. In recent years this principle has
been used in designing so-called complete treatment or "total oxidation"
units. In practice these units, utilizing long mixed liquor detention times,
have been able to limit, to a large extent, the accumulation of excess activated sludge. Recent practice has also shown that it is possible to utilize
auto-oxidation in the design of aerobic digestors as a separate treatment
unit.
The term "aerobic digestion" as used in this paper is related to a process
wherein thickened sludges separated from their associated liquors undergo
stabilization during prolonged aeration.
In order to establish the feasibility of employing aerobic digestion in the
treatment of the wide variety of sludges produced by industrial waste treatment, numerous laboratory and pilot plant studies have been conducted. This
paper presents the results of these studies.
The problems encountered in the digestion of sludge are many and varied;
by and large they do not lend themselves to complete investigation at laboratory scale. It is believed, however, that the following parameters may be
properly considered in laboratory studies: the ultimate reduction of volatile
suspended solids and the approximate time required for that reduction; the
oxygen required to bring about this solids reduction; and the susceptibility of
the digested sludge to various forms of dewatering.
Other parameters such as effluent, pH, alkalinity, nutrient value, and
BOD are also important and the information available on these factors will
also be discussed.
SOLIDS REDUCTION
It has been shown by several investigators that (1-2) the rate of auto-
oxidation in a continuous activated sludge system may be expressed as a
constant per cent of the volatile suspended solids. Studies on prolonged aeration of sludges under limited food supply have described the solids degrading
in terms of a retardant reaction, the rate of reaction decreasing with aeration
time.
Previous work also indicates that, for any sludge, there is a definite limit
to the reduction in volatile solids which may be expected. This observation
is confirmed by Jaworski et al. (3) who reports that the solids reduction in 60
days is not significantly more than after 15 days aeration time. Present data,
Figure 1, further illustrates this point.
For the purposes of these studies it was desirable to be able to compare
easily the results obtained on many different sludges. Therefore, simpler
formulations than the retardant form were investigated.   These investigations
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