The Effect of Temperature and Oxygen
Tension on One of the Microorganisms
Responsible for Sludge Bulking
PHILIP H. JONES, Graduate Student
Department of Civil Engineering
Northwestern University
Evanston, Illinois
INTRODUCTION
Various mechanisms have been proposed to explain sludge bulking; such as,
bound water causing dispersion and consequent reduction in specific gravity of
the sludge (1), the presence of a waste rich in carbohydrates favoring the growth of
filamentous microorganisms, reduced oxygen tension, and even low temperature
air supply to the aeration tanks (2). In fact, quite a large number of seemingly
unrelated^ factors have been accused of contributing to bulking.
Microscopic examination of bulking sludge usually reveals the presence of a
loose mesh of filamentous microorganisms. These filamentous microorganisms
were originally known by the general term of "sewage fungus." More recently,
the filamentous organism most consistently reported as being dominant in bulking
sludge has been identified as sphaerotilus natans. Sphaerotilus is, of course, a
higher bacterium and not a fungus, even though on occasions it displays many
fungal properties.
In most reports which have been published on the subject of bulking, it can
be noted that some fungus was usually reported to be present also. It seems likely
from the great diversity of reports on the subject, that a fairly large group of
morphologically similar, filamentous microorganisms could, and probably have
been mistaken for Sphaerotilus. In some of the early reports (3), Smit published
some photomicrographs of a variety of filamentous microorganisms isolated from
bulking sludge which he preferred not to name specifically. In one print, he
showed a filamentous growth which he proposed to be named Geotrichoides palu-
dosus, a new species offungus.
The writer developed an interest in sludge bulking almost two years ago and,
using various" enrichment techniques, described in the literature for the isolation
of Sphaerotilus, isolated a filamentous microorganism. It was not until a little
less than a year had gone by that the organism was finally identified as a member
of the Geotrichum   genus.
Laboratory experiments were performed attempting to simulate bulking conditions, using several of the causative factors suggested in the literature as being
instrumental in bringing about bulking. In these studies, it was found that
Sphaerotilus could occasionally be induced to grow in relatively large numbers,
but it was further noticed that one other organism was found more frequently and
in larger numbers.    The filamentous microorganism in question was isolated and
identified as Geotrichum candidum    (4, 5).
Reference to the literature will readily reveal that this organism has rarely
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