The Effect of Nitrogen and Phosphorous
Compounds on One of the Micro-organisms
Responsible for Sludge Bulking
PHILIP H. JONES, Assistant Professor
Civil Engineering Department
University of Toronto
Toronto, Canada
INTRODUCTION
Under certain circumstances activated sludge may develop poor settling
properties; when this occurs the sludge is said to be "bulking." Bulking is a biophysical response to some upset in the ecological balance of a mixed population
of micro-organisms. Several mechanisms have been proposed to explain this
phenomenon, such as bound water causing dispersion of floe (1) low temperature
air supply to aeration tanks, and the presence of a waste rich in carbohydrates (2).
This latter cause is perhaps the most commonly occurring in municipal sewage-treatment plants. Wastes from breweries or canneries might be considered as
high carbohydrate wastes. One of the interesting features of wastes rich in carbohydrates is that frequently they are relatively poor in nitrogen and phosphorous
compounds. Thus without actually changing the quantities of nitrogen and phosphorous present in a plant the net effect is a N or P deficient waste due to the
change in BOD/N and BOD/P ratios. Bacteria has been reported to contain 10 to
12 per cent nitrogen, whereas fungi contain only four to six per cent.
In view of the fact that sludge bulking is frequently associated with high
carbohydrates in the waste it was decided to investigate the possibility that tne
filamentous micro-organisms actually become dominant due to the indirect effect of increasing BOD/N or BOD/P ratios and not directly as a result of the carbohydrate in the waste.
Microscopic examination of bulking sludge usually reveals a fairly loose mesh
of filamentous micro-organisms. In earlier reports (3) these micro-organisms were
generally described as sewage fungus." The tendency today is to apply generic
names to the organisms where possible. This may have led to a certain amount of
confusion, due to misidentification on the basis of gross similarities between different micro-organisms. The micro-organism most consistently associated with
sludge bulking has been Spkaerotilus natans. Spbaerotilus is defined as a higher
bacterium and not a fungus even though occasionally it displays many fungal
properties.
Using techniques described in the literature to isolate Sphaerotilus natans,
the writer isolated a filamentous micro-organism which was subsequently identified (4,5) as Geotrichum candidwn. Reference to the literature will reveal that
this organism has rarely been reported as being present in significant numbers in
bulking sludge. This, however, may be because (1) the microbiologists and sanitary engineers have not been alerted to its possible presence, ana (2) its gross
morphological similarities to Sphaerotilus spp. Hawkes(6) reported the presence
of >Geotricbum\ in an occurrence of bulking in an activated sludge plant in Birmingham England. In this case Geotrichum was suggested as the causative organism.
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