14    INFLUENCE OF So/Xo RATIO
AND MEDIUM COMPOSITION ON
ANAEROBIC BIODEGRADABILITY TEST
German Buitron, Research Leader
Gloria Moreno, Graduate Student
Environmental Bioprocesses Department,
Institute of Engineering
National University of Mexico
C.U., Apartado Postal 70-472
04510 Mexico, D.F., MEXICO
ABSTRACT
The effect of the composition of the culture medium and the variation of the substrate/microorganism ratio during the development of anaerobic biodegradability test was studied. An automatic monitoring system was developed for continuous and parallel monitoring of nine reactors
measuring the pressure generated by the biogas produced during the anaerobic biodegradation of
the substrate. With this system, the experimentation was carried out in serum bottles at 35°C
with anaerobic sludge acclimated to ethanol. The influence of three different culture media as
well as the effect of the initial substrate concentration to the initial biomass concentration ratio,
So/Xo, on the extent of anaerobic biodegradability was studied. It was observed that the composition of the medium has an important influence on the results. The degradation rate of substrate
increased when the So/Xo ratio increased (under agitation). Because of that, it is necessary to
make an effort to unify criteria including media composition and So/Xo ratio to achieve a possible standardization of the test.
INTRODUCTION
Biodegradability tests are conducted in order determine the susceptibility of an effluent to be
treated under specific environmental conditions; i.e., aerobic or anaerobic environment. In the
case of the anaerobic biodegradability tests, a common procedure utilized is the measure of the
biogas produced by the microorganisms during the organic substrate degradation. The quantification of the biogas production can be achieved with manual or automatic methodologies. Pressure
increase, due to the biogas generation in a closed bottle, is one of the techniques frequently used
to monitor online the extent of the biodegradation process.1-2
It has been observed that the results of the biodegradation tests could vary upon the methodology followed. The most important variables which influence the results are: the initial activity of
the microorganisms, the medium composition and the initial substrate concentration to the initial
biomass concentrations ratio, So/Xo. Some authors have concluded that the So/Xo ratio is the
most important factor in batch experiments.3-4 Chudoba et al.4 and Chang et al.5 have observed
that the substrate uptake kinetics depend on the initial So/Xo ratio and it has been demonstrated
that this ratio determines whether multiplication or biomass increase is taking place.6-7
In this work, the influence on the composition medium and the So/Xo ratio on the anaerobic
biodegradation extent was studied, using an inoculum with constant metabolic activity. An automated device measuring the pressure generated by the biogas during degradation was utilized.
52nd Purdue Industrial Waste Conference Proceedings, 1997. Ann Arbor Press, Chelsea, Michigan 48118. Printed in
U.S.A.
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