EVALUATION OF BIODEGRADATION KINETICS
FOR PRIORITY POLLUTANTS
David M. Philbrook, Environmental Engineer
Engineering-Science Inc.
Fairfax, Virginia 22030
C. P. Leslie Grady, Jr., Professor
Environmental Systems Engineering
Clemson University
Clemson, South Carolina 29631
INTRODUCTION
The wide-spread recognition of the presence in wastewaters of toxic and inhibitory organic compounds has changed the focus of wastewater treatment system design. Whereas it had previously been
assumed that a system designed to achieve good removal of conventional pollutants would also remove individual constituents to low levels, it is now necessary to demonstrate that specific contaminants, particularly the organic priority pollutants, will be removed to levels approaching the limits
of analytical detection, i.e., a few j.g/1. This provides a severe challenge to the environmental engineer because relatively little is known about the kinetics of biodegradation of individual constituents
in multicomponent waste streams.
Although research programs are underway which will eventually tell us more about the kinetics
of biodegradation, the engineer in the design office does not have the luxury of waiting for them to
be completed. Rather, techniques are needed now which will allow the engineer to obtain the maximum amount of information about the behavior of a pollutant with the minimum expenditure of
resources. In particular, the designer needs to focus on three questions. First, what is the minimum
achievable effluent concentration that can be attained with a given process? If the best that can be
done is not good enough, then that process must be eliminated from consideration. Second, what is
the likely variability in the effluent concentration of the pollutant of interest? It has recently been
recognized that there is a certain degree of variability inherent in the ability of biological processes
to remove priority pollutants [1,2]. Since that variability appears to be associated with the natural
dynamics of the microbial community it is important that its impact be analyzed explicitly during
design. Finally, what will be the impact of perturbations in waste strength and character upon the
removal of the target pollutant? This question is the most difficult to answer because less is known
about it.
After the design engineer's job has been completed and the plant is in operation, the operator
must continue to be concerned over questions very similar to those that the engineer faces. The
operator, however, has a more specific set of questions because they involve a particular plant with
a unique biomass. Nevertheless, the operator still needs to know how the capability of that biomass
to remove a particular pollutant varies with time as the character of the influent changes, as the
temperature changes, etc. Furthermore, the operator needs to know how the presence of other constituents in the wastewater impact upon the capability of the biomass to remove the target pollutant.
Are there particular constituents which either help or hinder removal?
Our ability to address questions like the ones enumerated above depends first and foremost upon
our ability to assess the kinetics of biodegradation. If such assessments can be made rapidly and
easily, then sufficient data can be collected to allow direct measurements of the time-dependent changes
in the capabilities of the biomass. The application of reactor engineering techniques with appropriately variable kinetic parameters will allow explicit evaluation of probable changes in removal efficiency. Once the engineer and operator can predict what is likely to occur, then they can use their
judgement and the art of engineering to do the best job possible. Without such analyses, however,
they cannot make rational decisions and the results of their efforts will be less than optimal. The
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