Section Five
WASTE TREATMENT PROCESSES
D. AEROBIC BIOLOGICAL PROCESSES
50    EFFECTS OF SORPTION ON THE EXTENT OF
MINERALIZATION OF BENZENE, TOLUENE AND
XYLENE DURING TRANSIENT STEP LOADING OF
BIOLOGICAL FLUIDIZED BED REACTORS
Xianda Zhao, Research Assistant
Kwunhwan Doh, Graduate Student
Craig S. Criddle, Assistant Professor
Thomas C. Voice, Associate Professor
Department of Civil and Environmental Engineering
Michigan State University
East Lansing, Michigan 48824
INTRODUCTION
The contamination of soil and groundwater by petroleum products is a common occurrence in
many parts of the country. Of the numerous pertroleum components, benzene, toluene and xylenes
are the most mobile, and thus the most likely to contaminate water supplies. One of most common
approaches to remediate contaminated groundwater involves a "pump and treat" procedure, in which
groundwater is removed from the subsurface, treated in an above-ground system and discharged to
either a publicly owned treatment works or a surface water, or is returned to the aquifer. Increasing
attention is being given to biological treatment because these systems can destroy the pollutant and are
often less expensive than physical-chemical treatment. Recent work has shown that biological activated carbon in fluidized bed reactor (BAC-FBR) systems provide a high level of removal efficiency
and stability in the treatment of BTX contaminated water.1"4
Since the early part of this century, it has been known that microorganisms are able to metabolize
benzene, toluene and xylene (BTX).5,6 Numerous investigations have been conducted on the degradation pathways, type of organisms, and the degradation rates of BTX. It is clear that the BTX can be
utilized as a sole energy and carbon source by various microorganisms. It is also clear that various
intermediate products (byproducts) will be generated as a part of the degradation reactions. The
partial oxidation products from the bio-transformation of BTX include catechol, cresols, and
dimethylpyrocatechol. In a study of cometabolic degradation of p-xylene by Pseudomonas sp. strain
Bl in fixed-bed biological activated carbon systems, Chang7 demonstrated the formation of byproducts in this pure culture system. These were identified as 3,6-dimethyl pyrocatechol and /^-xylene
dihydrodiol from the degradation of p-xylene when toluene was utilized as the carbon and energy
source. The byproducts were found in the effluents of biological activated carbon fix-bed and biological fixed-bed (without adsorption) reactors throughout the entire experimental period (about 30
days).
With a fully developed mixed-culture community, byproducts will not normally accumulate because
of the enzyme regulation systems in the microorganisms. In a continuous-flow bio-reactor, a community of microorganisms will develop to utilize the substrate resource for the maximum benefit. The
community will not produce more byproducts than are needed because such production costs, rather
than yields, energy. Under transient conditions, however, byproducts may be generated at a rate
higher than they are consumed, and these compounds may accumulate in the reactor or be washed out
in the effluent.
49th Purdue Industrial Waste Conference Proceedings, 1994 Lewis Publishers, Chelsea, Michigan 48118. Printed in
U.S.A.
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