19    FACTORS AFFECTING CONTINUOUS BIOREACTOR
TREATMENT OF DIESEL CONTAMINATED
SANDY SOILS
Ronald Britto, Doctoral Candidate
Joseph H. Sherrard, Professor and Head
Dennis D. Truax, Associate Professor,
Department of Civil Engineering
Mississippi State University
Mississippi State, Mississippi 39762
INTRODUCTION
Petroleum spills in aqueous territories have become the focus of considerable environmental concern in the last two decades. Petroleum contaminated soils, on the other hand, have become the focus
of research relatively recently. A wide range of options have been suggested to treat this latter
problem. These include both in-situ or ex-situ operations. Treatment processes may also be classified
as physical, chemical, biological phenomena, or a combination of these actions. Some of the physical-
chemical processes of remediation technologies are: volatilization or vacuum extraction; isolation or
containment; leaching or soil washing; asphalt incorporation; vitrification; thermal treatment; and
chemical oxidation. In recent years, bioremediation has been claimed to be an inexpensive, natural,
biological method of cleanup of petroleum contaminated soils. Both in-situ and ex-situ applications
have been fairly successful. Ex-situ processes include landfarming, composting, and bioreactors.
These contaminated sites, however, make up just 5 percent of the total number of sites targeted under
the U.S. EPA's superfund program.' One of the chief reasons for this low percentage is the lack of
comprehensive performance data on ex-situ bioremediation techniques.
Most research on off-site bioremediation is centered around batch-style techniques and is either
land farming or reactor-based. Since the promulgation of the final Land Disposal Regulations (LDR)
by the EPA,2 land farming is becoming restricted to a few very small sites. Recently, researchers in the
Delaware area,3 have developed full-scale, aerobic bioremediation technology in order to carry out
year-round, aerobic biodegradation of petroleum contaminated soils, while controlling any volatile
emissions. Unfortunately, precise removal mechanisms are lacking and further development of such
kinds of studies are necessary before full-scale acceptance and implementation.
Some of the insufficiencies in ex-situ bioremediation processes are the lack of a proper understanding of the role of microbes and the precise biokinetic mechanisms taking place. Added to these
shortcomings, is the difficulty of explaining microbial phenomena, while simultaneously accounting
for physical and chemical factors, if any. Some of these factors include volatilization of contaminant,
adsorption of contaminant onto soil, and transport limitations like advection and dispersion. Though
there is full agreement that microbes are capable of degrading almost all petroleum products, the
question remains as to how to model their growth and removal rates. A more exact mathematical basis
(with a clearer understanding of assumptions and limitations) for microbial removal of petroleum
contaminated soils by ex-situ treatment is essential. This necessity arises in order to predict remediation rates and process requirements for varying conditions.
Most bioreactors which have been used for treatment of petroleum contaminated soils are of the
batch-type and have suffered from drawbacks such as: (1) protracted periods of time for microbial
acclimation to take place; (2) extensive above ground surface area requirements; (3) lack of control of
organic and inorganic volatiles and hence imperfect material balances; and (4) poor understanding of
microbial mechanisms and biokinetics, and the factors influencing them.
Several experiments with well-mixed batch reactors have demonstrated that the classical growth
curve (including the lag and log growth phases, the stationary and endogenous phases) can be
recorded over a period of time for petroleum contaminated soils. Deviations from these phenomena
have been attributed to characteristics such as: (1) adsorption of substrate to inaccessible sites and
48th Purdue Industrial Waste Conference Proceedings, 1993 Lewis Publishers, Chelsea, Michigan 48118. Printed in
U.S.A.
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