Biomedical Engineering Reference
In-Depth Information
CHAPTER 8
BIOAUGMENTATION FOR THE IN SITU AEROBIC
COMETABOLISM OF CHLORINATED SOLVENTS
Lewis Semprini
Oregon State University, Corvallis, OR 97331
8.1 INTRODUCTION
In situ aerobic cometabolism is a method for remediating groundwater contaminated with
chlorinated aliphatic hydrocarbons (CAHs) (Semprini et al., 1990 ; Hopkins and McCarty, 1995 ;
McCarty et al., 1998a ). The process relies on the fortuitous transformation of the CAHs by
nonspecific oxygenase enzymes. The process most commonly applied in the field is to stimulate
the indigenous microorganisms through additions of an appropriate cometabolic growth
substrate as the electron donor, and oxygen as the electron acceptor. Field studies of this
biostimulation strategy have been performed at the pilot scale and at the full scale, as described
by Semprini ( 1997 ).
A few field trials also have evaluated different approaches to enhancing in situ aerobic
cometabolism through bioaugmentation. To date, bioaugmentation has provided little benefit in
these field trials, although a great deal has been learned about the reasons for its limited success
so far. An earlier review of the older bioaugmentation field trials was provided by ESTCP
( 2005 ). Presented here is a review of these and other more recent field trials, as well as support
work performed in microcosms and column studies. The process of cometabolism also is
discussed, to provide insight into the potential benefits of bioaugmentation for aerobic come-
tabolism, the challenges faced when attempting to bioaugment for this process, and the reasons
that several different approaches to bioaugmentation have been developed.
8.2 AEROBIC COMETABOLIC PROCESSES
A detailed review of the kinetics of aerobic cometabolism of CAHs is provided by Alvarez-
Cohen and Speitel ( 2001 ), while Arp et al. ( 2001 ) review the complex biochemical processes.
McCarty ( 1997 ) and Semprini ( 1997 ) review processes and results of field evaluations of the
stimulation of indigenous microorganisms for enhanced aerobic cometabolism. Reviews of
modeling approaches are presented by Goltz et al. ( 2001 ) and Semprini et al. ( 1998 ). The key
concepts are presented in this chapter, but the reader should refer to these reviews for more detail.
Aerobic cometabolic transformation results from nonspecific enzymes fortuitously initiat-
ing the oxidation of a CAH. Presented in Figure 8.1 is a simplified representation of the process
of aerobic cometabolism. The initial oxidation of a substrate such as methane, propane or
butane requires a monooxygenase enzyme that also initiates the oxidation of the molecule.
A common feature of the enzymes shown capable of CAH cometabolism is that they are used
at the beginning of the pathway to harvest a growth substrate (Arp et al., 2001 ). Metabolism of
the hydrocarbon to carbon dioxide and water provides the energy and carbon for cell growth.
The monooxygenase enzyme initiates the oxidation of a chlorinated ethene, for example
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