Biomedical Engineering Reference
In-Depth Information
become an accepted approach (Atagana et al., 2003 ; Coppotelli et al., 2008 ; Tam and Wong,
2008 ). White rot fungi (notably isolates of Phanerochaete chrysosporium ) have been studied
as bioaugmentation agents for PAHs (and other recalcitrant compounds), but this approach
has had little field-scale success (e.g., Bumpus, 1989 ; Field et al., 1992 ; Pointing, 2001 ).
PAHs are often found in complex chemical mixtures, and a consortium of bacteria may be
better equipped to bioaugment such a mixture than a single culture inoculum (Jacques et al.,
2008 ). These larger PAH compounds, such as benzo[a]pyrene originally thought to be recalci-
trant, might be more effectively degraded by GEMs (Samanta et al., 2002 ).
1.7.3 Methyl Tert-Butyl Ether (MTBE)
MTBE is a gasoline additive that replaced tetraethyl lead as an antiknock agent in the 1980s
and that also serves as a fuel oxygenate. Consequently, many gasoline spills also are accom-
panied byMTBE contamination. Small amounts of MTBE in drinking water (lowmicrogram per
liter [ m g/L] concentrations) impart an unpleasant taste, and larger amounts pose a possible health
risk. The USEPA Federal Drinking Water Guideline for MTBE is 20-40 m g/L, although some
states have lower standards. The MTBE problem is exacerbated by its relatively high solubility in
water and the fact that it is biodegraded more slowly than other gasoline components, such as
BTEX compounds. As a result, MTBE plumes often can be larger and more persistent than
BTEX plumes. In this sense, MTBE can be a useful indicator of gasoline spills, preceding the
supposedly more harmful BTEX components. However, it also can result in a need to treat
significantly larger areas and greater volumes than the BTEX contamination alone.
MTBE is a relatively stable compound that is difficult to degrade due to its ether bond.
Various reviews detail remediation efforts on MTBE, stressing that aerobic conditions are ideal
for MTBE degradation (Deeb et al., 2000 ; Stocking et al., 2000 ; Zanardini et al., 2002 ;
H¨ggblom et al., 2007 ), although anaerobic MTBE biodegradation also occurs (Finneran and
Lovley, 2001 ; Lopes Ferreira et al., 2006 ). One complication with MTBE contamination is that it
is usually accompanied by BTEX contamination and other gasoline products. Thus, any
remediation strategy should not interfere with the ability to degrade the other pollutants,
which are often more toxic than MTBE. Biodegradation of MTBE - and its breakdown product
tert-butyl alcohol (TBA) - is clearly possible, but it has proven difficult to treat these
compounds in groundwater (Deeb et al., 2000 ). Further details on bioaugmentation of MTBE
are provided in Chapter 10 of this volume.
Field studies of bioaugmentation to degrade MTBE have demonstrated the need for
aerobic conditions. At Port Hueneme, California, an enriched mixed culture and oxygen
injection were combined to successfully remediate MTBE, although MTBE biodegradation
also occurred in the oxygen-only control plot after a lag period (Salanitro et al., 2000 ). In a
second study at the same site, only oxygen was needed to enhance remediation, and bioaug-
mentation did not increase effectiveness (Smith et al., 2005 ). This study employed a qPCR
method that had been developed to monitor the presence of a bioaugmentation strain (PM-1)
proven capable of rapid and complete MTBE degradation (Hristova et al., 2001 ).
1.7.4 Pesticides
Pesticides, particularly those of the organochlorine family, represent a generally more
xenobiotic class of compounds, having been manufactured and released into the environment
only recently. These compounds are often aromatic and chlorinated, thus being difficult to
degrade. Pesticides are able to seep through the soil to contaminate groundwater. The success
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