BIOAUGMENTATION FOR GROUNDWATER REMEDIATION: AN OVERVIEW - Bioaugmentation for Groundwater Remediation - page 7

a satisfactory rate, then the site organisms may be preadapted or enriched for use as an inoculum.
Presumably, themicroorganisms that exist at that site are already accustomed to the temperature,
pH and nutrient availability, and are therefore better suited for use at that site (Bento et al.,
2005 ). However, if there are no existing strains at the site that degrade the pollutant, or if the
numbers of indigenous degraders are low, or if there are multiple pollutants that must be
degraded sequentially, then it might be necessary to use a “foreign” inoculum, like an
enrichment from a different site or a commercial inoculum. For example, bioaugmentation
with Dehalococcoides is common at chlorinated ethene sites where indigenous degraders often
arepresentbutatverylownumbers.
Bacteria in the environment often form relationships with other bacteria in the system -
whether commensal or predatory. A consortium of bacteria often performs better as an
inoculum since the bacteria are already with a community of other bacteria that synergistically
support the activity of interest, namely pollutant degradation. For example, addition of a
consortium capable of PAH degradation resulted in more extensive degradation than any of
the strains individually (Jacques et al., 2008 ). Similar results have been reported for petroleum
hydrocarbons (Richard and Vogel, 1999 ). The bacteria do not need to be extracted and enriched;
the soil itself can be exposed to the contaminant and enriched for degradation to give an
inoculum called “activated soil” (Otte et al., 1994 ; Barbeau et al., 1997 ). The benefit of activated
soil is that it develops a consortium in the soil itself, thus negating the use of artificial media
and the biases that introduces.
1.3.1.2 Commercial Inocula
There are a number of commercially available inocula that target different pollutants
(Table 1.1 ). These inocula can be delivered by several methods including injection, mixing,
relying on bacterial chemotaxis, from a reactor on the surface or as a spray. The success of
these inocula depends partially on the application method and the strains therein, but it mainly
depends on the chemical and biological characteristics of the polluted site. In groundwater
applications, the focus of this volume, inocula are typically delivered via injection wells or
direct injection equipment such as Geoprobe # systems.
1.3.1.3 Bioaugmentation in Combination with Plants and Phytoaugmentation
Plants are already used in bioremediation in a process called phytoremediation, in which
plants either degrade pollutants (directly or indirectly through plant-associated bacteria),
volatilize or accumulate pollutants (Suresh and Ravishankar, 2004 ; Kramer, 2005 ).
This technique has been tested in a number of field studies (Vangronsveld et al., 2009 ; van
Aken and Geiger, 2011 ). Plants have the advantages of roots that reach into the subsurface
forming a system called the rhizosphere, and they have wide seed distribution capacities. Plants
naturally take up heavy metal pollutants through their roots during growth (Padmavathiamma
and Li, 2007 ). To expand on their intrinsic capabilities, genetic modification has been widely
considered, although rarely applied (Cherian and Oliveira, 2005 ).
The relationship between plants and bacteria can be manipulated to encourage pollutant
degradation. Plant growth promoting rhizobacteria (PGPR), reviewed recently by Zhuang et al.
( 2007 ), colonize the rhizosphere in either a symbiotic or free-living manner. They increase plant
growth by producing growth stimulating compounds, preventing disease and increasing nutri-
ent uptake. PGPR in combination with the plants are able to sequester metals more efficiently
than either plants or bacteria alone. Rhizoremediation uses plants to help support bacterial
growth during remediation (Kuiper et al., 2004 ; Cases and de Lorenzo, 2005 ). In recent trials,