7.2 In Situ Bioaugmentation
This entails identifying indigenous species of bacteria within the vicinity of the
contaminated site and determining the critical carbon sources and nutrients that
could be supplied to encourage the growth of the target species. When the selected
nutrients are introduced into the environment, either by injection into boreholes or by
spreading on the ground, the target species will out-compete other species and will be
able to degrade the contaminants. The potential problem with this form of
bioaugmentation is that the nutrients may be viewed as pollutants in their own right
especially at the beginning of the bioaugmentation process when microbial loading is
very low. The nutrients such as NO 3
À and SO 4
2À have an undesirable pollution effects
on receiving water bodies such as eutrophication of streams receiving the base flow
from the remediated areas. However, if applied successfully, in situ bioaugmentation
is the only environmentally nonintrusive process that avoids the introduction of alien
species into the environment. Most countries prohibit the introduction of organisms
foreign to a particular region or country (Federal Register 1999).
7.3 Ex Situ Bioremediation
Ex situ methods of remediation are conventionally used in the treatment of
contaminated groundwater through a pump-and-treat process which involves the
extraction of contaminated water from the aquifer, treatment above ground, and
injection of the treated water back into the aquifer (Milkey 2010; Wittbrodt and
Palmer 1992). The treatment above ground can be conducted via both chemical
and biological means. Chemical treatment processes employed in the remediation
of toxic metals produce unwanted chemical by-products which result in the
production of toxic sludge (Gonzalez et al. 2003; Shakoori et al. 2000). Therefore,
the chemical processes are viewed as costly and environmentally intrusive. Alternative biological treatment methods have been suggested using biotransformation
followed by bioseparation using biomass (Chirwa and Wang 1997; Chrysochoou
et al. 2013; Jin et al. 2008). These methods may be applied ex situ (Colica et al.
2010; Zakaria et al. 2007). The ex situ remediation processes are used rarely for
treatment of soil which requires excavation of large volumes of soil, treatment in
off-site facilities, and backfilling. The whole process for treatment of contaminated
soil is too expensive to be undertaken on a large scale. For the treatment of the
aquatic phase, experience at sites where pump-and-treat remediation groundwater
contaminated with metals and radionuclides showed that, although it is feasible to
remove large volumes of the contaminant from the subsurface, it becomes more
and more difficult to remove the remaining pollutants as concentration decreases
(EPA-Odessa 2005). The biological removal by reactors that can be applied ex situ
has been demonstrated at the laboratory scale for chromium, uranium, selenium,
and lead (Hawley et al. 2004; Mtimunye and Chirwa 2014; Wessels 2017; Chirwa
and Wang 2001; Brink et al. 2017).
42
E. M. Nkhalambayausi-Chirwa et al.
This entails identifying indigenous species of bacteria within the vicinity of the
contaminated site and determining the critical carbon sources and nutrients that
could be supplied to encourage the growth of the target species. When the selected
nutrients are introduced into the environment, either by injection into boreholes or by
spreading on the ground, the target species will out-compete other species and will be
able to degrade the contaminants. The potential problem with this form of
bioaugmentation is that the nutrients may be viewed as pollutants in their own right
especially at the beginning of the bioaugmentation process when microbial loading is
very low. The nutrients such as NO 3
À and SO 4
2À have an undesirable pollution effects
on receiving water bodies such as eutrophication of streams receiving the base flow
from the remediated areas. However, if applied successfully, in situ bioaugmentation
is the only environmentally nonintrusive process that avoids the introduction of alien
species into the environment. Most countries prohibit the introduction of organisms
foreign to a particular region or country (Federal Register 1999).
7.3 Ex Situ Bioremediation
Ex situ methods of remediation are conventionally used in the treatment of
contaminated groundwater through a pump-and-treat process which involves the
extraction of contaminated water from the aquifer, treatment above ground, and
injection of the treated water back into the aquifer (Milkey 2010; Wittbrodt and
Palmer 1992). The treatment above ground can be conducted via both chemical
and biological means. Chemical treatment processes employed in the remediation
of toxic metals produce unwanted chemical by-products which result in the
production of toxic sludge (Gonzalez et al. 2003; Shakoori et al. 2000). Therefore,
the chemical processes are viewed as costly and environmentally intrusive. Alternative biological treatment methods have been suggested using biotransformation
followed by bioseparation using biomass (Chirwa and Wang 1997; Chrysochoou
et al. 2013; Jin et al. 2008). These methods may be applied ex situ (Colica et al.
2010; Zakaria et al. 2007). The ex situ remediation processes are used rarely for
treatment of soil which requires excavation of large volumes of soil, treatment in
off-site facilities, and backfilling. The whole process for treatment of contaminated
soil is too expensive to be undertaken on a large scale. For the treatment of the
aquatic phase, experience at sites where pump-and-treat remediation groundwater
contaminated with metals and radionuclides showed that, although it is feasible to
remove large volumes of the contaminant from the subsurface, it becomes more
and more difficult to remove the remaining pollutants as concentration decreases
(EPA-Odessa 2005). The biological removal by reactors that can be applied ex situ
has been demonstrated at the laboratory scale for chromium, uranium, selenium,
and lead (Hawley et al. 2004; Mtimunye and Chirwa 2014; Wessels 2017; Chirwa
and Wang 2001; Brink et al. 2017).
42
E. M. Nkhalambayausi-Chirwa et al.