depends upon various factors including the bioavailability, accessibility, and the
potential of applied microorganisms for detoxifying, transforming to less harmful
chemicals, or degrading completely (Prasad and Aranda 2018). The various
reactions taking place during the biodegradation process include dehalogenation,
oxidation, sulfur replacement by oxygen, hydrolysis, reduction or oxidation of
nitrogen compounds, or OH radicle addition in benzene ring (Ortiz-Hernández
et al. 2013). Bioremediation can take place either aerobically or anaerobically. The
important factors that can affect the growth/metabolism of microorganisms include
pH, nutrient availability, temperature, moisture content, and substrate availability.
These factors in turn can affect the biodegradation process, thus limiting the efficiency of bioremediation. Some microorganisms also require other substrates for
degradation of pollutants which is termed as co-metabolism (Hayatsu et al. 2000).
Bioremediation applied for the removal of persistent organic pollutants can be
categorized as in situ and ex situ bioremediation. Treatment of the waste material at
contaminated site is known as in situ bioremediation whereas treating the
contaminated materials at some other place is known as ex situ bioremediation
(Aggarwal et al. 1990). In situ bioremediation involves the degradation of complex
and harmful organic pollutants into simpler compounds such as carbon dioxide and
water or other less harmful substances under natural conditions. In situ bioremediation is much more sustainable, cost effective, and eco-friendly technique for the
cleaning of the contaminated areas; however in case of ex situ bioremediation the
costs are generally higher because of the excavation and transfer of contaminated
materials. There are also differences in the biodegradation rates and uniformity of
both in situ and ex situ bioremediation processes; however these methods largely
depend upon metabolic potential of microorganisms (Jørgensen 2007). The in situ
bioremediation processes include bio-attenuation, bio-stimulation, and
bio-augmentation whereas the ex situ processes include composting, land farming,
phytoremediation, bio-stimulation, and bio-restoration (Madsen 1991; Alexander
1999). However, selecting and using a bioremediation technique broadly depends
upon the characteristics of pollutants, metabolic potential, and population of
microorganisms and site conditions (Megharaj et al. 2011).
The bioremediation process can be classified on the basis of organisms used as
bioremediator such as bacterial bioremediation (bacteria), mycoremediation (fungi)
(Prasad 2017, 2018; Prasad et al. 2021), phytoremediation (plants) (Sarma et al.
2021), and phycoremediation (algae) (Velázquez-Fernández et al. 2012). Although
every microorganism is capable of eliminating contaminants, some of the commonly
used microorganisms (naturally occurring or bioengineered) include Klebsiella,
Alcaligenes, Shigella, Escherichia, Streptococcus, Bacillus, Enterobacter, Corynebacterium, Phanerochaete chrysosporium, and Staphylococcus (Haritash and
Kaushik 2009).
8.6.1.1 In Situ Bioremediation
The techniques of in situ bioremediation include bio-attenuation, bio-stimulation,
and bio-augmentation. Bio-attenuation is the natural process of bioremediation in
which the toxic contaminants are degraded into the less harmful substances by
microorganisms, or by the reactions among naturally existing chemicals and
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potential of applied microorganisms for detoxifying, transforming to less harmful
chemicals, or degrading completely (Prasad and Aranda 2018). The various
reactions taking place during the biodegradation process include dehalogenation,
oxidation, sulfur replacement by oxygen, hydrolysis, reduction or oxidation of
nitrogen compounds, or OH radicle addition in benzene ring (Ortiz-Hernández
et al. 2013). Bioremediation can take place either aerobically or anaerobically. The
important factors that can affect the growth/metabolism of microorganisms include
pH, nutrient availability, temperature, moisture content, and substrate availability.
These factors in turn can affect the biodegradation process, thus limiting the efficiency of bioremediation. Some microorganisms also require other substrates for
degradation of pollutants which is termed as co-metabolism (Hayatsu et al. 2000).
Bioremediation applied for the removal of persistent organic pollutants can be
categorized as in situ and ex situ bioremediation. Treatment of the waste material at
contaminated site is known as in situ bioremediation whereas treating the
contaminated materials at some other place is known as ex situ bioremediation
(Aggarwal et al. 1990). In situ bioremediation involves the degradation of complex
and harmful organic pollutants into simpler compounds such as carbon dioxide and
water or other less harmful substances under natural conditions. In situ bioremediation is much more sustainable, cost effective, and eco-friendly technique for the
cleaning of the contaminated areas; however in case of ex situ bioremediation the
costs are generally higher because of the excavation and transfer of contaminated
materials. There are also differences in the biodegradation rates and uniformity of
both in situ and ex situ bioremediation processes; however these methods largely
depend upon metabolic potential of microorganisms (Jørgensen 2007). The in situ
bioremediation processes include bio-attenuation, bio-stimulation, and
bio-augmentation whereas the ex situ processes include composting, land farming,
phytoremediation, bio-stimulation, and bio-restoration (Madsen 1991; Alexander
1999). However, selecting and using a bioremediation technique broadly depends
upon the characteristics of pollutants, metabolic potential, and population of
microorganisms and site conditions (Megharaj et al. 2011).
The bioremediation process can be classified on the basis of organisms used as
bioremediator such as bacterial bioremediation (bacteria), mycoremediation (fungi)
(Prasad 2017, 2018; Prasad et al. 2021), phytoremediation (plants) (Sarma et al.
2021), and phycoremediation (algae) (Velázquez-Fernández et al. 2012). Although
every microorganism is capable of eliminating contaminants, some of the commonly
used microorganisms (naturally occurring or bioengineered) include Klebsiella,
Alcaligenes, Shigella, Escherichia, Streptococcus, Bacillus, Enterobacter, Corynebacterium, Phanerochaete chrysosporium, and Staphylococcus (Haritash and
Kaushik 2009).
8.6.1.1 In Situ Bioremediation
The techniques of in situ bioremediation include bio-attenuation, bio-stimulation,
and bio-augmentation. Bio-attenuation is the natural process of bioremediation in
which the toxic contaminants are degraded into the less harmful substances by
microorganisms, or by the reactions among naturally existing chemicals and
224
A. B. T. Akhtar et al.