ensure the transfer of the pollutant through the biofilter
and the optimal development of the microorganisms. To
avoid compaction, the height of the biofilter generally
does not exceed 1 m. This device is used when the ground
surface is not limited. In this case, the filter is flat and
allows the processing of large amounts of low gas pollutant concentration (treatment of odors from sewage treatment plants, water stations, or composting facilities). For
example, a 2,000 m
2 biofilter can handle about
160,000 m
3
/h of air; in some facilities, gas flow rates
can reach 360,000 m
3
/h. Biofilters can also be used to
treat either VOCs from contaminated sites (soil, groundwater) where a bioremediation is operated or industrial
gases containing, for example, BTEX (benzene, toluene,
ethylbenzene, xylenes). The main limitation of this treatment is the inability to control the pH.
(ii) Bioreactors: In a bioreactor, wherein the pH and the
temperature are regulated, the microorganisms
constituting the biofilm are immobilized on a column
(Fig. 16.20b) consisting of an inorganic carrier with
high packing vacuum level (e.g., Raschig rings,
ceramic, etc.). The polluted air passes through the filter
bed where biodegradation occurs. The liquid phase,
which is recycled, flows against the flow and allows a
nutrient input. Aeration is also operated.
(iii) Bioscrubbers: These devices have two compartments
(Fig. 16.20c). In the first compartment, the
contaminated gas is brought into contact with the liquid
phase (introduced by spraying). This phase is then
pumped into the second chamber (reactor) where the
biodegradation of dissolved pollutants take place by
suspended biomass (activated sludge). Bioscrubbers
that do not contain a solid phase are effective only for
soluble chemical compounds in the water.
16.7 Xenobiotics
16.7.1 Presentation
The term xenobiotic (in ancient greek xenos, stranger; bios,
life) includes all synthetic chemicals that do not occur naturally and are produced by human activity. In terms of environment hazard, polychlorinated biphenyls (PCBs),
chlorophenols, pesticides, chlorinated solvents, chlorinated
aliphatic hydrocarbons, plastics, ammunition, etc. are the
most problematic. PCBs and pesticides are discussed in
this chapter. These chemicals have the greatest impact on
ecosystems resulting from an extensive use and their inherent toxicity. Additional information regarding other
xenobiotics and their degradation can be found in the reference books listed at the end of the chapter (Leisinger 1983;
Connel 1997).
Since a century microorganisms have to face with these
new molecules that do not occur previously in the geological
eras. They have shown an extraordinary potential to attack
most of these compounds, even if biodegradation may be
extremely slow and take several years. Often biodegradation
cannot be performed by a single strain and involves microbial communities. Frequently cometabolism is the sole possible route of degradation. This is the case of trinitrotoluene
which is degraded in the presence of starch as a source of
carbon and energy and numerous halogenated aliphatic
compounds (dichloromethane and trichloroethene) which
are degraded in the presence of methane.
16.7.2 The Adaptation of Microorganisms
to Xenobiotic Compounds
The adaptation of microorganisms to the degradation of
xenobiotic compounds is based on a wide variety of enzymes
naturally distributed in microbial communities with a high
genome plasticity favorable to the appearance of novel gene
combinations or sequences suitable for the evolution of new
biochemical capacities. Considering the time scale required
for de novo appearance of a new metabolic capacity, it is
likely that, in the most general case, the transformation of
xenobiotic compounds can be initiated through the “recruitment” of enzymes assigned to the degradation of compounds
that are sources of nutrition for microorganisms (LeslieGrady 1985). The ability of an enzyme to be recruited
depends on its substrate specificity. These occasional
transformations have been collectively called “gratuitous”
(Knackmuss 1981) or “fortuitous” (Slater and Bull 1982)
metabolism.
Xenobiotic compounds bearing a common structural
basis with natural substrates may be misleading for the
enzyme which can accept them as new substrates. This is
indeed not sufficient. The presence of functional groups or
halogens may generate steric* and mesomeric effects* that
can affect the fixation of the substrate and/or the reaction
mechanisms at the active site of the enzyme. For instance,
dioxygenases involved in the cleavage of aromatic rings of
the soil organic matter cannot accept as substrates the
corresponding chlorinated compounds. They are degraded
by specific dioxygenases (Dagley 1978).
Enzyme recruitment is a key process to explain the
main features that control the degradation of xenobiotic
compounds in natural environments. Diverse situations
have been observed. They are not mutually exclusive. Sometimes, degradation is brought about by enzymes belonging to
different functional classes or pathways producing different
reaction intermediates. For these chemicals, competitive
degradation may occur with parallel degradation pathways
running simultaneously. For instance, triazine herbicides can
684
J.-C. Bertrand et al.
and the optimal development of the microorganisms. To
avoid compaction, the height of the biofilter generally
does not exceed 1 m. This device is used when the ground
surface is not limited. In this case, the filter is flat and
allows the processing of large amounts of low gas pollutant concentration (treatment of odors from sewage treatment plants, water stations, or composting facilities). For
example, a 2,000 m
2 biofilter can handle about
160,000 m
3
/h of air; in some facilities, gas flow rates
can reach 360,000 m
3
/h. Biofilters can also be used to
treat either VOCs from contaminated sites (soil, groundwater) where a bioremediation is operated or industrial
gases containing, for example, BTEX (benzene, toluene,
ethylbenzene, xylenes). The main limitation of this treatment is the inability to control the pH.
(ii) Bioreactors: In a bioreactor, wherein the pH and the
temperature are regulated, the microorganisms
constituting the biofilm are immobilized on a column
(Fig. 16.20b) consisting of an inorganic carrier with
high packing vacuum level (e.g., Raschig rings,
ceramic, etc.). The polluted air passes through the filter
bed where biodegradation occurs. The liquid phase,
which is recycled, flows against the flow and allows a
nutrient input. Aeration is also operated.
(iii) Bioscrubbers: These devices have two compartments
(Fig. 16.20c). In the first compartment, the
contaminated gas is brought into contact with the liquid
phase (introduced by spraying). This phase is then
pumped into the second chamber (reactor) where the
biodegradation of dissolved pollutants take place by
suspended biomass (activated sludge). Bioscrubbers
that do not contain a solid phase are effective only for
soluble chemical compounds in the water.
16.7 Xenobiotics
16.7.1 Presentation
The term xenobiotic (in ancient greek xenos, stranger; bios,
life) includes all synthetic chemicals that do not occur naturally and are produced by human activity. In terms of environment hazard, polychlorinated biphenyls (PCBs),
chlorophenols, pesticides, chlorinated solvents, chlorinated
aliphatic hydrocarbons, plastics, ammunition, etc. are the
most problematic. PCBs and pesticides are discussed in
this chapter. These chemicals have the greatest impact on
ecosystems resulting from an extensive use and their inherent toxicity. Additional information regarding other
xenobiotics and their degradation can be found in the reference books listed at the end of the chapter (Leisinger 1983;
Connel 1997).
Since a century microorganisms have to face with these
new molecules that do not occur previously in the geological
eras. They have shown an extraordinary potential to attack
most of these compounds, even if biodegradation may be
extremely slow and take several years. Often biodegradation
cannot be performed by a single strain and involves microbial communities. Frequently cometabolism is the sole possible route of degradation. This is the case of trinitrotoluene
which is degraded in the presence of starch as a source of
carbon and energy and numerous halogenated aliphatic
compounds (dichloromethane and trichloroethene) which
are degraded in the presence of methane.
16.7.2 The Adaptation of Microorganisms
to Xenobiotic Compounds
The adaptation of microorganisms to the degradation of
xenobiotic compounds is based on a wide variety of enzymes
naturally distributed in microbial communities with a high
genome plasticity favorable to the appearance of novel gene
combinations or sequences suitable for the evolution of new
biochemical capacities. Considering the time scale required
for de novo appearance of a new metabolic capacity, it is
likely that, in the most general case, the transformation of
xenobiotic compounds can be initiated through the “recruitment” of enzymes assigned to the degradation of compounds
that are sources of nutrition for microorganisms (LeslieGrady 1985). The ability of an enzyme to be recruited
depends on its substrate specificity. These occasional
transformations have been collectively called “gratuitous”
(Knackmuss 1981) or “fortuitous” (Slater and Bull 1982)
metabolism.
Xenobiotic compounds bearing a common structural
basis with natural substrates may be misleading for the
enzyme which can accept them as new substrates. This is
indeed not sufficient. The presence of functional groups or
halogens may generate steric* and mesomeric effects* that
can affect the fixation of the substrate and/or the reaction
mechanisms at the active site of the enzyme. For instance,
dioxygenases involved in the cleavage of aromatic rings of
the soil organic matter cannot accept as substrates the
corresponding chlorinated compounds. They are degraded
by specific dioxygenases (Dagley 1978).
Enzyme recruitment is a key process to explain the
main features that control the degradation of xenobiotic
compounds in natural environments. Diverse situations
have been observed. They are not mutually exclusive. Sometimes, degradation is brought about by enzymes belonging to
different functional classes or pathways producing different
reaction intermediates. For these chemicals, competitive
degradation may occur with parallel degradation pathways
running simultaneously. For instance, triazine herbicides can
684
J.-C. Bertrand et al.