(d) Ozonolysis
Ozonolysis is one of the most useful pretreatment methods,
which uses ozone to corrupt the hemicellulose and lignin part
from lignocellulosic materials, for example bagasse, wheat
straw, pine, peanut, poplar sawdust, and cotton straw (Kumar
et al. 2009). Ozone, soluble in water acts as a powerful oxidant,
and is easily accessible. Ozone is extremely reactive in case of
compounds that posses functional groups and conjugated
double bonds with high electron densities. As a result, ozone
can be used to oxidize lignin part of biomass since it has higher
number of C = C bonds (García-Cubero et al. 2009). The
advantages of ozonolysis lie in the effective removal of lignin
without producing any toxic residues. Moreover, the reactions
are takeing place at standard pressure and temperature (Vidal
and Molinier 1988). However, the demand for more amount of
ozone makes it more expensive (Sun and Cheng 2002). Furthermore, it can be assumed that to reduce environmental
pollution, different processes can be proposed based on the fact
that ozone can easily be decaying by the use of high temperature and catalytic bed (Kumar et al. 2009).
(e) Organosolv
In organosolv pretreatment method, strong inorganic acids
are used as catalyst against lignocellulosics biomass (El
Hage et al. 2009). Use of acids breaks the carbohydrates–
lignin and lignin–lignin bonds of biomass. Volume of the
material and superficial area are improved on removal of the
lignin. Significantly, the process helps the ease of access of
enzyme and improves the effectiveness of the procedure to
get fermented sugars (Koo et al. 2011). In this process,
smaller amount of chemicals (viz. sodium sulfite (Na 2 SO 3 )
or sodium hydroxide (NaOH)) are used as catalyst and
generates less quantity of wastes as that of other pretreatments (Ruzene et al. 2007). During this process, high pressure of carbon dioxide along with high effectiveness for
lignin removal has been observed (Pasquini et al. 2005).
(f) Ionic Liquid (IL)
IL is known as “green solvents” made up of cellulose
without the formation of explosive or toxic gases. ILs is
made up of small inorganic anions and large organic cations
and remains in liquid state with minimum temperature
(<100 °C) (Elgharbawy et al. 2016). The various advantages
of ILs are as follows: (1) with non-volatile and no vapor
pressure; (2) on the basis of the design of alkyl constituents
of cation and anion, (3) has large stable range of temperature
(25–300 °C) and good chemical stability; there will be some
adjustments in case of acidity and polymer solubility,
organic and inorganic substance and water. Further, ILs can
make a dual phase system with the majority of solvents
(Ouellet et al. 2011).
ILs can dissolve both lignin and carbohydrates with anion
activity and forms H bonds among the sugar hydroxyl proton and non-hydrated chloride with 1:1 ratio. Thus, the
complex network structure of lignocellulose is broken as
well as decreases the production of degradation products.
Though IL pretreatment is a costly process but due to its
environment friendly nature, it can be developed gradually
(Chen et al. 2017).
(g) Wet Oxidation
This method is carried out in oxygen environment or air
mostly using sodium carbonate as catalyst. Wet oxidation
allows the conversion of biomass into monosaccharides with
small amount of phenolic aldehydes and furan. This method
is associated with the growing amounts of aliphatic acids
along with degradation of lignin. It is a costly pretreatment
method (Carvalheiro et al. 2008). The main advantage of this
method is to achieve free sugars together with alkalis with
no formation of 5-hydroxymethylfurfural and furfural
(Bjerre et al. 1996).
3.4 Biological Pretreatment
Another important category of pretreatment includes the
biological treatment that helps to alter the structure of lignocellulosic materials. In this method, lignin and hemicellulose are degraded, which makes the feedstock easily
available for enzyme digestion (Sarkar et al. 2012).
Advantages include mild environment friendly, operating
conditions, and low requirement of energy (Hamelinck et al.
2005). Low amount of toxic substance (hydroxymethyl
furfural, furfural, etc.) is formed. Fungi (viz. white, brown,
and soft rot fungi), bacteria, and actinomycetes are the main
driving forces of microbial treatment (Sarkar et al. 2012).
Brown rots are used to degrade lignin in polysaccharides
while soft and white affect lignin and carbohydrates. Mostly,
brown rot fungi degrade hemicellulose and cellulose more
quickly as compared to lignin. White rot fungi are found to
be very proficient in case of biological pretreatment of lignocelluloses due to degradation of carbohydrates and lignin
efficiently and generate enzymes like laccase, peroxidases,
etc. which are used to degrade lignin. Bacteria and actinomycetes have less efficiency compared to brown and white
rot fungi (Sun and Cheng 2002).
Bioconversion of Food Waste into Biogas
87
Ozonolysis is one of the most useful pretreatment methods,
which uses ozone to corrupt the hemicellulose and lignin part
from lignocellulosic materials, for example bagasse, wheat
straw, pine, peanut, poplar sawdust, and cotton straw (Kumar
et al. 2009). Ozone, soluble in water acts as a powerful oxidant,
and is easily accessible. Ozone is extremely reactive in case of
compounds that posses functional groups and conjugated
double bonds with high electron densities. As a result, ozone
can be used to oxidize lignin part of biomass since it has higher
number of C = C bonds (García-Cubero et al. 2009). The
advantages of ozonolysis lie in the effective removal of lignin
without producing any toxic residues. Moreover, the reactions
are takeing place at standard pressure and temperature (Vidal
and Molinier 1988). However, the demand for more amount of
ozone makes it more expensive (Sun and Cheng 2002). Furthermore, it can be assumed that to reduce environmental
pollution, different processes can be proposed based on the fact
that ozone can easily be decaying by the use of high temperature and catalytic bed (Kumar et al. 2009).
(e) Organosolv
In organosolv pretreatment method, strong inorganic acids
are used as catalyst against lignocellulosics biomass (El
Hage et al. 2009). Use of acids breaks the carbohydrates–
lignin and lignin–lignin bonds of biomass. Volume of the
material and superficial area are improved on removal of the
lignin. Significantly, the process helps the ease of access of
enzyme and improves the effectiveness of the procedure to
get fermented sugars (Koo et al. 2011). In this process,
smaller amount of chemicals (viz. sodium sulfite (Na 2 SO 3 )
or sodium hydroxide (NaOH)) are used as catalyst and
generates less quantity of wastes as that of other pretreatments (Ruzene et al. 2007). During this process, high pressure of carbon dioxide along with high effectiveness for
lignin removal has been observed (Pasquini et al. 2005).
(f) Ionic Liquid (IL)
IL is known as “green solvents” made up of cellulose
without the formation of explosive or toxic gases. ILs is
made up of small inorganic anions and large organic cations
and remains in liquid state with minimum temperature
(<100 °C) (Elgharbawy et al. 2016). The various advantages
of ILs are as follows: (1) with non-volatile and no vapor
pressure; (2) on the basis of the design of alkyl constituents
of cation and anion, (3) has large stable range of temperature
(25–300 °C) and good chemical stability; there will be some
adjustments in case of acidity and polymer solubility,
organic and inorganic substance and water. Further, ILs can
make a dual phase system with the majority of solvents
(Ouellet et al. 2011).
ILs can dissolve both lignin and carbohydrates with anion
activity and forms H bonds among the sugar hydroxyl proton and non-hydrated chloride with 1:1 ratio. Thus, the
complex network structure of lignocellulose is broken as
well as decreases the production of degradation products.
Though IL pretreatment is a costly process but due to its
environment friendly nature, it can be developed gradually
(Chen et al. 2017).
(g) Wet Oxidation
This method is carried out in oxygen environment or air
mostly using sodium carbonate as catalyst. Wet oxidation
allows the conversion of biomass into monosaccharides with
small amount of phenolic aldehydes and furan. This method
is associated with the growing amounts of aliphatic acids
along with degradation of lignin. It is a costly pretreatment
method (Carvalheiro et al. 2008). The main advantage of this
method is to achieve free sugars together with alkalis with
no formation of 5-hydroxymethylfurfural and furfural
(Bjerre et al. 1996).
3.4 Biological Pretreatment
Another important category of pretreatment includes the
biological treatment that helps to alter the structure of lignocellulosic materials. In this method, lignin and hemicellulose are degraded, which makes the feedstock easily
available for enzyme digestion (Sarkar et al. 2012).
Advantages include mild environment friendly, operating
conditions, and low requirement of energy (Hamelinck et al.
2005). Low amount of toxic substance (hydroxymethyl
furfural, furfural, etc.) is formed. Fungi (viz. white, brown,
and soft rot fungi), bacteria, and actinomycetes are the main
driving forces of microbial treatment (Sarkar et al. 2012).
Brown rots are used to degrade lignin in polysaccharides
while soft and white affect lignin and carbohydrates. Mostly,
brown rot fungi degrade hemicellulose and cellulose more
quickly as compared to lignin. White rot fungi are found to
be very proficient in case of biological pretreatment of lignocelluloses due to degradation of carbohydrates and lignin
efficiently and generate enzymes like laccase, peroxidases,
etc. which are used to degrade lignin. Bacteria and actinomycetes have less efficiency compared to brown and white
rot fungi (Sun and Cheng 2002).
Bioconversion of Food Waste into Biogas
87