for the conversion of biowaste to valued biofuels and
chemicals (de Paula et al. 2019). As discussed in the
aforementioned sections, the lignocellulosic biowaste conversion for biofuel production is a green substitute for fossil
fuels due to the high availability of biowastes and less
detrimental effect on the environment (Madurwar et al. 2013;
Heredia-Guerrero et al. 2017; Krishnan et al. 2010; Chandel
et al. 2012; Sarsaiya et al. 2019). The bioconversion of
lignocellulosic biowastes to bioethanol is more difficult
compared to the conversion of starch-based biowastes as it
involves four steps: (i) the pretreatment, (ii) hydrolysis of
cellulose and hemicelluloses to soluble monomers (pentoses
and hexoses), (iii) bioconversion of these monomers to
bioethanol through fermentation method, and (iv) purification of the final products. Bioethanol (biofuel) is one of the
most significant materials which can substitute petroleum
and is considered to be clean liquid fuel (Selim et al. 2018).
The foremost challenge for biowaste conversion to
bioethanol is attaining high yield which makes it
cost-competitive with conventional fossil fuels. Cellulose is
a main constituent of the lignocellulosic biowaste, however,
cellulases (enzymatic hydrolysis) cannot be used effectively
due to the low availability of crystalline cellulosic fibers and
presence of hemicellulose and lignin on the cellulose surface
(Dashtban et al. 2009). Hence, pretreatment is required to
hydrolyze lignocellulosic biowaste using various approaches. Usually, in industries, acid and high thermal treatments
are used for large-scale production. But, these approaches
are very slow, expensive, and unproductive (Rubin 2008);
however, those difficulties could be overcome by using
microbes, including bacteria, fungi, and yeasts. For instance,
the thermophilic fungal microbes, including Thielavia terrestris (Gilbert et al. 1993), Thermoascus aurantiacus
(Gomes et al. 2000; Schuerg et al. 2017), Sporotrichum
thermophile (Bhat and Maheshwari 1987; Singh 2016), etc.,
have thrived in literature as potential candidates for lignocellulosic waste bioconversion to sugars for large-scale
industrial usage. The xylanases produced fungal microorganisms such as Trichoderma, Penicillium, and Aspergillus
that possess elevated temperatures (approximately 60°C) in
bio-industries. Similarly, the bacteria such as Bacillus,
Clostridium, and Pseudomonas were used as promising
hydrolytic microbial enzymes (Nigam 2013). The primary
bottleneck problem is the conversion of biowaste into
oligosaccharides. In this path, novel biological innovations
Table 1 Biowastes from various sources and their potential usage
Type of product
Biowaste
Potential usage
References
Wood/paper industry
Sawdust or pulp
Biofuel and enzymes
Rathna et al. (2014)
Agriculture waste
Leaves, straw, husk, hull, stem,
nutshell, and bagasse, etc.
Biogas, enzyme, and
ethanol
Barakat et al. (2014)
Sugar industry
Molasses
Oligosaccharides and
enzymes
Ghazi et al. (2006)
Oil industry
Fibers, sludge, husks, and shells
Bioethanol
Cerveró et al. (2010), Jørgensen et al.
(2010)
Plant or animal waste
Fats, skin, fleshing wastage, horns,
and bones
Biofuel and enzymes
Yazid et al. (2017)
Domestic food and
municipal waste
Kitchen manure etc.
Biopesticides and organic
acids
Ohkouchi and Inoue (2007), Zhang et al.
(2015)
Industrial organic
waste
Various slaughterhouse leftovers
Lubricants, surfactants,
and fillers
Abraham et al. (2014), Yazid et al. (2016)
Fruits and vegetables
wastes
Peels, fiber, seeds Kernel and stones
Biofuels, enzymes, and
organic acids
Panda et al. (2016), Embaby et al. (2014),
Botella et al. (2007)
Poultry processing
waste
Feathers, skin, blood, fats, bones,
liver, and intestines
Enzymes, and
biofertilizers
Jayathilakan et al. (2012)
Nuts processing waste
Shells and pith
Bio-pulping, biochar,
activated carbon
Mtui (2009)
Commercial and hotels
waste
Coffee powder, waste tea leaves,
outdated and waste diets
Biopesticides, bioethanol,
and bioplastics
Jooste et al. (2013)
Fig. 5 Schematic illustration of various bioconversion techniques
Bioconversion of Biowastes for Energy Applications
7
chemicals (de Paula et al. 2019). As discussed in the
aforementioned sections, the lignocellulosic biowaste conversion for biofuel production is a green substitute for fossil
fuels due to the high availability of biowastes and less
detrimental effect on the environment (Madurwar et al. 2013;
Heredia-Guerrero et al. 2017; Krishnan et al. 2010; Chandel
et al. 2012; Sarsaiya et al. 2019). The bioconversion of
lignocellulosic biowastes to bioethanol is more difficult
compared to the conversion of starch-based biowastes as it
involves four steps: (i) the pretreatment, (ii) hydrolysis of
cellulose and hemicelluloses to soluble monomers (pentoses
and hexoses), (iii) bioconversion of these monomers to
bioethanol through fermentation method, and (iv) purification of the final products. Bioethanol (biofuel) is one of the
most significant materials which can substitute petroleum
and is considered to be clean liquid fuel (Selim et al. 2018).
The foremost challenge for biowaste conversion to
bioethanol is attaining high yield which makes it
cost-competitive with conventional fossil fuels. Cellulose is
a main constituent of the lignocellulosic biowaste, however,
cellulases (enzymatic hydrolysis) cannot be used effectively
due to the low availability of crystalline cellulosic fibers and
presence of hemicellulose and lignin on the cellulose surface
(Dashtban et al. 2009). Hence, pretreatment is required to
hydrolyze lignocellulosic biowaste using various approaches. Usually, in industries, acid and high thermal treatments
are used for large-scale production. But, these approaches
are very slow, expensive, and unproductive (Rubin 2008);
however, those difficulties could be overcome by using
microbes, including bacteria, fungi, and yeasts. For instance,
the thermophilic fungal microbes, including Thielavia terrestris (Gilbert et al. 1993), Thermoascus aurantiacus
(Gomes et al. 2000; Schuerg et al. 2017), Sporotrichum
thermophile (Bhat and Maheshwari 1987; Singh 2016), etc.,
have thrived in literature as potential candidates for lignocellulosic waste bioconversion to sugars for large-scale
industrial usage. The xylanases produced fungal microorganisms such as Trichoderma, Penicillium, and Aspergillus
that possess elevated temperatures (approximately 60°C) in
bio-industries. Similarly, the bacteria such as Bacillus,
Clostridium, and Pseudomonas were used as promising
hydrolytic microbial enzymes (Nigam 2013). The primary
bottleneck problem is the conversion of biowaste into
oligosaccharides. In this path, novel biological innovations
Table 1 Biowastes from various sources and their potential usage
Type of product
Biowaste
Potential usage
References
Wood/paper industry
Sawdust or pulp
Biofuel and enzymes
Rathna et al. (2014)
Agriculture waste
Leaves, straw, husk, hull, stem,
nutshell, and bagasse, etc.
Biogas, enzyme, and
ethanol
Barakat et al. (2014)
Sugar industry
Molasses
Oligosaccharides and
enzymes
Ghazi et al. (2006)
Oil industry
Fibers, sludge, husks, and shells
Bioethanol
Cerveró et al. (2010), Jørgensen et al.
(2010)
Plant or animal waste
Fats, skin, fleshing wastage, horns,
and bones
Biofuel and enzymes
Yazid et al. (2017)
Domestic food and
municipal waste
Kitchen manure etc.
Biopesticides and organic
acids
Ohkouchi and Inoue (2007), Zhang et al.
(2015)
Industrial organic
waste
Various slaughterhouse leftovers
Lubricants, surfactants,
and fillers
Abraham et al. (2014), Yazid et al. (2016)
Fruits and vegetables
wastes
Peels, fiber, seeds Kernel and stones
Biofuels, enzymes, and
organic acids
Panda et al. (2016), Embaby et al. (2014),
Botella et al. (2007)
Poultry processing
waste
Feathers, skin, blood, fats, bones,
liver, and intestines
Enzymes, and
biofertilizers
Jayathilakan et al. (2012)
Nuts processing waste
Shells and pith
Bio-pulping, biochar,
activated carbon
Mtui (2009)
Commercial and hotels
waste
Coffee powder, waste tea leaves,
outdated and waste diets
Biopesticides, bioethanol,
and bioplastics
Jooste et al. (2013)
Fig. 5 Schematic illustration of various bioconversion techniques
Bioconversion of Biowastes for Energy Applications
7