saccharification followed by fermentation of sugars. Different researches have been performed by using different FVWs
like potato peel, apple pomace, apple waste, banana peel,
banana waste, pineapple waste, soybean litter, and soybean
molasses for bioethanol production using Saccharomyces
cerevisiae. Mushimiyimana and Tallapragada also used
agro-waste including peel of carrot, onion, sugar beet, and
potato to produce bioethanol. In this process, Penicillium
sp. and Saccharomyces cerevisiae are used for hydrolysis
and fermentation to produce bioethanol, respectively
(Mushimiyimana and Tallapragada 2016). Ingale and friends
synthesized bioethanol from banana discards after
pre-treating with Aspergillus ellipticus and A. fumigatus
(Ingale et al. 2014). Bioethanol is consumed in fuel industries, pharmaceuticals, cosmetics, beverages, and chemical
industries. It has been used as adhesive in dyes and paints,
raw materials for plastics, preservative, solvent for spirits
industries, disinfectant, bleaching agent, and cleaning agent,
etc. (Chin and H’ng 2013).
4.6.2 Biohydrogen
Biohydrogen is universally recognized as complementary to
fossil fuels due to its non-polluting feature, less costly, and
renewable source. Hydrogen gas includes 2.75 times greater
energy yield than hydrocarbon fuels and it is carbon neutral.
This can be considered as a clean fuel and energy carrier
without CO 2 releases and can be easily operated in generating electricity (Kapdan and Kargi 2006). With the development of sustainable and minimization of waste policy,
biohydrogen production is realized from renewable sources,
also known as green technology. Hydrogen can be made by
different processes; electrolysis of water, biological processes, and thermocatalytic reformation of hydrogen-rich
organic compounds (Kapdan and Kargi 2006). Biological
processes for hydrogen generation using microorganisms is
an exciting approach and includes different methods
including direct biophotolysis, indirect biophotolysis, dark
fermentation, and photo fermentation. Biophotolysis refers
to breaking water molecules by microbes like green
microalgae and cyanobacteria into hydrogen and oxygen in
presence of sunlight, whereas fermentation process refers to
the production of biohydrogen by converting organic compounds as an energy source by microbes in the absence or
presence of light (Levin et al. 2004; Rahman et al. 2016).
Biohydrogen is produced as a secondary outcome during
anaerobic alteration of organic wastes, whereas in photosynthetic processes microorganisms use carbon dioxide and
water for hydrogen production (Levin et al. 2004). Different
wastes like potato waste, pumpkin waste, fennel waste, olive
pomace, leafy vegetables like cabbage, water celery, cauliflower, etc., can be a substrate for biohydrogen production
(Ghimire et al. 2015; Lee et al. 2010). Some biohydrogen
producers are Clostridium butyricum, Bacillus sp.,
Escherichia coli, Rhodobacter sphaeroides, Rhodopseudomonas palustris, R. faecalis, Rhodospirillum rubrum, etc.
(Rahman et al. 2016). The principal application of biohydrogen is utilization as a fuel cell for generating electricity,
however, during the production of biohydrogen other gas
such as ammonia, methane, hydrogen sulfite may be produced (Levin et al. 2004; Rahman et al. 2016) which can be
used for advantages.
4.6.3 Biomethane
Biomethane is a cheap form of bioenergy which can be
produced from anaerobic digestion of biogenic wastes by
different microbes. The practice of vegetable waste to generate biogas is environmentally friendly and resolves the
residual disposal problem, air and water pollution, soil
contamination, and lowers reliance on wood fuel. During
anaerobic digestion, the acidogenic microbes are responsible
to produce acetate, carbon dioxide, and hydrogen. This
produced hydrogen along with acetate is digested by
methanogens into water and methane. The charging rates of
biodegradable organic FVWs should be proper to produce
methane. For example, if loading of organic waste is high,
the digestion by acidogenic microbes increases, while
methanogenic microbes are unable to increase which results
in the termination of methane production. Biomethane production involves hydrolysis, methanogenesis, and acidogenesis that are completed by a sequence of microbial
interactions. However, the products differ with the type of
bacteria involved (Singh et al. 2012). A previous study used
vegetable waste like salad leaves, potato peelings, green
peas, and carrots remains in a number of phase transitioning
reactors and a focal reactor to produce biomethane (Raynal
et al. 1998). Organic waste influences excessive production
of methane and processed slurry formation. This processed
suspension can be applied in conditioning soil or biofertilizer
(Singh et al. 2012).
4.6.4 Biodiesel
Biodiesel is a renewable and clean-burning liquid biofuel
which consists of low aliphatic alcohols and esters of alkyl
groups having high fatty acids. Biodiesel can be considered
as “carbon neutral” because this biofuel produces no net
output of carbon dioxide. In addition, biodiesel is inexhaustible and perishable energy which reduces very fast
(4Â) than fossil fuel, has greasing assets that reduce engine
wear, and is secure for storing and management due to low
explosiveness and a high flash point of 100–170 °C
(Ramirez-Arias et al. 2018). Transesterification is a commonly applied procedure of producing biodiesel, requires
only low temperature and pressure, and produces 98%
conversion yield (Muniraj et al. 2015). However, supercritical fluid extraction methods can also be applied to extract
biodiesel from oilseed (Lee et al. 2010). In the
156
S. Shrestha et al.
like potato peel, apple pomace, apple waste, banana peel,
banana waste, pineapple waste, soybean litter, and soybean
molasses for bioethanol production using Saccharomyces
cerevisiae. Mushimiyimana and Tallapragada also used
agro-waste including peel of carrot, onion, sugar beet, and
potato to produce bioethanol. In this process, Penicillium
sp. and Saccharomyces cerevisiae are used for hydrolysis
and fermentation to produce bioethanol, respectively
(Mushimiyimana and Tallapragada 2016). Ingale and friends
synthesized bioethanol from banana discards after
pre-treating with Aspergillus ellipticus and A. fumigatus
(Ingale et al. 2014). Bioethanol is consumed in fuel industries, pharmaceuticals, cosmetics, beverages, and chemical
industries. It has been used as adhesive in dyes and paints,
raw materials for plastics, preservative, solvent for spirits
industries, disinfectant, bleaching agent, and cleaning agent,
etc. (Chin and H’ng 2013).
4.6.2 Biohydrogen
Biohydrogen is universally recognized as complementary to
fossil fuels due to its non-polluting feature, less costly, and
renewable source. Hydrogen gas includes 2.75 times greater
energy yield than hydrocarbon fuels and it is carbon neutral.
This can be considered as a clean fuel and energy carrier
without CO 2 releases and can be easily operated in generating electricity (Kapdan and Kargi 2006). With the development of sustainable and minimization of waste policy,
biohydrogen production is realized from renewable sources,
also known as green technology. Hydrogen can be made by
different processes; electrolysis of water, biological processes, and thermocatalytic reformation of hydrogen-rich
organic compounds (Kapdan and Kargi 2006). Biological
processes for hydrogen generation using microorganisms is
an exciting approach and includes different methods
including direct biophotolysis, indirect biophotolysis, dark
fermentation, and photo fermentation. Biophotolysis refers
to breaking water molecules by microbes like green
microalgae and cyanobacteria into hydrogen and oxygen in
presence of sunlight, whereas fermentation process refers to
the production of biohydrogen by converting organic compounds as an energy source by microbes in the absence or
presence of light (Levin et al. 2004; Rahman et al. 2016).
Biohydrogen is produced as a secondary outcome during
anaerobic alteration of organic wastes, whereas in photosynthetic processes microorganisms use carbon dioxide and
water for hydrogen production (Levin et al. 2004). Different
wastes like potato waste, pumpkin waste, fennel waste, olive
pomace, leafy vegetables like cabbage, water celery, cauliflower, etc., can be a substrate for biohydrogen production
(Ghimire et al. 2015; Lee et al. 2010). Some biohydrogen
producers are Clostridium butyricum, Bacillus sp.,
Escherichia coli, Rhodobacter sphaeroides, Rhodopseudomonas palustris, R. faecalis, Rhodospirillum rubrum, etc.
(Rahman et al. 2016). The principal application of biohydrogen is utilization as a fuel cell for generating electricity,
however, during the production of biohydrogen other gas
such as ammonia, methane, hydrogen sulfite may be produced (Levin et al. 2004; Rahman et al. 2016) which can be
used for advantages.
4.6.3 Biomethane
Biomethane is a cheap form of bioenergy which can be
produced from anaerobic digestion of biogenic wastes by
different microbes. The practice of vegetable waste to generate biogas is environmentally friendly and resolves the
residual disposal problem, air and water pollution, soil
contamination, and lowers reliance on wood fuel. During
anaerobic digestion, the acidogenic microbes are responsible
to produce acetate, carbon dioxide, and hydrogen. This
produced hydrogen along with acetate is digested by
methanogens into water and methane. The charging rates of
biodegradable organic FVWs should be proper to produce
methane. For example, if loading of organic waste is high,
the digestion by acidogenic microbes increases, while
methanogenic microbes are unable to increase which results
in the termination of methane production. Biomethane production involves hydrolysis, methanogenesis, and acidogenesis that are completed by a sequence of microbial
interactions. However, the products differ with the type of
bacteria involved (Singh et al. 2012). A previous study used
vegetable waste like salad leaves, potato peelings, green
peas, and carrots remains in a number of phase transitioning
reactors and a focal reactor to produce biomethane (Raynal
et al. 1998). Organic waste influences excessive production
of methane and processed slurry formation. This processed
suspension can be applied in conditioning soil or biofertilizer
(Singh et al. 2012).
4.6.4 Biodiesel
Biodiesel is a renewable and clean-burning liquid biofuel
which consists of low aliphatic alcohols and esters of alkyl
groups having high fatty acids. Biodiesel can be considered
as “carbon neutral” because this biofuel produces no net
output of carbon dioxide. In addition, biodiesel is inexhaustible and perishable energy which reduces very fast
(4Â) than fossil fuel, has greasing assets that reduce engine
wear, and is secure for storing and management due to low
explosiveness and a high flash point of 100–170 °C
(Ramirez-Arias et al. 2018). Transesterification is a commonly applied procedure of producing biodiesel, requires
only low temperature and pressure, and produces 98%
conversion yield (Muniraj et al. 2015). However, supercritical fluid extraction methods can also be applied to extract
biodiesel from oilseed (Lee et al. 2010). In the
156
S. Shrestha et al.