2015; Terrell and Theegala 2019). Mostly, unmanaged
biowastes are used for the landfill. Landfills generate mainly
methane and carbon dioxide gases which leads to global
warming. As the world is facing the problem of dumping of
biowastes, systematic efforts toward the landfill in the production of biogas may become boon for human beings.
Landfill gases have a promising potential as a sustainable
supply of green energy (Powell et al. 2016; Jaramillo and
Matthews 2005; Chen et al. 2015; Vrbová and Ciahotný
2017). Using these landfills, biogases can be generated
through anaerobic, aerobic, or hybrid type (anaerobicaerobic) bioreactors.
1.2 Energy Crops
Energy crops are defined as non-food crops useful for the
generation of bioenergy and materials with higher market
value. The presence of carbohydrates, oils, lipids, protein,
and fiber in energy crops is important for the production of
bioenergy for consumer and commercial applications. Generally, the yield of bioenergy depends on the amount of
fibers in these crops; more fiber yields more bioenergy
(Montross and Crofcheck 2010). Different types of bioenergy have been generated using diverse resources such as
bio-oils from switchgrass forage and two sets of alfalfa stem
(Mullen and Boateng 2008), short-rotation woody crops for
woody biomass (Zalesny et al. 2011), solid biofuel from
energy crops (Karampinis et al. 2012), bioenergy form
ryegrass, sugarcane, willow, sugar beet, and Miscanthus
Gigantes crops (Hattori and Morita 2010; Tonini et al.
2012). Agricultural waste is also used as energy crops
material for the production of bioenergy (Ferronato and
Torretta 2019). Bioenergy-generating crops are classified as
biofuel crops, biomass crops, bioethanol crops, biodiesel
crops, lignocellulosic crops, etc. Some countries, such as
Mexico, have a relative abundance of agricultural land for
the cultivation of energy crops because of the warm and
sunny climate. With well-developed infrastructure facilities
and easy availability of labor, it is possible to generate
bioenergy from energy crops for commercial applications
(Ruiz et al. 2016). Asian countries such as Japan, India, and
a few more have paddy land for the cultivation of rice as
energy crops (Hattori and Morita 2010). Energy crops are
standalone as an alternative source for fossil fuels.
In the present situation, the world generates two billion
tonnes of municipal waste every year. About 33% of the
generated municipal solid waste are not utilized in an
environmentally safe way. Worldwide, a person is responsible to produce about 0.74 kg of waste every day with
high-income countries generating about 34% of the world’s
waste. As the population increases, the waste generated is
anticipated to reach 3.40 billion tonnes by 2050. The projected waste generation, by region (millions of tonnes/year)
by the World Bank, is given in Fig. 3 (Kaza et al. 2018). The
conversion of biowaste into bioenergy is always a boon, but
there are certain adverse effects of biowaste on the atmosphere and human life. Generally, countries follow the 3R
rule: reduce, reuse, and recycle. If the wastes are disposed of
in an uncontrolled way, there will be an impact on air quality
and human life. Besides, uncontrolled disposing of wastes
will increase emissions of greenhouse gases, pollute the
marine system, pollute soil, and spread diseases (Ferronato
and Torretta 2019). Various reports also mention the impact
of biowastes on the environment, for example, air pollution
due to the simple process of bioconversion such as the
production of biochar from biowaste (Sparrevik et al. 2014).
Biomedical waste dumping sites cause emissions of hazardous gases and an increase in heavy metal contents like
chromium, zinc, nickel, lead, and copper (Manzoor and
Sharma 2019; Chionyedua Theresa 2014). Municipal biowaste generates gases such as methane, carbon dioxide,
carbon monoxide which lead to global warming issues
(Vieira and Matheus 2019). Municipal waste also causes
soil, water, and air pollution (Colón et al. 2012). Animal
biowaste may impact the processing of other biowastes
Fig. 2 Schematics for the
biosynthesis of biomethanol.
Adapted with permission (Paliwal
et al. 2019). Copyright (2019),
Springer, Singapore
Bioconversion of Biowastes for Energy Applications
3
biowastes are used for the landfill. Landfills generate mainly
methane and carbon dioxide gases which leads to global
warming. As the world is facing the problem of dumping of
biowastes, systematic efforts toward the landfill in the production of biogas may become boon for human beings.
Landfill gases have a promising potential as a sustainable
supply of green energy (Powell et al. 2016; Jaramillo and
Matthews 2005; Chen et al. 2015; Vrbová and Ciahotný
2017). Using these landfills, biogases can be generated
through anaerobic, aerobic, or hybrid type (anaerobicaerobic) bioreactors.
1.2 Energy Crops
Energy crops are defined as non-food crops useful for the
generation of bioenergy and materials with higher market
value. The presence of carbohydrates, oils, lipids, protein,
and fiber in energy crops is important for the production of
bioenergy for consumer and commercial applications. Generally, the yield of bioenergy depends on the amount of
fibers in these crops; more fiber yields more bioenergy
(Montross and Crofcheck 2010). Different types of bioenergy have been generated using diverse resources such as
bio-oils from switchgrass forage and two sets of alfalfa stem
(Mullen and Boateng 2008), short-rotation woody crops for
woody biomass (Zalesny et al. 2011), solid biofuel from
energy crops (Karampinis et al. 2012), bioenergy form
ryegrass, sugarcane, willow, sugar beet, and Miscanthus
Gigantes crops (Hattori and Morita 2010; Tonini et al.
2012). Agricultural waste is also used as energy crops
material for the production of bioenergy (Ferronato and
Torretta 2019). Bioenergy-generating crops are classified as
biofuel crops, biomass crops, bioethanol crops, biodiesel
crops, lignocellulosic crops, etc. Some countries, such as
Mexico, have a relative abundance of agricultural land for
the cultivation of energy crops because of the warm and
sunny climate. With well-developed infrastructure facilities
and easy availability of labor, it is possible to generate
bioenergy from energy crops for commercial applications
(Ruiz et al. 2016). Asian countries such as Japan, India, and
a few more have paddy land for the cultivation of rice as
energy crops (Hattori and Morita 2010). Energy crops are
standalone as an alternative source for fossil fuels.
In the present situation, the world generates two billion
tonnes of municipal waste every year. About 33% of the
generated municipal solid waste are not utilized in an
environmentally safe way. Worldwide, a person is responsible to produce about 0.74 kg of waste every day with
high-income countries generating about 34% of the world’s
waste. As the population increases, the waste generated is
anticipated to reach 3.40 billion tonnes by 2050. The projected waste generation, by region (millions of tonnes/year)
by the World Bank, is given in Fig. 3 (Kaza et al. 2018). The
conversion of biowaste into bioenergy is always a boon, but
there are certain adverse effects of biowaste on the atmosphere and human life. Generally, countries follow the 3R
rule: reduce, reuse, and recycle. If the wastes are disposed of
in an uncontrolled way, there will be an impact on air quality
and human life. Besides, uncontrolled disposing of wastes
will increase emissions of greenhouse gases, pollute the
marine system, pollute soil, and spread diseases (Ferronato
and Torretta 2019). Various reports also mention the impact
of biowastes on the environment, for example, air pollution
due to the simple process of bioconversion such as the
production of biochar from biowaste (Sparrevik et al. 2014).
Biomedical waste dumping sites cause emissions of hazardous gases and an increase in heavy metal contents like
chromium, zinc, nickel, lead, and copper (Manzoor and
Sharma 2019; Chionyedua Theresa 2014). Municipal biowaste generates gases such as methane, carbon dioxide,
carbon monoxide which lead to global warming issues
(Vieira and Matheus 2019). Municipal waste also causes
soil, water, and air pollution (Colón et al. 2012). Animal
biowaste may impact the processing of other biowastes
Fig. 2 Schematics for the
biosynthesis of biomethanol.
Adapted with permission (Paliwal
et al. 2019). Copyright (2019),
Springer, Singapore
Bioconversion of Biowastes for Energy Applications
3