developing systematic processes for the conversion of biowaste into bioenergy (Farmanbordar et al. 2018; Cardoen
et al. 2015). The conversion method is largely determined by
the type of biowastes used for the production of bioenergy.
Generally, biowastes are subdivided into two main types:
(i) waste biomass and (ii) energy crops. The details of these
biowastes are discussed in the following sections.
1.1 Waste Biomass
This type of biomass is generated through forest residues,
agriculture, animals, food waste, and municipal waste
(Bhatia et al. 2018). If all these bioresources are left to
biodegrade in an uncontrolled way, it may be very harmful
to the environment. This would lead to an increase in pollution, which transforms into contaminants and produces
climate affecting gases. The ultimate effect of uncontrolled
waste management is global warming (Idehai and Akujieze
2015; Wang et al. 2017). Conversion of these biowastes to
commercial applications is necessary for sustainability in the
present era. The biowastes can be transformed into
high-value materials such as biogas, biodiesel, bioalcohols
(methanol, ethanol, propanol, etc.), electricity, charcoal,
syngas, and heat energy. In the conversion of biowastes to
value-added products, different methodologies have been
developed. The process of conversion of biowastes into
value-added products can be divided into two groups: the
biological and the physicochemical methods (Fig. 1).
The biological methods involve an anaerobic digestion,
microbial enzymatic, fermentation and composting, microbial fuel cell, etc. The anaerobic digestion process is mostly
used in the generation of methane and hydrogen gases
(Wang et al. 2016; Borja et al. 2003). The anaerobic
digestion process can be combined with other processes to
produce biofuel as shown in Fig. 2.
Liu et al. used zero-valent iron for improved methane
generation by using wastewater sludge (Liu et al. 2015). It
was observed that the chemical properties of iron have a
significant effect on the production of methane. The production of methane was higher when either clean or rusty
iron was used than iron powder. A few reports also suggested an improvement in methane production using different parameters and methods such as alkaline microwaving
pretreatment (Yu et al. 2017), the effect of sonication (Aldin
et al. 2010), solid concentration, and temperature (Paritosh
et al. 2019). The transesterification method is the main
method applied for the generation of biodiesel from various
sources like jatropha oil (Lau et al. 2016), waste cooking oil
(Degfie et al. 2019), frying oil (Vastano et al. 2019), etc.
Also, a few innovative techniques such as a fluidic system
(Yeh et al. 2016) and co-solvent method (Thanh et al. 2013)
have been employed for the enhancement of bioenergy
production. Nowadays, a few nanocatalysts are also
employed for extensive and greener creation of biodiesel
such as CaO (Degfie et al. 2019), carbon nanotubes with iron
oxide (Fan et al. 2017), and iron-doped zinc oxide (Baskar
and Soumiya 2016). Bioalcohols such as methanol, ethanol,
and butanol are produced using the fermentation process
(Bušić et al. 2018; Luque et al. 2008). The innovative
organisms named electricigens offer the possibility of efficiently converting organic compounds into electricity (Liu
et al. 2014; Lovley 2006). The assessment of the finest use
of biomasses for heat, electricity, and transport purpose has
been investigated in Europe (Steubing et al. 2012). An
energy system model was proposed and various methodologies for the generation of energy using fossil and biomass
were compared. It was observed that conversion efficiency
was the key factor for the optimal use of biomass for these
applications. It was further noted that woody biomass is the
paramount material for energy generation if fossil fuels have
to replace.
The physicochemical process involves pyrolysis,
hydrothermal carbonization, gasification, landfill, and
incineration. For example, biochar and charcoal are being
mostly produced using the pyrolysis method (Santín et al.
2017). Biochar and activated carbon produced via pyrolysis
are cost-effective and can be used for the removal of
micropollutants in wastewater, however, their effectiveness
as micropollutants removal depends on biochar production
conditions and treatment capabilities (Thompson et al.
2016). The carbon and charcoal are also prepared using
hydrothermal carbonization of different biowastes like
biorefinery waste (Ho et al. 2018), human waste (Afolabi
et al. 2017), carbohydrates, and organic molecules (Hu et al.
2010). Biomass can be used to generate syngas, methane,
and hydrogen via the gasification method (Richardson et al.
Fig. 1 Graphical illustration of the use of biowastes for bioenergy
using biological and physicochemical processes
2
A. M. Palve et al.
et al. 2015). The conversion method is largely determined by
the type of biowastes used for the production of bioenergy.
Generally, biowastes are subdivided into two main types:
(i) waste biomass and (ii) energy crops. The details of these
biowastes are discussed in the following sections.
1.1 Waste Biomass
This type of biomass is generated through forest residues,
agriculture, animals, food waste, and municipal waste
(Bhatia et al. 2018). If all these bioresources are left to
biodegrade in an uncontrolled way, it may be very harmful
to the environment. This would lead to an increase in pollution, which transforms into contaminants and produces
climate affecting gases. The ultimate effect of uncontrolled
waste management is global warming (Idehai and Akujieze
2015; Wang et al. 2017). Conversion of these biowastes to
commercial applications is necessary for sustainability in the
present era. The biowastes can be transformed into
high-value materials such as biogas, biodiesel, bioalcohols
(methanol, ethanol, propanol, etc.), electricity, charcoal,
syngas, and heat energy. In the conversion of biowastes to
value-added products, different methodologies have been
developed. The process of conversion of biowastes into
value-added products can be divided into two groups: the
biological and the physicochemical methods (Fig. 1).
The biological methods involve an anaerobic digestion,
microbial enzymatic, fermentation and composting, microbial fuel cell, etc. The anaerobic digestion process is mostly
used in the generation of methane and hydrogen gases
(Wang et al. 2016; Borja et al. 2003). The anaerobic
digestion process can be combined with other processes to
produce biofuel as shown in Fig. 2.
Liu et al. used zero-valent iron for improved methane
generation by using wastewater sludge (Liu et al. 2015). It
was observed that the chemical properties of iron have a
significant effect on the production of methane. The production of methane was higher when either clean or rusty
iron was used than iron powder. A few reports also suggested an improvement in methane production using different parameters and methods such as alkaline microwaving
pretreatment (Yu et al. 2017), the effect of sonication (Aldin
et al. 2010), solid concentration, and temperature (Paritosh
et al. 2019). The transesterification method is the main
method applied for the generation of biodiesel from various
sources like jatropha oil (Lau et al. 2016), waste cooking oil
(Degfie et al. 2019), frying oil (Vastano et al. 2019), etc.
Also, a few innovative techniques such as a fluidic system
(Yeh et al. 2016) and co-solvent method (Thanh et al. 2013)
have been employed for the enhancement of bioenergy
production. Nowadays, a few nanocatalysts are also
employed for extensive and greener creation of biodiesel
such as CaO (Degfie et al. 2019), carbon nanotubes with iron
oxide (Fan et al. 2017), and iron-doped zinc oxide (Baskar
and Soumiya 2016). Bioalcohols such as methanol, ethanol,
and butanol are produced using the fermentation process
(Bušić et al. 2018; Luque et al. 2008). The innovative
organisms named electricigens offer the possibility of efficiently converting organic compounds into electricity (Liu
et al. 2014; Lovley 2006). The assessment of the finest use
of biomasses for heat, electricity, and transport purpose has
been investigated in Europe (Steubing et al. 2012). An
energy system model was proposed and various methodologies for the generation of energy using fossil and biomass
were compared. It was observed that conversion efficiency
was the key factor for the optimal use of biomass for these
applications. It was further noted that woody biomass is the
paramount material for energy generation if fossil fuels have
to replace.
The physicochemical process involves pyrolysis,
hydrothermal carbonization, gasification, landfill, and
incineration. For example, biochar and charcoal are being
mostly produced using the pyrolysis method (Santín et al.
2017). Biochar and activated carbon produced via pyrolysis
are cost-effective and can be used for the removal of
micropollutants in wastewater, however, their effectiveness
as micropollutants removal depends on biochar production
conditions and treatment capabilities (Thompson et al.
2016). The carbon and charcoal are also prepared using
hydrothermal carbonization of different biowastes like
biorefinery waste (Ho et al. 2018), human waste (Afolabi
et al. 2017), carbohydrates, and organic molecules (Hu et al.
2010). Biomass can be used to generate syngas, methane,
and hydrogen via the gasification method (Richardson et al.
Fig. 1 Graphical illustration of the use of biowastes for bioenergy
using biological and physicochemical processes
2
A. M. Palve et al.