inhibiting compounds. Features and advantages of different
types of biodigesters are also presented, as well as laboratory
analyses of the organic waste used in the biogas production
unit and the importance of the composition of the substrate
in the establishment of stable and productive systems.
2 Biogas Production Steps from Organic
Waste
Within the biodigester, there are a series of reactions that occur
for the production of biogas. These reactions are caused by
thousands of bacteria that need ideal conditions for their performance. The biodigestion process is influenced by several
factors, such as temperature, substrate composition, presence
of oxygen, a model of the biodigester, operational conditions
and the presence of materials of a toxic nature, among others.
In comparison to other types of treatment, the anaerobic
process is satisfactory in relation to fluctuations in load. It is
thus necessary to select the residues that feed in the biodigester and pre-process them to ensure that the ideal conditions for the microbial activity is reached (Ferguson et al.
2016). Techniques for the treatment of these organic residues are necessary before the biodigestion process itself
takes place, avoiding problems in the operation of the plant
and optimizing biogas production.
2.1 Raw Materials
The main advantage of using organic waste in biogas plants
is to combine proper disposal with the concomitant generation of biofertilizer and biogas, which is a renewable and
low-cost energy source. However, the biggest obstacle is the
variable composition of the waste that goes through the
biodigestion process. The range of organic residues is vast
and in addition to the possibility of containing substances
toxic to bacteria, their poor segregation can cause operational problems for the biodigester.
In the case of organic waste from various sources, the
control of the separation of organic waste is more complicated and the presence of contaminants such as plastic, glass,
metals, stones, sand, among others, is more common
(Wojnowska-Baryla et al. 2020). The segregation of organic
waste in a shopping mall and a food industry, for example,
would be different. In the shopping mall, the consumers are
responsible for the segregation, who often discard the metal
cutlery, plastics and other waste together with the organic. In
the food industry, however, the employees are constantly
trained to segregate correctly, reducing the possibility of
contaminating organic waste with other materials.
Due to this problem, greater attention is needed for the
reception and segregation process of these residues before
inserting them in the process called pre-treatment.
2.2 Pre-treatment of Organic Wastes
Different pre-treatment techniques are used to process
organic waste before the biodigestion process. Pre-treatment
allows the removal of undesirable inorganic elements and
guarantees a suitable compound for biodigestion. This stage
is divided into receiving organic waste and crushing it.
According to Tabatabaei and Ghavanati (2018), organic
waste pre-treatment systems must meet the following
essential requirements: (i) digestion of a variety of organic
wastes; (ii) substrate homogenization; (iii) removal of contaminants; and (iv) high production of biogas in anaerobic
digesters. The composition of the waste is the most essential
criterion for selecting an appropriate pre-treatment technology, followed by the model of the biodigester and whether
wet or dry digestion system is used.
Wet anaerobic systems are operated at lower concentrations of solids, with a solids content between 4 and 8%
(Leite and Lopes 2009) and use pre-treatment systems to
remove unwanted contaminants prior fermentation. The
digestate after biodigestion can be used directly as a
high-quality fertilizer and no additional treatment is usually
required. Dry systems are operated at higher concentrations
of solids, with solids content around 20% (LEITE et al.
2009) and they depend on elementary pre-treatment systems.
Consequently, the digestate needs additional treatment to be
used as fertilizer.
Notably, organic waste is heterogeneous, varying both in
the moisture content and the level of contamination.
Therefore, pre-treatment preceding biodigestion is of great
importance, resulting in increased biogas production and
yield and high quality of biofertilizers.
2.3 Reception of Organic Wastes
A suitable place for receiving waste is essential. As previously mentioned, many inorganic wastes arrive at the plants
mixed with organic wastes. The reception of residues must
be simple so that the process does not become impracticable,
but it should be efficient so that most of the residues undesirable to biodigestion are removed.
In some cases, the waste arrives at the unit in plastic
packaging, so the first stage of segregation is necessary; this
stage is called the bag tearing. Equipment called bag breaker
is ideal for removing the contents of the bags in an
Bioconversion of Food Waste to Biogas
97
types of biodigesters are also presented, as well as laboratory
analyses of the organic waste used in the biogas production
unit and the importance of the composition of the substrate
in the establishment of stable and productive systems.
2 Biogas Production Steps from Organic
Waste
Within the biodigester, there are a series of reactions that occur
for the production of biogas. These reactions are caused by
thousands of bacteria that need ideal conditions for their performance. The biodigestion process is influenced by several
factors, such as temperature, substrate composition, presence
of oxygen, a model of the biodigester, operational conditions
and the presence of materials of a toxic nature, among others.
In comparison to other types of treatment, the anaerobic
process is satisfactory in relation to fluctuations in load. It is
thus necessary to select the residues that feed in the biodigester and pre-process them to ensure that the ideal conditions for the microbial activity is reached (Ferguson et al.
2016). Techniques for the treatment of these organic residues are necessary before the biodigestion process itself
takes place, avoiding problems in the operation of the plant
and optimizing biogas production.
2.1 Raw Materials
The main advantage of using organic waste in biogas plants
is to combine proper disposal with the concomitant generation of biofertilizer and biogas, which is a renewable and
low-cost energy source. However, the biggest obstacle is the
variable composition of the waste that goes through the
biodigestion process. The range of organic residues is vast
and in addition to the possibility of containing substances
toxic to bacteria, their poor segregation can cause operational problems for the biodigester.
In the case of organic waste from various sources, the
control of the separation of organic waste is more complicated and the presence of contaminants such as plastic, glass,
metals, stones, sand, among others, is more common
(Wojnowska-Baryla et al. 2020). The segregation of organic
waste in a shopping mall and a food industry, for example,
would be different. In the shopping mall, the consumers are
responsible for the segregation, who often discard the metal
cutlery, plastics and other waste together with the organic. In
the food industry, however, the employees are constantly
trained to segregate correctly, reducing the possibility of
contaminating organic waste with other materials.
Due to this problem, greater attention is needed for the
reception and segregation process of these residues before
inserting them in the process called pre-treatment.
2.2 Pre-treatment of Organic Wastes
Different pre-treatment techniques are used to process
organic waste before the biodigestion process. Pre-treatment
allows the removal of undesirable inorganic elements and
guarantees a suitable compound for biodigestion. This stage
is divided into receiving organic waste and crushing it.
According to Tabatabaei and Ghavanati (2018), organic
waste pre-treatment systems must meet the following
essential requirements: (i) digestion of a variety of organic
wastes; (ii) substrate homogenization; (iii) removal of contaminants; and (iv) high production of biogas in anaerobic
digesters. The composition of the waste is the most essential
criterion for selecting an appropriate pre-treatment technology, followed by the model of the biodigester and whether
wet or dry digestion system is used.
Wet anaerobic systems are operated at lower concentrations of solids, with a solids content between 4 and 8%
(Leite and Lopes 2009) and use pre-treatment systems to
remove unwanted contaminants prior fermentation. The
digestate after biodigestion can be used directly as a
high-quality fertilizer and no additional treatment is usually
required. Dry systems are operated at higher concentrations
of solids, with solids content around 20% (LEITE et al.
2009) and they depend on elementary pre-treatment systems.
Consequently, the digestate needs additional treatment to be
used as fertilizer.
Notably, organic waste is heterogeneous, varying both in
the moisture content and the level of contamination.
Therefore, pre-treatment preceding biodigestion is of great
importance, resulting in increased biogas production and
yield and high quality of biofertilizers.
2.3 Reception of Organic Wastes
A suitable place for receiving waste is essential. As previously mentioned, many inorganic wastes arrive at the plants
mixed with organic wastes. The reception of residues must
be simple so that the process does not become impracticable,
but it should be efficient so that most of the residues undesirable to biodigestion are removed.
In some cases, the waste arrives at the unit in plastic
packaging, so the first stage of segregation is necessary; this
stage is called the bag tearing. Equipment called bag breaker
is ideal for removing the contents of the bags in an
Bioconversion of Food Waste to Biogas
97