substrate (Szamosi and Siménfalvi 2020). Another benefit of
agitation is the prevention of sedimentation and dead areas
inside the biodigester, making all the organic matter available to the microorganisms (Gueri 2017). The agitation can
be operated by automated devices, recirculation of the
digestate or the produced biogas (Karim et al. 2005).
3.7 Inhibitors
There is a wide variety of compounds that inhibit the
biodigestion that can be introduced through feeding or is
formed during the process. Upon entering the process, substances such as oil, grease, phenols, paracetamol, caffeine,
ibuprofen, triclosan, volatile aromatic and heavy metals,
antibiotics, detergents, chlorine, among others, are able to
inhibit the anaerobic process, ceasing microbial activity
(Haak et al. 2016). Other agents such as nitrates, cyanides,
phenols, sodium, potassium, calcium, magnesium, ammoniacal nitrogen, oxygen and heavy metals can also inhibit
partial or complete the production of methane. The level of
inhibition depends on (i) the concentration of the inhibitory
compounds, (ii) the ability of bacteria to adapt to it (Deublein and Steinhauser 2008), (iii) the process conditions and
(iv) the presence of synergistic compounds (when the effect
of the inhibitory agents combined is greater than the sum of
individual effects).
3.8 Temperature
The anaerobic digestion process is greatly influenced by
temperature. The sudden variation of this parameter can lead
to the destabilization of the reactor and death of microorganisms (Singh et al. 2017). The microbial population of the
biodigester is defined according to the temperature range used
in the system. The methanogenic archaea are divided into
mesophilic (20–40 °C) and the thermophilic (50–60 °C).
Mesophilic processes are generally operated between 35
and 37 °C (Van and Fujiwara 2019). They are the most used
due to installation and lower energy consumption. In addition, mesophilic biodigestion presents greater process stability (Gebreeyessus and Jenicek 2016) because the
inhibition by ammonium is substantially reduced due to the
preferred formation of free ammonia (Leite 2015).
Thermophilic systems have some advantages, such as
faster biogas production and efficient destruction of pathogens (Ruffino and Campo 2015). Thermophilic digesters are
usually smaller and feed at higher rates (De La Rubia et al.
2002). In addition, oxygen is less soluble in the thermophilic
temperature range so that ideal anaerobic operating
conditions are reached more quickly (Leite 2015). On the
other hand, maintaining high temperatures requires greater
energy expenditure, especially in colder regions, which can
overcome the cited advantages of the process. Moreover, the
thermophilic process is more sensitive to environmental
variations, such as interruptions in food, temperature and
load (Kim et al. 2006; Parawira et al. 2004). The thermophilic system is most indicated when the substrate used is
generated at high temperatures or when pathogens are present (Ruffino and Campo 2015). Two stages of biodigestion
systems in which methanogenesis and hydrolysis are operated at different temperatures exist (Liao et al. 2018) (usually
mesophilic hydrolysis and thermophilic methanogenesis).
3.9 Types of Biodigesters
The choice of the most suitable biodigester depends on
several factors, including the amount of total solids of the
substrate, the need to control parameters such as agitation,
heating and economic factors. The biodigesters mostly used
for the treatment of organic solid waste are presented.
Continuous Flow Stirred Tank Reactor (CSTR): It is
the type of biodigester most used with high organic loads
and high content of solids (5–15%) (Furst et al. 2019).
Generally, it results in reduced hydraulic retention time (14–
28 days, depending on the substrate and the operating temperature) in comparison to other types of biodigesters
(Verma 2002).
Batch: Biodigestion carried in batch mode is characterized by the addition of a certain amount of biomass to the
biodigester, which is then controlled until no more gas is
produced. Then, the reactor is opened, cleaned and a new
batch is started. The new batch is usually inoculated with the
mixture of solid digestate formed in the previous batch and
the liquid digestate is recirculated to increase the contact of
the microorganisms with the biomass. The amount of total
solids for the use of this type of biodigester is above 30%
(Furst et al. 2019).
Piston flow: It is indicated for a wide variety of organic
substrates with solids content between 15 and 45%. The
biomass is transported (vertically or horizontally) from the
beginning to the end of the process by means of pumps,
being able to operate in parallel with other digesters and
generally operates in the mesophilic or thermophilic phases
(Furst et al. 2019).
Covered pond biodigester [BLC]: This model is widely
used in rural areas to treat animal waste, industrial and
agricultural waste with low solids concentrations (0.5–3%)
and hydraulic retention times between 30 and 60 days
(Probiogas 2015).
Bioconversion of Food Waste to Biogas
101
agitation is the prevention of sedimentation and dead areas
inside the biodigester, making all the organic matter available to the microorganisms (Gueri 2017). The agitation can
be operated by automated devices, recirculation of the
digestate or the produced biogas (Karim et al. 2005).
3.7 Inhibitors
There is a wide variety of compounds that inhibit the
biodigestion that can be introduced through feeding or is
formed during the process. Upon entering the process, substances such as oil, grease, phenols, paracetamol, caffeine,
ibuprofen, triclosan, volatile aromatic and heavy metals,
antibiotics, detergents, chlorine, among others, are able to
inhibit the anaerobic process, ceasing microbial activity
(Haak et al. 2016). Other agents such as nitrates, cyanides,
phenols, sodium, potassium, calcium, magnesium, ammoniacal nitrogen, oxygen and heavy metals can also inhibit
partial or complete the production of methane. The level of
inhibition depends on (i) the concentration of the inhibitory
compounds, (ii) the ability of bacteria to adapt to it (Deublein and Steinhauser 2008), (iii) the process conditions and
(iv) the presence of synergistic compounds (when the effect
of the inhibitory agents combined is greater than the sum of
individual effects).
3.8 Temperature
The anaerobic digestion process is greatly influenced by
temperature. The sudden variation of this parameter can lead
to the destabilization of the reactor and death of microorganisms (Singh et al. 2017). The microbial population of the
biodigester is defined according to the temperature range used
in the system. The methanogenic archaea are divided into
mesophilic (20–40 °C) and the thermophilic (50–60 °C).
Mesophilic processes are generally operated between 35
and 37 °C (Van and Fujiwara 2019). They are the most used
due to installation and lower energy consumption. In addition, mesophilic biodigestion presents greater process stability (Gebreeyessus and Jenicek 2016) because the
inhibition by ammonium is substantially reduced due to the
preferred formation of free ammonia (Leite 2015).
Thermophilic systems have some advantages, such as
faster biogas production and efficient destruction of pathogens (Ruffino and Campo 2015). Thermophilic digesters are
usually smaller and feed at higher rates (De La Rubia et al.
2002). In addition, oxygen is less soluble in the thermophilic
temperature range so that ideal anaerobic operating
conditions are reached more quickly (Leite 2015). On the
other hand, maintaining high temperatures requires greater
energy expenditure, especially in colder regions, which can
overcome the cited advantages of the process. Moreover, the
thermophilic process is more sensitive to environmental
variations, such as interruptions in food, temperature and
load (Kim et al. 2006; Parawira et al. 2004). The thermophilic system is most indicated when the substrate used is
generated at high temperatures or when pathogens are present (Ruffino and Campo 2015). Two stages of biodigestion
systems in which methanogenesis and hydrolysis are operated at different temperatures exist (Liao et al. 2018) (usually
mesophilic hydrolysis and thermophilic methanogenesis).
3.9 Types of Biodigesters
The choice of the most suitable biodigester depends on
several factors, including the amount of total solids of the
substrate, the need to control parameters such as agitation,
heating and economic factors. The biodigesters mostly used
for the treatment of organic solid waste are presented.
Continuous Flow Stirred Tank Reactor (CSTR): It is
the type of biodigester most used with high organic loads
and high content of solids (5–15%) (Furst et al. 2019).
Generally, it results in reduced hydraulic retention time (14–
28 days, depending on the substrate and the operating temperature) in comparison to other types of biodigesters
(Verma 2002).
Batch: Biodigestion carried in batch mode is characterized by the addition of a certain amount of biomass to the
biodigester, which is then controlled until no more gas is
produced. Then, the reactor is opened, cleaned and a new
batch is started. The new batch is usually inoculated with the
mixture of solid digestate formed in the previous batch and
the liquid digestate is recirculated to increase the contact of
the microorganisms with the biomass. The amount of total
solids for the use of this type of biodigester is above 30%
(Furst et al. 2019).
Piston flow: It is indicated for a wide variety of organic
substrates with solids content between 15 and 45%. The
biomass is transported (vertically or horizontally) from the
beginning to the end of the process by means of pumps,
being able to operate in parallel with other digesters and
generally operates in the mesophilic or thermophilic phases
(Furst et al. 2019).
Covered pond biodigester [BLC]: This model is widely
used in rural areas to treat animal waste, industrial and
agricultural waste with low solids concentrations (0.5–3%)
and hydraulic retention times between 30 and 60 days
(Probiogas 2015).
Bioconversion of Food Waste to Biogas
101