very important at this stage, as it prevents pH drop inside the
biodigester that could kill the methanogenic archaea.
2.5.3 Acetogenesis
In this phase, organic acids and alcohols are converted to
acetate, carbon dioxide and hydrogen substrates that are used
by methanogenic bacteria. The hydrogen produced in acetogenesis plays a relevant intermediate role, since the reaction occurs when the partial pressure of the hydrogen is low
enough. The decrease in partial pressure is performed by
bacteria that degrade hydrogen (Yousuf 2019). Acetogenesis
is a fundamental step to efficiently produce biogas because
approximately 70% of methane originates from the acetate
reduction process (Zieminski and Frac 2012).
The symbiotic activity among microorganisms is essential
in this phase, as they perform anaerobic oxidation reactions
together with methanogenic microorganisms (Yousuf 2019).
Acetogenesis produces hydrogen, which can generate
excessive partial pressure that is harmful to acetogenic
microorganisms. However, due to the presence of methanogens, hydrogen can be consumed quickly, maintaining
partial hydrogen pressures to favor acetogenesis (Stams and
Plugge 2009).
2.5.4 Methanogenesis
In this last stage of biogas production, organic acids and H 2
are converted into methane. The efficiency of methanogenesis is highly dependent on the last three steps and the
methane production microbial community. Methanogenesis
is a slow and sensible step severely influenced by operational conditions (Durruty and Gonzalez 2015).
3 Conditions and Parameters
Because some parameters influence the efficiency of the
process, appropriate conditions must be provided for
anaerobic microorganisms. The growth and activity of
anaerobic microorganisms is significantly influenced by O 2 ,
temperature, pH, availability of nutrients, agitation and
presence and quantity of inhibitors (Whitman et al. 2006).
3.1 Nutrients
The presence of nutrients such as carbon, nitrogen, potassium, phosphorus and sulfur, and some mineral micronutrients, vitamins and amino acids are necessary for the
development of methanogenic bacteria. Microelements are
as important as the macroelements for the growth and survival of the microorganisms involved in the process. The
insufficient supply of these nutrients can cause inhibition and
disturbances in the process (Al Seadi et al. 2008), so
knowing the complete composition of the substrate is
important. Apart from micronutrients, the C:N ratio of the
substrate must be in the range of 1:30–35 (Deublein and
Steinhauser 2008). If necessary, the dosage of specific
nutrients and chemical activators can be carried out so that a
good biomass fermentation occurs.
3.2 pH
Microbial growth in an anaerobic digester is largely
dependent on pH (Yang et al. 2015). The ideal growth of the
hydrolytic microorganisms involved in the biodigestion
process occurs preferentially from 5.0 to 6.0, while the best
pH for the methanogenic bacteria is between 6.5 and 8.0
(Kundu et al. 2017).
In one-stage operation, the favorable pH range to meet
the requirements of most microbial groups involved in the
process must be between 6.8 and 7.4 (Tabatabaei and
Ghavanati 2018). At pH below 6.5, the interruption of
methane production can be noticed (Ostrem 2004). However, fermentation usually continues but generating other
products. Hernandez and Rodrigues (2013) analyzed the
impact of low pH values and observed that hydrogen was
preferentially produced at pH < 6, with methane representing less than 1%.
There are two systems to ensure the pH balance: bicarbonate and ammonium alkalinity. Alkalinity is important
because it represents the capacity of the digester to neutralize
the organic acids formed during the acidogenic phase.
Bicarbonate buffering systems occur in pH close to 7.0,
while ammonium buffering occurs at pH close to 9.25
(Deublein and Steinhauser 2008) and is not recommended
for biodigestion (Rabii et al. 2019).
For a stable methane production, average alkalinity
should be between 200 and 5,000 mg/L. However, due to
the organic acid formation within the biodigester, pH tends
to decrease. Trusting in pH control can be tricky because
drastic changes in pH values can occur when the buffer
capacity of the substrate is outstripped. The time necessary
to reestablish the buffering capacity of the system may not be
enough to avoid the collapse of the biodigester. In anaerobic
digesters with low buffering capacity, pH, partial alkalinity
and AGVs are reliable indicators for process imbalance. In
these cases where the substrate has recognizable low
buffering properties, the codigestion with high alkalinity
substrates is recommended (Trabold and Babbitt 2018). On
the other hand, in highly buffered systems, pH changes can
be small even when the process suffers turbulences, then
AGVs can be considered as the unique reliable parameters
for monitoring processes (Murto et al. 2004).
Because the solubility of the gases depends on the temperature, the buffering capacity of thermophilic and
Bioconversion of Food Waste to Biogas
99
biodigester that could kill the methanogenic archaea.
2.5.3 Acetogenesis
In this phase, organic acids and alcohols are converted to
acetate, carbon dioxide and hydrogen substrates that are used
by methanogenic bacteria. The hydrogen produced in acetogenesis plays a relevant intermediate role, since the reaction occurs when the partial pressure of the hydrogen is low
enough. The decrease in partial pressure is performed by
bacteria that degrade hydrogen (Yousuf 2019). Acetogenesis
is a fundamental step to efficiently produce biogas because
approximately 70% of methane originates from the acetate
reduction process (Zieminski and Frac 2012).
The symbiotic activity among microorganisms is essential
in this phase, as they perform anaerobic oxidation reactions
together with methanogenic microorganisms (Yousuf 2019).
Acetogenesis produces hydrogen, which can generate
excessive partial pressure that is harmful to acetogenic
microorganisms. However, due to the presence of methanogens, hydrogen can be consumed quickly, maintaining
partial hydrogen pressures to favor acetogenesis (Stams and
Plugge 2009).
2.5.4 Methanogenesis
In this last stage of biogas production, organic acids and H 2
are converted into methane. The efficiency of methanogenesis is highly dependent on the last three steps and the
methane production microbial community. Methanogenesis
is a slow and sensible step severely influenced by operational conditions (Durruty and Gonzalez 2015).
3 Conditions and Parameters
Because some parameters influence the efficiency of the
process, appropriate conditions must be provided for
anaerobic microorganisms. The growth and activity of
anaerobic microorganisms is significantly influenced by O 2 ,
temperature, pH, availability of nutrients, agitation and
presence and quantity of inhibitors (Whitman et al. 2006).
3.1 Nutrients
The presence of nutrients such as carbon, nitrogen, potassium, phosphorus and sulfur, and some mineral micronutrients, vitamins and amino acids are necessary for the
development of methanogenic bacteria. Microelements are
as important as the macroelements for the growth and survival of the microorganisms involved in the process. The
insufficient supply of these nutrients can cause inhibition and
disturbances in the process (Al Seadi et al. 2008), so
knowing the complete composition of the substrate is
important. Apart from micronutrients, the C:N ratio of the
substrate must be in the range of 1:30–35 (Deublein and
Steinhauser 2008). If necessary, the dosage of specific
nutrients and chemical activators can be carried out so that a
good biomass fermentation occurs.
3.2 pH
Microbial growth in an anaerobic digester is largely
dependent on pH (Yang et al. 2015). The ideal growth of the
hydrolytic microorganisms involved in the biodigestion
process occurs preferentially from 5.0 to 6.0, while the best
pH for the methanogenic bacteria is between 6.5 and 8.0
(Kundu et al. 2017).
In one-stage operation, the favorable pH range to meet
the requirements of most microbial groups involved in the
process must be between 6.8 and 7.4 (Tabatabaei and
Ghavanati 2018). At pH below 6.5, the interruption of
methane production can be noticed (Ostrem 2004). However, fermentation usually continues but generating other
products. Hernandez and Rodrigues (2013) analyzed the
impact of low pH values and observed that hydrogen was
preferentially produced at pH < 6, with methane representing less than 1%.
There are two systems to ensure the pH balance: bicarbonate and ammonium alkalinity. Alkalinity is important
because it represents the capacity of the digester to neutralize
the organic acids formed during the acidogenic phase.
Bicarbonate buffering systems occur in pH close to 7.0,
while ammonium buffering occurs at pH close to 9.25
(Deublein and Steinhauser 2008) and is not recommended
for biodigestion (Rabii et al. 2019).
For a stable methane production, average alkalinity
should be between 200 and 5,000 mg/L. However, due to
the organic acid formation within the biodigester, pH tends
to decrease. Trusting in pH control can be tricky because
drastic changes in pH values can occur when the buffer
capacity of the substrate is outstripped. The time necessary
to reestablish the buffering capacity of the system may not be
enough to avoid the collapse of the biodigester. In anaerobic
digesters with low buffering capacity, pH, partial alkalinity
and AGVs are reliable indicators for process imbalance. In
these cases where the substrate has recognizable low
buffering properties, the codigestion with high alkalinity
substrates is recommended (Trabold and Babbitt 2018). On
the other hand, in highly buffered systems, pH changes can
be small even when the process suffers turbulences, then
AGVs can be considered as the unique reliable parameters
for monitoring processes (Murto et al. 2004).
Because the solubility of the gases depends on the temperature, the buffering capacity of thermophilic and
Bioconversion of Food Waste to Biogas
99