mesophilic systems differs. The pH value of thermophilic
systems is usually higher due to the capacity of carbon
dioxide to dissolve in water at high temperatures (Pareek and
Pareek 2019). According to Tabatabaei and Ghavanati
(2018a), other factors can interfere in the buffering system,
such as high organic feeding rates, the presence of toxic
compounds, temperature decrease, high organic load, high
sulfide content, pH increase as a consequence of H 2 S formation and an increase of free ammonia. When acidification
is detected, one or more of the following strategies should be
considered: stop feeding, increase the retention time,
increase the buffering capacity, add pH neutralizing agents
and add water.
3.3 FOS/TAC
FOS/TAC is proved to be a critical parameter that serves to
quickly assess the stability of a biodigester, as it provides
data in an easy and continuous way. FOS stands for volatile
fatty acids, while TAC stands for total inorganic carbon
(carbon buffering capacity) (Logan et al. 2019). It is considered a more reliable parameter than pH because it gives
important information on the increase of FOS even when the
biodigester’s pH is within the optimal range; thus, allowing
interventions that anticipate pH variation.
FOS/TAC values should be ideally between 0.1 and 0.3
(Fachagentur Nachwachsende Rohstoffe 2010). Values
above the limit indicate that the proportion of organic acids
is very high, which means that acidogenesis is being dominant. The accumulation of acids causes a drop in pH and the
consequent inactivation of methanogenic bacteria, leading to
inhibition of methane production. Values below 0.1 means
system’s alkalosis, which is the result of impaired hydrolysis
and/or acidogenesis. The most common causes are lack of
macro and/or micronutrients, excess feeding with easily
degradable material (simple carbohydrates) and constant
changes of the substrate composition (Chavarria et al. 2018).
FOS/TAC should be regularly measured to identify any
deviations and implement countermeasures if necessary
early. However, FOS/TAC is highly influenced by feeding;
measurements made right before and after substrate addition
provides significantly different results (Voss et al. 2009).
Thus, stating clear rules for sampling, preparation and
titration of samples is highly suggested.
3.4 Organic Load Rate (OLR) and Solid Content
Another important parameter in the operation of biodigesters
is the added mass of organic waste per unit volume of the
reactor per time, which is known as organic load rate (OLR).
Theoretically, as much substrate is fed to the biodigester so
that methane production should be higher. However, higher
OLR results in acidification and system collapse. An ideal
OLR in CSTR-type mesophilic biodigesters varies between
3 and 5 kg VS /m
3 /d, according to the substrate (Drosg 2013),
while the system failure is observed at rates above 6.4 kg VS /
m
3 /d (Moriarty 2013).
Most biogas plants are operated, for safety, with OLR
lower than the optimum. This strategy minimizes possible
errors and process fluctuations, especially when complex
substrates such as food wastes are used. On the other hand,
the safety margin used is indirectly proportional to the system’s efficiency and operational costs (Tabatabaei and
Ghavanati 2018). Biogas plants are generally designed to
convert 75% of the maximum degradable organic matter
(Deublein and Steinhauser 2008).
3.5 Inoculum
Biodigestion depends directly on the action of microorganisms. A microbial community can grow naturally in a
biodigestion system or can be artificially added to it. The
addition of an inoculum helps to accelerate the process since
the development and stabilization of a natural microflora can
take months. The presence of a good microbial density in the
biodigester favors access to the substrate, facilitating its
degradation (Castro and Mateus 2016). Considering that at
the beginning of the process naturally occurring acidogenic
bacteria (producing acids and hydrogen) first develop the
process control, the addition of methanogenic organisms can
prevent the imbalance of the system and reduce the time for
process stabilization.
Digested sewage sludge, UASB sludge, manure, digested
and leached waste are usually used as inoculants (Estoppey
2010). The choice of an inoculum must consider the physical, chemical and nutritional characteristics of the place of
origin (Barcelos 2009). As the ideal amount of inoculum (in
percentage terms) to be added to the biodigester cannot be
easily stated, it depends on the methanogenic activity of the
microbial community (Araujo 2017). In continuous feeding
systems, the addition of the inoculum occurs only at the start
of the biodigester. In many biogas plants, part of the
digestate is recirculated to the process in order to take
advantage of the stabilized microbial community to
increasing the density of microorganisms in the biodigester
and, consequently, the efficiency of the process.
3.6 Agitation
The agitation of the substrate in the biodigester is very
significant for the process as it favors the transfer of mass,
energy and optimizes the access of microorganisms for the
100
E. B. Sydney et al.
systems is usually higher due to the capacity of carbon
dioxide to dissolve in water at high temperatures (Pareek and
Pareek 2019). According to Tabatabaei and Ghavanati
(2018a), other factors can interfere in the buffering system,
such as high organic feeding rates, the presence of toxic
compounds, temperature decrease, high organic load, high
sulfide content, pH increase as a consequence of H 2 S formation and an increase of free ammonia. When acidification
is detected, one or more of the following strategies should be
considered: stop feeding, increase the retention time,
increase the buffering capacity, add pH neutralizing agents
and add water.
3.3 FOS/TAC
FOS/TAC is proved to be a critical parameter that serves to
quickly assess the stability of a biodigester, as it provides
data in an easy and continuous way. FOS stands for volatile
fatty acids, while TAC stands for total inorganic carbon
(carbon buffering capacity) (Logan et al. 2019). It is considered a more reliable parameter than pH because it gives
important information on the increase of FOS even when the
biodigester’s pH is within the optimal range; thus, allowing
interventions that anticipate pH variation.
FOS/TAC values should be ideally between 0.1 and 0.3
(Fachagentur Nachwachsende Rohstoffe 2010). Values
above the limit indicate that the proportion of organic acids
is very high, which means that acidogenesis is being dominant. The accumulation of acids causes a drop in pH and the
consequent inactivation of methanogenic bacteria, leading to
inhibition of methane production. Values below 0.1 means
system’s alkalosis, which is the result of impaired hydrolysis
and/or acidogenesis. The most common causes are lack of
macro and/or micronutrients, excess feeding with easily
degradable material (simple carbohydrates) and constant
changes of the substrate composition (Chavarria et al. 2018).
FOS/TAC should be regularly measured to identify any
deviations and implement countermeasures if necessary
early. However, FOS/TAC is highly influenced by feeding;
measurements made right before and after substrate addition
provides significantly different results (Voss et al. 2009).
Thus, stating clear rules for sampling, preparation and
titration of samples is highly suggested.
3.4 Organic Load Rate (OLR) and Solid Content
Another important parameter in the operation of biodigesters
is the added mass of organic waste per unit volume of the
reactor per time, which is known as organic load rate (OLR).
Theoretically, as much substrate is fed to the biodigester so
that methane production should be higher. However, higher
OLR results in acidification and system collapse. An ideal
OLR in CSTR-type mesophilic biodigesters varies between
3 and 5 kg VS /m
3 /d, according to the substrate (Drosg 2013),
while the system failure is observed at rates above 6.4 kg VS /
m
3 /d (Moriarty 2013).
Most biogas plants are operated, for safety, with OLR
lower than the optimum. This strategy minimizes possible
errors and process fluctuations, especially when complex
substrates such as food wastes are used. On the other hand,
the safety margin used is indirectly proportional to the system’s efficiency and operational costs (Tabatabaei and
Ghavanati 2018). Biogas plants are generally designed to
convert 75% of the maximum degradable organic matter
(Deublein and Steinhauser 2008).
3.5 Inoculum
Biodigestion depends directly on the action of microorganisms. A microbial community can grow naturally in a
biodigestion system or can be artificially added to it. The
addition of an inoculum helps to accelerate the process since
the development and stabilization of a natural microflora can
take months. The presence of a good microbial density in the
biodigester favors access to the substrate, facilitating its
degradation (Castro and Mateus 2016). Considering that at
the beginning of the process naturally occurring acidogenic
bacteria (producing acids and hydrogen) first develop the
process control, the addition of methanogenic organisms can
prevent the imbalance of the system and reduce the time for
process stabilization.
Digested sewage sludge, UASB sludge, manure, digested
and leached waste are usually used as inoculants (Estoppey
2010). The choice of an inoculum must consider the physical, chemical and nutritional characteristics of the place of
origin (Barcelos 2009). As the ideal amount of inoculum (in
percentage terms) to be added to the biodigester cannot be
easily stated, it depends on the methanogenic activity of the
microbial community (Araujo 2017). In continuous feeding
systems, the addition of the inoculum occurs only at the start
of the biodigester. In many biogas plants, part of the
digestate is recirculated to the process in order to take
advantage of the stabilized microbial community to
increasing the density of microorganisms in the biodigester
and, consequently, the efficiency of the process.
3.6 Agitation
The agitation of the substrate in the biodigester is very
significant for the process as it favors the transfer of mass,
energy and optimizes the access of microorganisms for the
100
E. B. Sydney et al.