67
Employed to adjust the properties of the substrate and fulfil the requirements of
an efficient bioconversion in anaerobic digestion processes, co-digestion of organic
materials has also proven a good strategy. Co-digestion can enhance the digester’s
performance by its influence on the cumulative biogas/biomethane production, lag
phase time length and production rate.
Some inhibitions may occur in the fermentative processes due to unappropriate
nutrients necessary for metabolic growth of useful microflora. For instance, Fe, Ni
and Co are essential nutrients for high conversion of acetate into methane by acetoclastic bacteria. Fe and Ni microelements were similarly reported to enhance biohydrogen production. Light and heavy metal ions, phenolic compounds, ammonia and
other metabolites or toxic compounds brought within the substrate may negatively
affect the biogas and biohydrogen productions alike, depending on their concentration [24, 87].
Thermal regime is particularly a relevant factor highly altering the fuel gas production in anaerobic digestion and dark fermentation processes. Generally, higher
temperatures have been associated with increased fuel generation, but in such case
the biochemical processes may become more prone to imbalances at even small
variations of environmental conditions within the digester. Increasing temperature
is reflected in higher costs for heating and requires more careful process monitoring
and control. Fermentation can take place at various temperature ranges, conventionally classified in several categories. Most fermentative processes are developed at
ambient (15–27 °C) temperatures, at mesophilic (30–45 °C) temperatures or moderate thermophilic (50–60 °C) temperatures. Cryophilic (<10 °C), extreme thermophilic (65–80 °C) and hyperthermophilic (>80 °C) conditions are scarcely used [44,
87, 95]. It has been noticed that to a certain level, temperature augmentation
improves the reactions kinetics, but the optimal digestion temperature for each project should be set for every specific configuration as it may vary with the composition of raw materials, reactor type, bacterial specificity from inoculum, etc. [44].
Thus, even though the fermentation at higher temperatures is typically more efficient than at lower temperatures, a temperature range that simultaneously preserves
the advantages of fast microbial multiplication and high process efficiency should
be pursued.
The optimal pH for methanogenesis under anaerobic digestion should range
between 6 and 7.5, while the dark fermentation requires optimal pH values from 5.5
to 6.0; for this low pH, repression of methanogenic microorganisms is promoted
while indirectly increasing H 2 production [34].
Organic loading in the digester is a good indicator of nutrient level availability
for consumption by microorganisms, and it is also directly linked to the reactor
performance. The operator responsible for the biomethane/biohydrogen production
facility should have minimal information on the biodegradability of the substrates
so as not to overload the system nor to operate the reactor at under-capacity. For
continuous systems, the organic loading rate, which may be defined as the mass of
volatile solids entering the digestion tank in unit time, should take into account the
limits of the reactor. For example, a sudden increase in the organic loading rate may
cause a fast release of volatile organic acids, which accumulate, leading to
Biochemical Conversion of Residual Biomass: An Approach to Fuel Gas and Green…
Employed to adjust the properties of the substrate and fulfil the requirements of
an efficient bioconversion in anaerobic digestion processes, co-digestion of organic
materials has also proven a good strategy. Co-digestion can enhance the digester’s
performance by its influence on the cumulative biogas/biomethane production, lag
phase time length and production rate.
Some inhibitions may occur in the fermentative processes due to unappropriate
nutrients necessary for metabolic growth of useful microflora. For instance, Fe, Ni
and Co are essential nutrients for high conversion of acetate into methane by acetoclastic bacteria. Fe and Ni microelements were similarly reported to enhance biohydrogen production. Light and heavy metal ions, phenolic compounds, ammonia and
other metabolites or toxic compounds brought within the substrate may negatively
affect the biogas and biohydrogen productions alike, depending on their concentration [24, 87].
Thermal regime is particularly a relevant factor highly altering the fuel gas production in anaerobic digestion and dark fermentation processes. Generally, higher
temperatures have been associated with increased fuel generation, but in such case
the biochemical processes may become more prone to imbalances at even small
variations of environmental conditions within the digester. Increasing temperature
is reflected in higher costs for heating and requires more careful process monitoring
and control. Fermentation can take place at various temperature ranges, conventionally classified in several categories. Most fermentative processes are developed at
ambient (15–27 °C) temperatures, at mesophilic (30–45 °C) temperatures or moderate thermophilic (50–60 °C) temperatures. Cryophilic (<10 °C), extreme thermophilic (65–80 °C) and hyperthermophilic (>80 °C) conditions are scarcely used [44,
87, 95]. It has been noticed that to a certain level, temperature augmentation
improves the reactions kinetics, but the optimal digestion temperature for each project should be set for every specific configuration as it may vary with the composition of raw materials, reactor type, bacterial specificity from inoculum, etc. [44].
Thus, even though the fermentation at higher temperatures is typically more efficient than at lower temperatures, a temperature range that simultaneously preserves
the advantages of fast microbial multiplication and high process efficiency should
be pursued.
The optimal pH for methanogenesis under anaerobic digestion should range
between 6 and 7.5, while the dark fermentation requires optimal pH values from 5.5
to 6.0; for this low pH, repression of methanogenic microorganisms is promoted
while indirectly increasing H 2 production [34].
Organic loading in the digester is a good indicator of nutrient level availability
for consumption by microorganisms, and it is also directly linked to the reactor
performance. The operator responsible for the biomethane/biohydrogen production
facility should have minimal information on the biodegradability of the substrates
so as not to overload the system nor to operate the reactor at under-capacity. For
continuous systems, the organic loading rate, which may be defined as the mass of
volatile solids entering the digestion tank in unit time, should take into account the
limits of the reactor. For example, a sudden increase in the organic loading rate may
cause a fast release of volatile organic acids, which accumulate, leading to
Biochemical Conversion of Residual Biomass: An Approach to Fuel Gas and Green…