65
available, inexpensive, high-carbohydrate content and easily biodegradable fermentation materials are used [96].
Apart from dark fermentation, some other biochemical processes for biomass-tobiohydrogen conversion are used at smaller extent, as shown in Fig. 5. Biophotolysis
of water is a technique in which green algae and cyanobacteria decompose water
into hydrogen and oxygen. Within the direct biophotolysis, H 2 -producing green
algae generate hydrogen gas mostly with the aid of hydrogenase enzyme (e.g.
Chlamydomonas reinhardtii, Platymonas subcordiformis, Chlorella fusca, etc.),
under anaerobic conditions. But there are also some other types of algae (e.g.
Dunaliella salina) that do not use hydrogenase enzyme for biophotolysis [97]. By
biophotolysis, cyanobacteria, which are photoautotrophic microorganisms and contain photosynthetic pigments, use hydrogenase enzyme as well as nitrogenase
enzyme to produce biohydrogen. Some examples of such cyanophytes include
Calothrix sp., Oscillatoria sp., Anabaena sp., etc., from which Anabaena variabilis
has recently got increased attention due to its high hydrogen production potential.
Under nitrogen-deficient conditions, biohydrogen is produced by photosynthetic
bacteria from reduced organic acids, catalysed by nitrogenase enzyme, and using
light energy. The potential of photoheterotrophic bacteria to use light energy in
decomposing organic compounds for hydrogen production has also been investigated [34].
In hybrid systems using fermentative and photosynthetic bacteria, the anaerobic
fermentation of the substrate produces low molecular weight intermediates that are
then submitted to photosynthetic bacterial attack in a photo-bioreactor. In the case
of glucose, after the first stage of the process (dark fermentation), the metabolite
product of the bacteria is acetic acid which is further reacted to carbon dioxide and
hydrogen. In total, 12 moles of biohydrogen are theoretically expected, as seen in
Fig. 5—Hybrid systems (a).
Bioelectrochemical bioreactor-assisted conversion is another type of hybrid system in which microbial fuel cells generate electrons and protons during organic
matter decomposition by bacteria. The electrolyte carries the protons towards the
cathode, while electrons travel towards the cathode via a circuit, generating electrical current, and oxygen reacts to electrons and protons forming reduced compounds.
In a bioelectrochemical microbial reactor, in order to decompose acetate, a theoretical voltage of 0.11 V was applied to eliminate oxygen and evolve hydrogen. Even
though the real necessary voltage was found higher (0.25 V) to counter system
resistance, this voltage is significantly lower than what is required for water electrolysis with hydrogen production in alkaline media (1.8–2 V) [92, 98].
5 Conversion Technological Parameters
The proper running of the fuel gas production processes in both anaerobic digestion
and dark fermentation processes relies on the system capability to prove suitable
environmental conditions for specific microbial communities’ growth. This is a very
Biochemical Conversion of Residual Biomass: An Approach to Fuel Gas and Green…
available, inexpensive, high-carbohydrate content and easily biodegradable fermentation materials are used [96].
Apart from dark fermentation, some other biochemical processes for biomass-tobiohydrogen conversion are used at smaller extent, as shown in Fig. 5. Biophotolysis
of water is a technique in which green algae and cyanobacteria decompose water
into hydrogen and oxygen. Within the direct biophotolysis, H 2 -producing green
algae generate hydrogen gas mostly with the aid of hydrogenase enzyme (e.g.
Chlamydomonas reinhardtii, Platymonas subcordiformis, Chlorella fusca, etc.),
under anaerobic conditions. But there are also some other types of algae (e.g.
Dunaliella salina) that do not use hydrogenase enzyme for biophotolysis [97]. By
biophotolysis, cyanobacteria, which are photoautotrophic microorganisms and contain photosynthetic pigments, use hydrogenase enzyme as well as nitrogenase
enzyme to produce biohydrogen. Some examples of such cyanophytes include
Calothrix sp., Oscillatoria sp., Anabaena sp., etc., from which Anabaena variabilis
has recently got increased attention due to its high hydrogen production potential.
Under nitrogen-deficient conditions, biohydrogen is produced by photosynthetic
bacteria from reduced organic acids, catalysed by nitrogenase enzyme, and using
light energy. The potential of photoheterotrophic bacteria to use light energy in
decomposing organic compounds for hydrogen production has also been investigated [34].
In hybrid systems using fermentative and photosynthetic bacteria, the anaerobic
fermentation of the substrate produces low molecular weight intermediates that are
then submitted to photosynthetic bacterial attack in a photo-bioreactor. In the case
of glucose, after the first stage of the process (dark fermentation), the metabolite
product of the bacteria is acetic acid which is further reacted to carbon dioxide and
hydrogen. In total, 12 moles of biohydrogen are theoretically expected, as seen in
Fig. 5—Hybrid systems (a).
Bioelectrochemical bioreactor-assisted conversion is another type of hybrid system in which microbial fuel cells generate electrons and protons during organic
matter decomposition by bacteria. The electrolyte carries the protons towards the
cathode, while electrons travel towards the cathode via a circuit, generating electrical current, and oxygen reacts to electrons and protons forming reduced compounds.
In a bioelectrochemical microbial reactor, in order to decompose acetate, a theoretical voltage of 0.11 V was applied to eliminate oxygen and evolve hydrogen. Even
though the real necessary voltage was found higher (0.25 V) to counter system
resistance, this voltage is significantly lower than what is required for water electrolysis with hydrogen production in alkaline media (1.8–2 V) [92, 98].
5 Conversion Technological Parameters
The proper running of the fuel gas production processes in both anaerobic digestion
and dark fermentation processes relies on the system capability to prove suitable
environmental conditions for specific microbial communities’ growth. This is a very
Biochemical Conversion of Residual Biomass: An Approach to Fuel Gas and Green…