68
acidification in the fermentation medium and causing process imbalance. For undisturbed fermentation and optimum production of fuel gas, it is necessary that constant organic loadings should be provided [87].
The fermentation time (in batch operation) and the hydraulic retention time (in
continuous operation) are process parameters with decisive influence on the production yield. Thus, under batch operation mode, the biogas production in anaerobic
digesters is ceasing when the substrate has been exhausted. The complete fermentation process could last from several days to weeks or months depending on the fermentation temperature [101].
For biohydrogen, the duration of dark fermentation in batch regime is usually
short, of about 2–3 days on average, but some experimental results indicate good
hydrogen yields at hydraulic retention time shorter than 14 h [102–105]. In selecting the hydraulic retention time lengths for continuous processes, several factors
should be considered. The optimal period for mesophilic anaerobic digesters is
between 10 and 20 days of fermentation, but this could depend on the reactor design
or particularly on the substrate biodegradability [106]. For example, a short hydraulic retention time, usually less than 3 days, may induce reduction of the methanogens which are slowly growing and promote acidogenic bacteria growth, thus
favouring the hydrogen production. On the other hand, a too high feeding rate would
lead to poor hydrolysis of the substrate [94, 107].
The influence of homogenization on the fermentation outcome has been studied
at a larger extent for anaerobic digestion than for dark fermentation. Mechanical
stirring inside the reactor can improve the availability of microorganisms in the
substrate by increasing their contact with the substrate. Homogenization could be
also ensured by feedstock hydrodynamics as well as by the gas flowing inside the
reactor, in connexion to the hydraulic retention time [108]. In case of mass mixing,
the hydraulic retention time is reduced, and reactions in the fermenter are fostered.
On the other side, the need for mixing should be well assessed since excessive
mixing increases the energy consumption, but mass agitation may negatively influence the balance of microorganisms, with effect on the fuel gas production. Besides
ensuring the substrate homogenization within the reactor, mass mixing is also useful for balancing the temperature and for preventing scum, sediment or any inactive
areas to be developed. Slight mixing can ensure the homogeneous concentration of
nutrients and metabolic products of microorganisms [87].
Pressure influence on biomethane/biohydrogen production has not been largely
investigated, neither explained from a biochemical perspective. The pressure effect
is supposed to be associated to various syntrophic phenomena between microbial
species like acetogens and methanogens and usually held responsible for hydrogen
exchange between species [109]. Some experiments demonstrated that by increasing the hydrostatic pressure inside the reactor, the methanogenesis was negatively
affected [91].
Regarding dark fermentation processes in continuous biohydrogen synthesis, the
biochemical pathways were found sensitive to hydrogen concentration increments
which switched the metabolic pathways towards the formation of other chemical
products such as ethanol, butanol, propanone, lactate, etc. System response to the
C. Mateescu and A.-D. Dima
acidification in the fermentation medium and causing process imbalance. For undisturbed fermentation and optimum production of fuel gas, it is necessary that constant organic loadings should be provided [87].
The fermentation time (in batch operation) and the hydraulic retention time (in
continuous operation) are process parameters with decisive influence on the production yield. Thus, under batch operation mode, the biogas production in anaerobic
digesters is ceasing when the substrate has been exhausted. The complete fermentation process could last from several days to weeks or months depending on the fermentation temperature [101].
For biohydrogen, the duration of dark fermentation in batch regime is usually
short, of about 2–3 days on average, but some experimental results indicate good
hydrogen yields at hydraulic retention time shorter than 14 h [102–105]. In selecting the hydraulic retention time lengths for continuous processes, several factors
should be considered. The optimal period for mesophilic anaerobic digesters is
between 10 and 20 days of fermentation, but this could depend on the reactor design
or particularly on the substrate biodegradability [106]. For example, a short hydraulic retention time, usually less than 3 days, may induce reduction of the methanogens which are slowly growing and promote acidogenic bacteria growth, thus
favouring the hydrogen production. On the other hand, a too high feeding rate would
lead to poor hydrolysis of the substrate [94, 107].
The influence of homogenization on the fermentation outcome has been studied
at a larger extent for anaerobic digestion than for dark fermentation. Mechanical
stirring inside the reactor can improve the availability of microorganisms in the
substrate by increasing their contact with the substrate. Homogenization could be
also ensured by feedstock hydrodynamics as well as by the gas flowing inside the
reactor, in connexion to the hydraulic retention time [108]. In case of mass mixing,
the hydraulic retention time is reduced, and reactions in the fermenter are fostered.
On the other side, the need for mixing should be well assessed since excessive
mixing increases the energy consumption, but mass agitation may negatively influence the balance of microorganisms, with effect on the fuel gas production. Besides
ensuring the substrate homogenization within the reactor, mass mixing is also useful for balancing the temperature and for preventing scum, sediment or any inactive
areas to be developed. Slight mixing can ensure the homogeneous concentration of
nutrients and metabolic products of microorganisms [87].
Pressure influence on biomethane/biohydrogen production has not been largely
investigated, neither explained from a biochemical perspective. The pressure effect
is supposed to be associated to various syntrophic phenomena between microbial
species like acetogens and methanogens and usually held responsible for hydrogen
exchange between species [109]. Some experiments demonstrated that by increasing the hydrostatic pressure inside the reactor, the methanogenesis was negatively
affected [91].
Regarding dark fermentation processes in continuous biohydrogen synthesis, the
biochemical pathways were found sensitive to hydrogen concentration increments
which switched the metabolic pathways towards the formation of other chemical
products such as ethanol, butanol, propanone, lactate, etc. System response to the
C. Mateescu and A.-D. Dima