78
investigations regarding possible joinder of dark fermentation with other waste-toenergy management options, such as anaerobic digestion, are required in order to
get a better substrate conversion [96].
In both cases, the metabolic engineering of microbial strains emerges as an innovative new research direction. For instance, developing thermophilic bacteria strains
able to ferment lignocellulosic or other heavily biodegradable biomass, also able to
withstand environmental conditions’ sharp variations during fermentation stages,
and have a better behaviour against competing species could be a good approach to
increase fuel gas production in fermentative processes. Moreover, gene encoding
could be used to develop microorganisms which selectively follow biochemical
pathways in order to obtain higher production yields and a cleaner gas requiring less
purification or upgrading [157].
The development of some new degradation biocatalysts such as different types of
enzymes with broader spectrum of applications and more effective in the hydrolysis
of biopolymers should be explored in further research. Thus, enzyme-producing
microorganisms could be successfully used to increase the biochemical process performance, being able to act as microbial catalysts of biochemical reactions. The
type and characteristics, but also the amount of biocatalysts, may highly influence
the fermentation process stability and the conversion rates, which make the cost of
their production to be considered evenly [40]. On the other hand, prior or along with
developing advanced biocatalysts, a higher comprehension of biochemical processes that affect anaerobic digestion and dark fermentation should be followed. A
deeper understanding of microorganisms’ interactions among each other and with
their environment, for example, by molecular analysis, enables the development of
a knowledge-based process control which uses reproducible forecasts on how the
fermentation systems reacts to varying external influences for process optimization [159].
Pushing forward the existing technology for obtaining fuel gas is also an important challenge, being correlated to the need of improving bioreactor designs, reducing the hydraulic retention time without affecting the gas production rate, enhancing
gas storage volume, etc. [160]. Moreover, advances in the fermentation process
monitoring and control are still to be made. The existing limitations in bioprocesses
monitoring are considered one of the major challenges opposing progress in the fuel
gas production field.
Developing new monitoring techniques and equipment or improving the existent
ones (such as biosensors, optical sensors, immunosensors, etc.) could lead to adequate operation of bioreactors and higher production yields along with higher financial gain and process sustainability [161]. Additionally, developing integrated or
multi-stage processes is a promising possibility for obtaining enhanced conversion
yields of organic matter to fuel gas. An example of such a system is the coupling of
dark fermentation with biomethane production in anaerobic digesters, providing a
better use of the discharged substrate [93, 96].
The need to obtain pure gases represents another challenge of the bioconversion
processes, as either high financial investments in maintaining optimum operational
conditions or enhanced upgrading techniques for gas purification and elimination of
C. Mateescu and A.-D. Dima
investigations regarding possible joinder of dark fermentation with other waste-toenergy management options, such as anaerobic digestion, are required in order to
get a better substrate conversion [96].
In both cases, the metabolic engineering of microbial strains emerges as an innovative new research direction. For instance, developing thermophilic bacteria strains
able to ferment lignocellulosic or other heavily biodegradable biomass, also able to
withstand environmental conditions’ sharp variations during fermentation stages,
and have a better behaviour against competing species could be a good approach to
increase fuel gas production in fermentative processes. Moreover, gene encoding
could be used to develop microorganisms which selectively follow biochemical
pathways in order to obtain higher production yields and a cleaner gas requiring less
purification or upgrading [157].
The development of some new degradation biocatalysts such as different types of
enzymes with broader spectrum of applications and more effective in the hydrolysis
of biopolymers should be explored in further research. Thus, enzyme-producing
microorganisms could be successfully used to increase the biochemical process performance, being able to act as microbial catalysts of biochemical reactions. The
type and characteristics, but also the amount of biocatalysts, may highly influence
the fermentation process stability and the conversion rates, which make the cost of
their production to be considered evenly [40]. On the other hand, prior or along with
developing advanced biocatalysts, a higher comprehension of biochemical processes that affect anaerobic digestion and dark fermentation should be followed. A
deeper understanding of microorganisms’ interactions among each other and with
their environment, for example, by molecular analysis, enables the development of
a knowledge-based process control which uses reproducible forecasts on how the
fermentation systems reacts to varying external influences for process optimization [159].
Pushing forward the existing technology for obtaining fuel gas is also an important challenge, being correlated to the need of improving bioreactor designs, reducing the hydraulic retention time without affecting the gas production rate, enhancing
gas storage volume, etc. [160]. Moreover, advances in the fermentation process
monitoring and control are still to be made. The existing limitations in bioprocesses
monitoring are considered one of the major challenges opposing progress in the fuel
gas production field.
Developing new monitoring techniques and equipment or improving the existent
ones (such as biosensors, optical sensors, immunosensors, etc.) could lead to adequate operation of bioreactors and higher production yields along with higher financial gain and process sustainability [161]. Additionally, developing integrated or
multi-stage processes is a promising possibility for obtaining enhanced conversion
yields of organic matter to fuel gas. An example of such a system is the coupling of
dark fermentation with biomethane production in anaerobic digesters, providing a
better use of the discharged substrate [93, 96].
The need to obtain pure gases represents another challenge of the bioconversion
processes, as either high financial investments in maintaining optimum operational
conditions or enhanced upgrading techniques for gas purification and elimination of
C. Mateescu and A.-D. Dima