Despite all these difficulties, many authors maintain that practical application of
MES can become a reality in the medium term in sectors linked to waste biorefineries, or power-to-gas [132]. Here MES can contribute to the desired shift from a
fossil fuel based economy to an electricity-based one, by providing a link between
chemical and electrical energy. In this context, the use of MES for biogas upgrading
through electromethanogenesis represents perhaps the most straightforward
approach for quick market implementation of MES [123, 133, 134]. Raw biogas
typically contains 30–40% CO 2 and usually needs refinement before its use as a fuel
[135]. Electromethanogenesis directly converts the CO 2 present in the biogas into
methane which increases the net content of the later thus avoiding the need to
manage CO 2 side streams, which is one of the disadvantages of conventional
methods for biogas upgrading [103]. The use of MES as a post-treatment for biogas
leads to a versatile and easy-to-operate system, and recent studies reported content
in CO 2 below than 10% still with high coulombic efficiencies (above 80%) [133].
The explosive growth of intermittent renewable energy sources (such as solar
and wind power) has been the main drive for the increased interest in electrical
energy storage systems [136]. The ability of MES to convert electrical energy into
chemical energy (e.g.: methane) can make of it, at least potentially, an alternative
energy storage system similar to the more mature technology of PEM electrolysers
[103]. However, it is still not clear how MES would perform and how the microbial
biofilms would respond to periodic power cuts (resulting from the unpredictable
nature of renewable energy supplies). In this regard, a few works have already give
attention to this question showing that acetate producing MES are resilient to
short-term to energy fluctuations and that performance and productivities can be
maintained after continuous and scheduled power disconnections, reaching titers
and production rates comparable with stably powered systems [101, 102]. However, when the power cuts are relatively large (6 weeks), shifts may occur both on
the microbial populations and on the final end-product [102].
Finally, another challenging question is product inhibition in batch-operated
systems. The accumulation of VFA’s or alcohols on the catholite can inhibit the
metabolism of the electrotrophic microorganisms, leading to a decrease of the
production rates thus threatening practical applicability of MES. An obvious
solution to this problem is the operation of the MES in continuous mode, which
prevents the accumulation of metabolites in the broth. This alternative however has
its pros and cons. On the one hand, it brings the additional advantage of providing
higher production rates; on the other hand it can result in lower product titers, which
increases downstream post-processing costs [137]. Another approach to limit
excessive product build-up in batch systems is the implementation of in-situ separation technologies such as membrane processes, distillation, adsorption/
adsorption and extraction [108, 138, 139]. Gildemyn et al. [108] explored this
option in a MES that incorporated an extraction compartment connected to the
cathodic chamber by means of an anion exchange membrane (AEM) that allowed
for the extraction and recovery of acetate. Although titers and production rates were
in the range of those obtained in batch systems, the relatively high energy
requirement, together with the risk of membrane fouling and the complexity of the
overall set-up requires a further optimization of this approach.
98
C. M. Cordas et al.
MES can become a reality in the medium term in sectors linked to waste biorefineries, or power-to-gas [132]. Here MES can contribute to the desired shift from a
fossil fuel based economy to an electricity-based one, by providing a link between
chemical and electrical energy. In this context, the use of MES for biogas upgrading
through electromethanogenesis represents perhaps the most straightforward
approach for quick market implementation of MES [123, 133, 134]. Raw biogas
typically contains 30–40% CO 2 and usually needs refinement before its use as a fuel
[135]. Electromethanogenesis directly converts the CO 2 present in the biogas into
methane which increases the net content of the later thus avoiding the need to
manage CO 2 side streams, which is one of the disadvantages of conventional
methods for biogas upgrading [103]. The use of MES as a post-treatment for biogas
leads to a versatile and easy-to-operate system, and recent studies reported content
in CO 2 below than 10% still with high coulombic efficiencies (above 80%) [133].
The explosive growth of intermittent renewable energy sources (such as solar
and wind power) has been the main drive for the increased interest in electrical
energy storage systems [136]. The ability of MES to convert electrical energy into
chemical energy (e.g.: methane) can make of it, at least potentially, an alternative
energy storage system similar to the more mature technology of PEM electrolysers
[103]. However, it is still not clear how MES would perform and how the microbial
biofilms would respond to periodic power cuts (resulting from the unpredictable
nature of renewable energy supplies). In this regard, a few works have already give
attention to this question showing that acetate producing MES are resilient to
short-term to energy fluctuations and that performance and productivities can be
maintained after continuous and scheduled power disconnections, reaching titers
and production rates comparable with stably powered systems [101, 102]. However, when the power cuts are relatively large (6 weeks), shifts may occur both on
the microbial populations and on the final end-product [102].
Finally, another challenging question is product inhibition in batch-operated
systems. The accumulation of VFA’s or alcohols on the catholite can inhibit the
metabolism of the electrotrophic microorganisms, leading to a decrease of the
production rates thus threatening practical applicability of MES. An obvious
solution to this problem is the operation of the MES in continuous mode, which
prevents the accumulation of metabolites in the broth. This alternative however has
its pros and cons. On the one hand, it brings the additional advantage of providing
higher production rates; on the other hand it can result in lower product titers, which
increases downstream post-processing costs [137]. Another approach to limit
excessive product build-up in batch systems is the implementation of in-situ separation technologies such as membrane processes, distillation, adsorption/
adsorption and extraction [108, 138, 139]. Gildemyn et al. [108] explored this
option in a MES that incorporated an extraction compartment connected to the
cathodic chamber by means of an anion exchange membrane (AEM) that allowed
for the extraction and recovery of acetate. Although titers and production rates were
in the range of those obtained in batch systems, the relatively high energy
requirement, together with the risk of membrane fouling and the complexity of the
overall set-up requires a further optimization of this approach.
98
C. M. Cordas et al.