Since fossil sources for fuel and platform chemicals will become limited in the near future, it is important to develop new concepts for energy supply and production of basic reagents for chemical industry. One alternative to crude oil and fossil natural gas could be the biological conversion of CO2 or small organic molecules to methane via methanogenic archaea. This process has been known from biogas plants, but recently, new insights into the methanogenic metabolism, technical optimizations and new technology combinations were gained, which would allow moving beyond the mere conversion of biomass. In biogas plants, steps have been undertaken to increase yield and purity of the biogas, such as addition of hydrogen or metal granulate. Furthermore, the integration of electrodes led to the development of microbial electrosynthesis (MES). The idea behind this technique is to use CO2 and electrical power to generate methane via the microbial metabolism. This review summarizes the biochemical and metabolic background of methanogenesis as well as the latest technical applications of methanogens. As a result, it shall give a sufficient overview over the topic to both, biologists and engineers handling biological or bioelectrochemical methanogenesis.

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The objective of microbial electrosynthesis (MES) is to combine the advantages of electrochemistry and biotechnology in order to produce chemicals and fuels. This combination enables resource-efficient processes by using renewable raw materials and regenerative energies. In the last decade, different MES processes have been described, for example, MES based on biofilms or mediators, electro-fermentation, and secondary MES. This review compares the MES technologies with regard to the reached process performances in terms of key process indicators (i.e., coulombic efficiency (CE), product titre, productivity) and technology readiness level (TRL). Often the underlying mechanism of electron transfer in biofilm-based processes has not been elucidated and can therefore not be optimized. Similarly, technical aspects of electro-fermentation processes and processes with soluble mediators are under investigation and techno-economic assessments are missing. In contrast, the electrochemical production of microbial substrates in secondary MES or hybrid systems show high key process indicators and TRLs up to 7. In summary, the different types of MES processes offer options for today's industrial use, as well as an exciting and future-oriented technology that can be applied in a medium-term perspective.

From the first electromicrobial experiment to a sophisticated microbial electrochemical process - it all takes place in a reactor. Whereas the reactor design and materials used strongly influence the obtained results, there are no common platforms for MES reactors. This is a critical convention gap, as cross-comparison and benchmarking among MES as well as MES vs. conventional biotechnological processes is needed. Only knowledge driven engineering of MES reactors will pave the way to application and commercialization. In this chapter we first assess the requirements on reactors to be used for bioelectrochemical systems as well as potential losses caused by the reactor design. Subsequently, we compile the main types and designs of reactors used for MES so far, starting from simple H-cells to stirred tank reactors. We conclude with a discussion on the weaknesses and strengths of the existing types of reactors for bioelectrochemical systems that are scored on design criteria and draw conclusions for the future engineering of MES reactors.

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Facing energy problems, there is a strong demand for new technologies dealing with the replacement of fossil fuels. The emerging fields of biotechnology, photobiotechnology and electrobiotechnology, offer solutions for the production of fuels, energy, or chemicals using renewable energy sources (light or electrical current e.g. produced by wind or solar power) or organic (waste) substrates. From an engineering point of view both technologies have analogies and some similar challenges, since both light and electron transfer are primarily surface‐dependent. In contrast to that, bioproduction processes are typically volume dependent. To allow large scale and industrially relevant applications of photobiotechnology and electrobiotechnology, this opinion first gives an overview over the current scales reached in these areas. We then try to point out the challenges and possible methods for the scale up or numbering up of the reactors used. It is shown that the field of photobiotechnology is by now much more advanced than electrobiotechnology and has achieved industrial applications in some cases. We argue that transferring knowledge from photobiotechnology to electrobiotechnology can speed up the development of the emerging field of electrobiotechnology. We believe that a combination of scale up and numbering up, as it has been shown for several photobiotechnological reactors, may well lead to industrially relevant scales in electrobiotechnological processes allowing an industrial application of the technology in near future.

Since their first use in the 1930s, shake flasks have been a widely used bioreactor type for screening and process development due to a number of advantages. However, the limited gas-liquid mass transfer capacities—resulting from practical operation limits regarding shaking frequency and filling volumes—are a major drawback. The common way to increase the gas-liquid mass transfer in shake flasks with the implementation of baffles is generally not recommended as it comes along with several severe disadvantages. Thus, a new design principle for shaken bioreactors that aims for improving the gas-liquid mass transfer without losing the positive characteristics of unbaffled shake flasks is introduced. The flasks consist of cylindrical glass vessels with implemented perforated concentric ring walls. The ring walls improve the gas-liquid mass transfer via the formation of additional liquid films on both of its sides, whereas the perforations allow for mixing between the compartments. Sulfite oxidation experiments revealed over 200% higher maximum oxygen transfer capacities (OTRmax) compared to conventional shake flasks. In batch cultivations of Escherichia coli BL21 in mineral media, unlimited growth until glucose depletion and oxygen transfer rates (OTR) of up to 138 mmol/L/h instead of an oxygen limitation at 57 mmol/L/h as in normal shake flasks under comparable conditions could be achieved. Even overflow metabolism could be prevented due to sufficient oxygen supply without the use of unconventional shaking conditions or oxygen enrichment. Therefore, we believe that the new perforated ring flask principle has a high potential to considerably improve biotechnological screening and process development steps.