Microbiome based anaerobic digestion combined with microbial electrochemical technologies exploits biomass efficiently and flexibly by concurrent conversion to methane and electrons.

Anaerobic digestion (AD) and microbial electrochemical technologies (MET) can be combined in manifold ways. Recent studies show negative influences of AD effluents on the performance of pre-grown Geobacter spp.-dominated biofilm anodes. In this study, it was investigated how such biofilm anodes are affected by AD effluents. Therefore, experiments using AD effluents in different concentrations (0−100%) in combination with biofilms of different ages were performed. Furthermore, the activity of methanogens was inhibited and minimized by application of 2-bromoethanesulfonate (2-BES) and microfiltration, respectively. Biofilms pre-grown for 5 weeks show higher resistance against AD effluents compared to biofilms pre-grown for only 3 weeks. Nevertheless, adaptation of biofilms to AD effluents was not successful. Biofilm activity in terms of coulombic efficiency and maximum current density (j max ) dropped by factor 32.2 ± 3.2 and 38.9 ± 8.4, respectively. The application of 2-BES and microfiltration had positive effects on the biofilm activity. The results support the assumption that methanogens or further compounds not studied here, for example, protozoans, which may have been inhibited or removed by 2-BES application or microfiltration, have an immediate influence on the stability of Geobacter spp.-dominated biofilms and may limit their practical application in AD environments.

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The autonomous growth of Geobacter sp.‐dominated biofilms on electrodes and their sensitivity on volatile fatty acids (VFAs) allows their application as biosensor. In this study, the implementation of a microbial electrochemical sensor in the anaerobic digestion (AD) process is reported. The amperometric sensor signal follows the acetate concentration profile in the AD process. Yet, the sensor's stability is impaired by the process conditions. Therefore, potential biofilm inhibitors are individually examined, i.e., high salt and NH4+ concentrations as well as fumarate as potential alternative electron acceptor. The results show no influence of high salinity on the signal, whereas ammonia at a concentration ≥ 5 g L−1 NH4+ acts as inhibitor, and fumarate is used as substrate for current generation.

Geobacter spp. enrichment biofilms were cultivated in batch using one-chamber and twochamber bioelectrochemical reactors. Time-resolved substrate quantification was performed to derive physiological parameters as well as incremental coulombic efficiency (i.e., coulombic efficiency during one batch cycle, here every 6h) during early stage biofilm development. The results of one-chamber reactors revealed an intermediate acetate increase putatively due to the presence of acetogens. Total coulombic efficiencies of two-chamber reactors were considerable lower (19.6±8.3% and 49.3±13.2% for 1 st and 2 nd batch cycle, respectively) compared to usually reported values of mature Geobacter spp. enrichment biofilms presumably reflecting energetic requirements for biomass production (i.e., cells and extracellular polymeric substances) during early stages of biofilm development. The incremental coulombic efficiency exhibits considerable changes during batch cycles indicating shifts between phases of maximizing metabolic rates and maximizing biomass yield. Analysis based on Michaelis-Menten kinetics yielded maximum substrate uptake rates (v max,Ac , v max,I) and half-saturation concentration coefficients (K M,Ac ,K M,I) based on acetate uptake or current production, respectively. The latter is usually reported in literature but neglects energy demands for biofilm growth and maintenance as well as acetate and electron storage. From 1 st to 2 nd batch cycle, v max,Ac and K M,Ac , decreased from 0.0042-0.0051 mmol Ac − h −1 cm −2 to 0.0031-0.0037 mmol Ac − h −1 cm −2 and 1.02-2.61 mM Ac − to 0.28-0.42 mM Ac − , respectively. Furthermore, differences between K M,Ac /K M,I and v max,Ac /v max,I were observed providing insights into the physiology of Geobacter spp. enrichment biofilms. Notably, K M,I considerably scattered while v max,Ac /v max,I and K M,Ac remained rather stable indicating that acetate transport within biofilm only marginally affects reaction rates. The observed data variation mandates the requirement of a more detailed analysis with an improved experimental system, e.g., using flow conditions and a comparison with Geobacter spp. pure cultures.