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Microbial electrolysis cells (MECs) have been proven effective for oxidizing ammonium (NH 4 + ), where the anode acts as an electron acceptor, reducing the energy input by substituting oxygen (O 2 ). However, O 2 has been proved to be essential for achieving high removal rates MECs. Thus, precise control of oxygen supply is crucial for optimizing treatment performance and minimizing energy consumption. Unlike previous studies focusing on dissolved oxygen (DO) levels, this study introduces the O 2 /NH 4 + -N ratio as a novel control parameter for balancing oxidation rates and the selectivity of NH 4 + oxidation towards dinitrogen gas (N 2 ) under limited oxygen condition. Our results demonstrated that the O 2 /NH 4 + -N ratio is a more relevant oxygen supply indicator compared to DO level. Oxygen served as a more favorable electron acceptor than the electrode, increasing NH 4 + oxidation rates but also resulting in more oxidized products such as nitrate (NO 3 - ). Additionally, nitrous oxide (N 2 O) and N 2 production were higher with the electrode as the electron acceptor compared to oxygen alone. An O 2 /NH 4 + -N ratio of 0.5 was found to be optimal, achieving a balance between product selectivity for N 2 (51.4 % ± 4.5 %) and oxidation rates (344.6 ± 14.7 mg-N/L*d), with the columbic efficiency of 30.7 % ± 2.0 %. Microbial community analysis revealed that nitrifiers and denitrifiers were the primary bacteria involved, with oxygen promoting the growth of nitrite-oxidizing bacteria, thus facilitating complete NH 4 + oxidation to NO 3 - . Our study provides new insights and guidelines on the appropriate oxygen dosage, offering strategies into optimizing operational conditions for NH 4 + removal using MECs., Competing Interests: Declaration of competing interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper., (Copyright © 2024. Published by Elsevier Ltd.)

Microbial conversion of CO 2 to multi-carbon compounds such as acetate and butyrate is a promising valorisation technique. For those reactions, the electrochemical supply of hydrogen to the biocatalyst is a viable approach. Earlier we have shown that trace metals from microbial growth media spontaneously form in situ electro-catalysts for hydrogen evolution. Here, we show biocompatibility with the successful integration of such metal mix-based HER catalyst for immediate start-up of microbial acetogenesis (CO 2 to acetate). Also, n-butyrate formation started fast (after twenty days). Hydrogen was always produced in excess, although productivity decreased over the 36 to 50 days, possibly due to metal leaching from the cathode. The HER catalyst boosted microbial productivity in a two-step microbial community bioprocess: acetogenesis by a BRH-c20a strain and acetate elongation to n-butyrate by Clostridium sensu stricto 12 (related) species. These findings provide new routes to integrate electro-catalysts and micro-organisms showing respectively bio and electrochemical compatibility., Competing Interests: Declaration of competing interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper., (Copyright © 2024 The Authors. Published by Elsevier B.V. All rights reserved.)

Bioelectrochemical systems (BESs) are considered to be energy-efficient to convert ammonium, which is present in wastewater. The application of BESs as a technology to treat wastewater on an industrial scale is hindered by the slow removal rate and lack of understanding of the underlying ammonium conversion pathways. This study shows ammonium oxidation rates up to 228 ± 0.4 g-N m -3 d -1 under microoxic conditions (dissolved oxygen at 0.02-0.2 mg-O 2 /L), which is a significant improvement compared to anoxic conditions (120 ± 21 g-N m -3 d -1 ). We found that this enhancement was related to the formation of hydroxylamine (NH 2 OH), which is rate limiting in ammonium oxidation by ammonia-oxidizing microorganisms. NH 2 OH was intermediate in both the absence and presence of oxygen. The dominant end-product of ammonium oxidation was dinitrogen gas, with about 75% conversion efficiency in the presence of a microoxic level of dissolved oxygen and 100% conversion efficiency in the absence of oxygen. This work elucidates the dominant pathways under microoxic and anoxic conditions which is a step toward the application of BESs for ammonium removal in wastewater treatment.

We developed a technique based on the use of microsensors to measure pH and H 2 gradients during microbial electrosynthesis. The use of 3D electrodes in (bio)electrochemical systems likely results in the occurrence of gradients from the bulk conditions into the electrode. Since these gradients, e.g., with respect to pH and reactant/product concentrations determine the performance of the electrode, it is essential to be able to accurately measure them. Apart from these parameters, also local oxidation-reduction potential and electric field potential were determined in the electrolyte and throughout the 3D porous electrodes. Key was the realization that the presence of an electric field disturbed the measurements obtained by the potentiometric type of microsensor. To overcome the interference on the pH measure, a method was validated where the signal was corrected for the local electric field measured with the electric potential microsensor. The developed method provides a useful tool for studies about electrode design, reactor engineering, measuring gradients in electroactive biofilms, and flow dynamics in and around 3D porous electrodes of (bio)electrochemical systems.

Background: Chain elongation forms a new platform technology for the circular production of biobased chemicals from renewable carbon and energy sources. This study aimed to develop a continuous methanol-based chain elongation process for the open-culture production of a new-generation biofuel precursor and potential platform chemical: n -valerate. Propionate was used as a substrate for chain elongation to n -valerate in an anaerobic open-culture bioreactor. In addition, the co-production of n - and iso-butyrate in addition to n -valerate via, respectively, acetate and propionate elongation was investigated., Results: n -Valerate was produced during batch and continuous experiments with a pH in the range 5.5-5.8 and a hydraulic retention time of 95 h. Decreasing the pH from 5.8 to 5.5 caused an increase of the selectivity for n -valerate formation (from 58 up to 70 wt%) during methanol-based propionate elongation. n -Valerate and both n - and iso-butyrate were produced during simultaneous methanol-based elongation of propionate and acetate. Propionate was within the open-culture preferred over acetate as a substrate with 10-30% more consumption. Increasing the methanol concentration in the influent (from 250 to 400 mM) resulted in a higher productivity (from 45 to 58 mmol C/L/day), but a lower relative product selectivity (from 49 to 43 wt%) of n -valerate. The addition of acetate as a substrate did not change the average n -valerate productivities. Within the continuous bioreactor experiments, 6 to 17 wt% of formed products was methane. The microbial community during all steady-states in both methanol-based elongation bioreactors was dominated by species related to Clostridium luticellarii and Candidatus Methanogranum . C. luticellarii is the main candidate for n -valerate formation from methanol and propionate., Conclusions: n -Valerate was for the first time proven to be produced from propionate and methanol by an open-culture bioreactor. Methanogenic activity can be inhibited by decreasing the pH, and the n -valerate productivity can be improved by increasing the methanol concentration. The developed process can be integrated with various biorefinery processes from thermochemical, (bio)electrochemical, photovoltaic and microbial technologies. The findings from this study form a useful tool to steer the process of biological production of chemicals from biomass and other carbon and energy sources., Competing Interests: Competing interestsThe authors declare that they have no competing interests.