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• MES was proposed as a route to convert CO 2 to commodities. • Performance indicators of MES are plateauing and far from economic competitiveness. • Effective water electrolysis in microbial-compatible electrolytes is challenging. • Severe trade-off between production rate and energy conversion efficiency. • Improvement of MES, applicability and possible directions are discussed. Microbial electrosynthesis (MES) is an electrochemical process used to drive microbial metabolism for bio-production, such as the reduction of CO 2 into industrially relevant organic products as an alternative to current fossil-fuel-derived commodities. After a decade of research on MES from CO 2 , figures of merit have increased significantly but are plateauing yet far from those expected to allow competitiveness for synthesis of commodity chemicals. Here we discuss the substantial technological shortcomings still associated with MES and evoke possible ways to mitigate them. It appears particularly challenging to obtain both relevant production rates (driven by high current densities) and energy conversion efficiency (i.e. low cell voltage) in microbial-compatible electrolytes. More competitive processes could arise by decoupling effective abiotic electroreductions (e.g. CO 2 to CO or ethanol; H 2 evolution) with subsequent fermentation processes. [ABSTRACT FROM AUTHOR]

• Integrated fixed-film activated sludge tested for shortcut nitrogen removal. • Combining strategies selectively suppressed activity nitrite oxidizing bacteria. • Flocs acted as nitrite source, while carriers were nitrite sinks. • Optimal nitrite source: low, but sufficient aerobic floc retention time (±7 days) • Optimal sink: dissolved oxygen < 0.47 mg O 2 L−1 and N loading rate > 150 mg N L−1 d−1. Implementation of mainstream partial nitritation/anammox (PN/A) can lead to more sustainable and cost-effective sewage treatment. For mainstream PN/A reactor, an integrated fixed-film activated sludge (IFAS) was operated (26 °C). The effects of floccular aerobic sludge retention time (AerSRT floc), a novel aeration strategy, and N-loading rate were tested to optimize the operational strategy. The best performance was observed with a low, but sufficient AerSRT floc (~7d) and continuous aeration with two alternating dissolved oxygen setpoints: 10 min at 0.07–0.13 mg O 2 L−1 and 5 min at 0.27–0.43 mg O 2 L−1. Nitrogen removal rates were 122 ± 23 mg N L−1 d−1, and removal efficiencies 73 ± 13%. These conditions enabled flocs to act as nitrite sources while the carriers were nitrite sinks, with low abundance of nitrite oxidizing bacteria. The operational strategies in the source-sink framework can serve as a guideline for successful operation of mainstream PN/A reactors. [ABSTRACT FROM AUTHOR]

Waste gas fermentation powered by renewable H 2 is reaching kiloton scale. The presence of sulfide, inherent to many waste gases, can cause inhibition, requiring additional gas treatment. In this work, acetogenesis and methanogenesis inhibition by sulfide were studied in a 10-L mixed-culture fermenter, supplied with CO 2 and connected with a water electrolysis unit for electricity-powered H 2 supply. Three cycles of inhibition (1.3 mM total dissolved sulfide (TDS)) and recovery were applied, then the fermenter was operated at 0.5 mM TDS for 35 days. During operation at 0.5 mM TDS the acetate production rate reached 7.1 ± 1.5 mmol C L−1 d−1. Furthermore, 43.7 ± 15.6% of the electrons, provided as H 2 , were distributed to acetate and 7.7 ± 4.1% to butyrate, the second most abundant fermentation product. Selectivity of sulfide as inhibitor was demonstrated by a 7 days lag-phase of methanogenesis recovery, compared to 48 h for acetogenesis and by the less than 1% electrons distribution to CH 4 , under 0.5 mM TDS. The microbial community was dominated by Eubacterium , Proteiniphilum and an unclassified member of the Eggerthellaceae family. The taxonomic diversity of the community decreased and conversely the phenotypic diversity increased, during operation. This work illustrated the scale-up potential of waste gas fermentations, by elucidating the effect of sulfide as a common gas impurity, and by demonstrating continuous, potentially renewable supply of electrons. [Display omitted] • A 10-L scale CO 2 fermentation was tested using reducing power obtained by electrolysis. • Sulfide at 1.3 mM inhibits acetogenesis but the effect is reversible. • 1.3 mM TDS irreversibly suppressed methanogenesis in H 2 /CO 2 fermentation. • Under sulfide stress acetogenesis shifts towards higher butyrate production. [ABSTRACT FROM AUTHOR]