Coupling dark fermentation (DF) with microbial electrolysis cells (MECs) can raise biohydrogen recovery from agro-industrial wastewaters up to 13-fold at bench scale (Marone et al., 2017), but the cascade has not been demonstrated beyond the laboratory, and its mechanistic basis remains correlational. The open problem is to mature this cascade into a deployable, mechanistically understood technology: (i) demonstrate coupled DF-MEC behavior, energy balances, and robustness at pilot or industrial scale; (ii) determine whether the taxa whose abundances predicted performance - an OTU related to Geovibrio ferrireducens and an uncultured Coriobacteriaceae OTU - actually mediate anodic electron transfer; (iii) make MEC polishing effective on complex or inhibitory effluents such as paper-mill wastewater; (iv) identify the redox-active species and cathodic side reactions behind hydrogen-recovery losses; and (v) quantify the economic and logistic tradeoff of redirecting by-products with existing uses (e.g., vinasse used as fertilizer) into hydrogen production.

Background

Hydrogen is an energy carrier whose biological production from wastewater couples pollution control with renewable fuel generation. Dark fermentation (DF) is the anaerobic microbial conversion of sugars and other organic matter to hydrogen gas plus volatile fatty acids (VFAs, mainly acetate and butyrate) and alcohols; it is robust and fast but thermodynamically limited, leaving most of the substrate energy in these reduced metabolites, whose aggregate amount is measured as chemical oxygen demand (COD). A microbial electrolysis cell (MEC) is a bioelectrochemical reactor in which anode-respiring bacteria (exoelectrogens, classically Geobacter species) oxidize organic acids at an anode and transfer the released electrons to the electrode; an applied voltage of a few hundred millivolts then drives hydrogen evolution at the cathode. A MEC can therefore recover the residual COD of a DF effluent as additional hydrogen, and in a two-stage DF-MEC cascade the MEC acts as a polishing step that raises total hydrogen recovery per unit of organic matter while removing COD. Key performance metrics are the hydrogen yield per g of COD consumed, the coulombic efficiency (fraction of electrons in the removed substrate recovered as current), the cathodic hydrogen recovery (fraction of that current recovered as hydrogen), and the ratio of hydrogen combustion energy to electrical input.

After early two-stage demonstrations on cheese whey (Moreno et al., 2015), Marone et al. (2017) provided the first systematic comparison of the coupled cascade across six real agro-industrial wastewaters and by-products - including deproteinized cheese whey, fruit-juice and fruit-processing wastewaters, paper-mill wastewater, and spirits-distillation vinasse - under identical bench-scale conditions (batch DF at 37 °C and pH 5.5; two-chamber MECs with carbon-felt anodes poised at +0.2 V vs. a reference electrode, an anion-exchange membrane (AEM) separating the chambers, and Pt-Ir cathodes). Coupling increased total hydrogen recovery up to 13-fold relative to DF alone, with a maximum overall yield of 1608.6 ± 266.2 mL H2_2 per g of COD consumed, COD removal up to 78.5 ± 5.7%, and hydrogen combustion energy exceeding the electrical input by factors of 3.4 to 7.3. Performance was strongly substrate-dependent: simple effluents rich in acetate and butyrate performed best, while complex effluents such as paper-mill wastewater performed poorly and variably. High-throughput 16S rRNA gene sequencing (a marker-gene survey of microbial community composition; an operational taxonomic unit, OTU, is a DNA-sequence-based surrogate for a taxon) showed that the best-performing anode was dominated (50.3%) by an OTU with 99% sequence similarity to Geovibrio ferrireducens, a dissimilatory Fe(III)-reducing bacterium (Caccavo et al., 1996) previously detected in microbial fuel cell anodes by Katuri et al. (2012) and Ortega-Martinez et al. (2013), and that a Coriobacteriaceae OTU correlated with coulombic efficiency. These correlations were computed from only six biofilm samples.

The paper itself states that all results were obtained at bench scale and that scaling up the coupled system is an outstanding task. Subsequent reviews of DF-MEC coupling (Bakonyi et al., 2018; Lee et al., 2022; Srivastava et al., 2024) and of MEC scale-up generally (Jiang et al., 2023) treat both the translation to pilot scale and the kinetics of the anodic biofilm as the field's central unresolved challenges, while pilot-scale MEC development to date (Heidrich et al., 2014; Singh et al., 2021; Guerrero-Sodric et al., 2023, 2025) has addressed single-stage MECs treating urban or domestic wastewater, not the coupled cascade on fermentation effluents.

Problem Statement

Can the two-stage dark fermentation-microbial electrolysis (DF-MEC) cascade demonstrated at bench scale by Marone et al. (2017) be advanced to a deployable, mechanistically understood technology for biohydrogen production from agro-industrial wastewaters and by-products? The source work leaves five linked questions open, all serving this single objective: (1) Scale-up - how does the coupled cascade behave at pilot or industrial scale (at the 100 L to 1 m3^3 scale of published pilot MECs and beyond), and do its energy balances, hydrogen yields, and robustness over extended operation on real effluents survive the transition from batch bench reactors? (2) Microbial mechanism - do the taxa whose abundances statistically predicted performance, namely the OTU related to Geovibrio ferrireducens and the uncultured Coriobacteriaceae OTU, actually mediate anodic electron transfer from fermentation metabolites? (3) Robustness on complex effluents - which toxic or inhibitory compounds and substrate-complexity factors (e.g., lignocellulosic fractions in paper-mill-type wastewater) limit MEC polishing, and which pretreatments, bioaugmentations, or reactor designs overcome them so the cascade works across the six agro-industrial wastewaters and by-products of Marone et al. (2017) plus at least one additional complex effluent? (4) Loss mechanisms - which redox-active species and which cathodic side reactions (such as reduction of metal ions in the cathodic chamber, as discussed for the source system) account for the observed cathodic hydrogen-recovery losses, and which membrane selections or operational controls mitigate them? (5) Valorization tradeoff - for by-products with existing uses, such as concentrated vinasse currently valorized as fertilizer, does treating the stream with a DF-MEC cascade upstream of concentration improve the overall economic and logistic outcome? These five components are jointly posed parts of this single overarching question.

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