Microbial electrolysis cells (MECs) can recover hydrogen from wastewater organics, but in single-chamber designs the produced hydrogen is consumed by scavengers — methanogens and homoacetogens — that proliferate in the mixed culture, and nitrogen sparging has proven insufficient to stop them. The source study (Montpart et al., 2015) showed that a syntrophic anodic consortium can convert complex substrates (glycerol, milk, starch) to hydrogen, with milk and tri-substrate co-digestion sustaining production for three months, but its conclusions and outlook left several issues open. This problem asks whether single-chamber MECs can sustain high-recovery, high-purity hydrogen production from real complex dairy-type wastewater: (1) quantitatively attribute the methane formed among fermentation-hydrogen consumption, acetoclastic conversion, and cathode-driven hydrogenotrophic methanogenesis, with characterization of the responsible microbial communities during long-term operation; (2) develop scavenger-control strategies that are effective, scalable, economical, and compatible with continuous-flow operation, without harming the anodic consortium; and (3) confirm on actual dairy plant effluent, under long-term continuous operation directed at future process scale-up, the potential demonstrated at bench scale with model substrates to produce hydrogen without proliferation of methanogenic archaea. All three parts are required jointly.

Background

A microbial electrolysis cell (MEC) is a bioelectrochemical reactor in which anode-respiring bacteria (also called exoelectrogens, e.g. Geobacter species) oxidize organic matter (quantified as chemical oxygen demand, COD, the standard wastewater measure of organic pollution) and release electrons to an anode; because the electrons cannot reduce protons spontaneously at useful rates, a small applied voltage (typically 0.5–1 V, far below the ~1.8 V of abiotic water electrolysis in practical cells) drives hydrogen evolution at the cathode. In a single-chamber MEC there is no membrane separating the cathode from the mixed microbial culture, which lowers cost and internal resistance but exposes the cathodically produced hydrogen to biological scavenging: hydrogenotrophic methanogens (archaea that convert hydrogen and carbon dioxide to methane) and homoacetogens (bacteria that convert hydrogen and carbon dioxide to acetate) consume the product gas, and acetoclastic methanogens convert accumulated acetate to methane, so gas purity and recovery collapse. Coulombic efficiency (the fraction of substrate electrons recovered as current) and cathodic gas recovery (the fraction of those electrons recovered as hydrogen gas) are the standard performance metrics. Complex substrates such as dairy wastes require a syntrophic consortium in which fermentative bacteria hydrolyze and ferment the organics to volatile fatty acids that the anode respirers can oxidize. Montpart, Rago, Baeza and Guisasola (2015, Water Research 68:601–615) operationally enriched such a consortium and showed, in single-chamber MECs fed glycerol, milk, and starch, that hydrogen production was sustained over three months only with milk and with simultaneous degradation of all three substrates (best case: 150 A m⁻³, 0.94 m³ H₂ m⁻³ d⁻¹, 91% cathodic gas recovery at 0.8 V applied voltage), while with glycerol and starch hydrogen scavengers proliferated even under continuous nitrogen sparging. The authors' conclusions and outlook identified, as separate remarks: (i) validation with real industrial dairy wastewater rather than model substrates such as reconstituted milk; (ii) microbial characterization during MEC operation — an acknowledged limitation, since no anode samples were taken during the MEC phase and community shifts were inferred only from electrochemical and gas-composition data, leaving the dynamics of hydrogen scavengers, homoacetogens, and anode-respiring bacteria under long-term MEC conditions unmeasured; (iii) quantitative partitioning of methane sources among hydrogen produced in fermentation, acetate conversion, and possible direct cathode-driven methanogenesis — their open-circuit and hydrogen-bubbling tests were explicitly qualitative, and the contribution of fermentation-produced hydrogen was not experimentally assessed; (iv) control strategies beyond nitrogen sparging — e.g. reduced hydraulic residence time to wash out non-favored microorganisms, periodic exposure to air, or optimized sparging — that are feasible in continuous-flow operation and scale-up, with economical feasibility studied in detail; and (v) continuous-flow operation and future process scale-up of single-chamber MECs, which the bench-scale study did not test. These items are distinct outlook remarks and acknowledged limitations of the source study, all directed at the same overarching need: making single-chamber MEC hydrogen recovery from real complex wastewater robust, mechanistically understood, and scalable.

Problem Statement

Can a single-chamber microbial electrolysis cell (MEC) sustain high-recovery, high-purity hydrogen production from real complex dairy-type wastewater, in a way that is mechanistically understood and operationally scalable? Concretely, determine, with justification, whether — and if so how — a single-chamber MEC can sustain such production; if it can, the answer should demonstrate and explain a process that: (1) quantitatively partitions the methane formed among its sources — hydrogen produced during fermentation and subsequently consumed, acetoclastic conversion of accumulated acetate, and cathode-driven hydrogenotrophic methanogenesis (including any cathodic biofilm contribution) — going beyond the source study's qualitative open-circuit and hydrogen-bubbling tests, and characterizes the abundance and dynamics of hydrogen scavengers, homoacetogens, and anode-respiring bacteria during long-term MEC operation; (2) suppresses methanogens and homoacetogens effectively, economically, and scalably — nitrogen sparging alone having proven insufficient — in a way compatible with continuous-flow operation and scale-up and without damaging the fermentative–anode-respiring syntrophic consortium; and (3) confirms on actual dairy plant effluent (not model substrates such as reconstituted milk or cheese whey alone) the potential demonstrated at bench scale with milk and tri-substrate co-digestion to sustain net hydrogen production without proliferation of methanogenic archaea and other hydrogen scavengers, under long-term continuous operation of the kind needed for future process scale-up. For any of these parts, a supported finding of either outcome — success, or a demonstration that it cannot be achieved under the stated conditions — resolves that part. For concreteness, high recovery and high purity are benchmarked against the source study's best case (cathodic gas recovery of about 91%, with hydrogen the dominant product-gas constituent so that net hydrogen production is not scavenged to methane).

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