Guiding Microbial Crossroads: Syngas-Driven Valorisation of Anaerobic-Digestion Intermediates into Bio-Hydrogen and Volatile Fatty Acids

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Anaerobic digestion (AD) has long been valued for producing a biogas–digestate pair, yet its profitability is tightening. Next-generation AD biorefineries now position syngas both as a supplementary feedstock and as a springboard to capture high-value intermediates, hydrogen (H2) and volatile fatty acids (VFA). This review dissects how complex natural consortia “decide” between hydrogenogenesis and acetogenesis when CO, H2, and CO2 co-exist in the feedstocks, bridging molecular mechanisms with process-scale levers. The map of the bioenergetic contest between the biological water–gas shift reaction and Wood–Ljungdahl pathways is discussed, revealing how electron flow, thermodynamic thresholds, and enzyme inhibition dictate microbial “decision”. Kinetic evidence from pure and mixed cultures is integrated with practical operating factors (gas composition and pressure, pH–temperature spectrum, culture media composition, hydraulic retention time, and cell density), which can bias consortia toward the desired product.