BACKGROUNDAnaerobic, upflow fixed-bed reactors have been considered suitable for the production of H-2 from wastewater because, in contrast to completely mixed reactors, they avoid the washout of H-2-producing microorganisms. However, the excessive growth and accumulation of biomass on the bed are unfavorable and cause a drastic drop in biogas production, even though the fermentative process in the liquid medium continues unimpeded. This paper proposes a novel anaerobic fixed-bed reactor configuration intended to control the accumulation of biomass by means of structuring the bed longitudinally and inverting the flow (down-flow). RESULTSThree anaerobic, down-flow structured-bed reactors (ADSBRs) with different support materials for the attachment of microorganisms were operated continuously with and without frequent biomass discharges. The H-2 flow was continuously maintained for 120 days, resulting in a volumetric production of 0.6 L H-2 L-1 d(-1) and a yield of 0.5 mol H-2 mol(cs)(-1). Compared with the anaerobic, upflow fixed-bed reactor, the ADSBR increased by 10% the natural removal of biomass induced by the effluent and decreased by 15% the accumulated biomass in the bed. CONCLUSIONHence, this study demonstrates that the ADSBR produces H-2 continuously and stably due to the constant renewal of biomass; furthermore, young biomass exhibited more H-2-producing microorganisms than either non-producing or H-2-consuming microorganisms. (c) 2015 Society of Chemical Industry