Biohydrogen production with fixed-bed bioreactors

An investigation on anaerobic hydrogen production was conducted in fixed-bed bioreactors containing hydrogen-producing bacteria originated from domestic sewage sludge. Three porous materials, loofah sponge (LS), expanded clay (EC) and activated carbon (AC), were used as the support matrix to allow retention of the hydrogen-producing bacteria within the fixed-bed bioreactors. The carriers were assessed for their effectiveness in biofilm formation and hydrogen production in batch and continuous modes. It was found that LS was inefficient for biomass immobilization, while EC and AC exhibited better biomass yields. The fixed-bed reactors packed with EC or AC (denote as EC or AC reactors) were thus used for continuous hydrogen fermentation at a hydraulic retention time (HRT) of 0.5-5 h. Sucrose was utilized as the major carbon source. With a sucrose concentration of ca. 20 g COD/l in the feed, the EC reactor (workingvolume = 300 ml) was able to produce H₂ at an optimal rate of 0.415 l/h/l at HRT = 2 h. In contrast, the AC reactor (300 ml in volume) exhibited a better hydrogen production rate of 1.32 l/h/l, which occurred at HRT = 1 h. When the AC reactor was scaled up to 3 l, the hydrogen production rate was nearly 0.53-0.68 l/h/l for HRT = 1-3 h, but after a short thermal treatment (75°C, 1 h) the rate rose to ca. 1.21 l/h/l at HRT = 1 h. The biogas produced with EC and AC reactors typically contained 25-35% of H₂ and the rest was mainly CO₂, while production of methane was negligible (less than 0.1%). During the efficient hydrogen production stage, the major soluble metabolite was butyric acid, followed by propionic acid, acetic acid, and ethanol.