Kuo Shing Lee

Biohydrogen production with fixed-bed bioreactors

Abstract 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 ( working volume =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.

Microbial hydrogen production with immobilized sewage sludge

Municipal sewage sludge was immobilized to produce hydrogen gas under anaerobic conditions. Cell immobilization was essentially achieved by gel entrapment approaches, which were physically or chemically modified by addition of activated carbon (AC), polyurethane (PU), and acrylic latex plus silicone (ALSC). The performance of hydrogen fermentation with a variety of immobilized‐cell systems was assessed to identify the optimal type of immobilized cells for practical uses. With sucrose as the limiting carbon source, hydrogen production was more efficient with the immobilized‐cell system than with the suspended‐cell system, and in both cases the predominant soluble metabolites were butyric acid and acetic acid. Addition of activated carbon into alginate gel (denoted as CA/AC cells) enhanced the hydrogen production rate ( vH2) and substrate‐based yield ( YH2/sucrose) by 70% and 52%, respectively, over the conventional alginate‐immobilized cells. Further supplementation of polyurethane or acrylic latex/silicone increased the mechanical strength and operation stability of the immobilized cells but caused a decrease in the hydrogen production rate. Kinetic studies show that the dependence of specific hydrogen production rates on the concentration of limiting substrate (sucrose) can be described by Michaelis‐Menten model with good agreement. The kinetic analysis suggests that CA/AC cells may contain higher concentration of active biocatalysts for hydrogen production, while PU and ALSC cells had better affinity to the substrate. Acclimation of the immobilized cells led to a remarkable enhancement in vH2 with a 25‐fold increase for CA/AC and ca. 10‐ to 15‐fold increases for PU and ALSC cells. However, the ALSC cells were found to have better durability than PU and CA/AC cells as they allowed stable hydrogen production for over 24 repeated runs.

Biomass based hydrogen production by dark fermentation — recent trends and opportunities for greener processes

The generation of biohydrogen as source of biofuel/bioenergy from the wide variety of biomass has gathered a substantial quantum of research efforts in several aspects. One of the major thrusts in this field has been the pursuit of technically sound and effective methods and/or approaches towards significant improvement in the bioconversion efficiency and enhanced biohydrogen yields. In this perspective, the present contribution showcases the views formulated based on the latest advances reported in dark fermentative biohydrogen production (DHFP), which is considered as the most feasible route for commercialization of biohydrogen. The potential prospects and future research avenues are also presented.