Repeated-batch production of hydrogen using Rhodobacter sphaeroides S10

Photofermentation of acid hydrolyzed oil palm empty fruit bunch is reported for hydrogen production in repeated-batch fermentations using the bacterium Rhodobacter sphaeroides S10. Photofermentations were carried out at 35 °C at an incident light level of 10 klux. At specified times, different specified volumes of the culture broth were removed and replaced with an equal volume of the fresh medium. The initial mixed carbon (glucose, xylose, acetic acid) content in the medium of the repeated-batch reactors was adjusted to 20 mM. The kinetics of hydrogen production were evaluated in repeated-batch fermentations carried out in various ways: different volume exchange levels, different switch times from batch to repeated-batch operation, and different cycle times.A 90% replacement of the broth volume with the fresh medium proved to be the best, resulting in a maximum hydrogen production rate of 27.7 ± 1.7 mL H2/L h with a conversion efficiency of 33.7 ± 0.9%. At a fixed broth exchange volume of 90%, experiments with varying intervals between volume exchanges, showed an exchange interval of 84 h to be the best. At this exchange interval, the maximum hydrogen production rate was 35.9 ± 1.6 mL H2/L h and the conversion efficiency was 42.6 ± 1.0%. With the broth exchange volume fixed at 90%, the batch to repeated-batch switch time fixed at 84 h, four repeated-batch cycles were monitored, each with a cycle time of 60 h. For the five cycles (including the first batch phase), the average conversion efficiency was 51.8 ± 4.8%; the average substrate specific hydrogen production rate was 13.8 ± 2.4 mL H2/g h; and the average maximum hydrogen production rate was 32.3 ± 1.1 mL H2/L h. Thus, the kinetics of hydrogen production for the five cycles were highly consistent.

► Hydrogen is produced via a novel repeated-batch photofermentation of Rhodobacter sphaeroides.
► A pretreated agroindustry waste stream is used as the sole carbon source.
► An optimal operational strategy is identified for the photofermentation.
►  The average substrate conversion exceeded 51% in repeated cycles.