Comparison of Photofermentative Hydrogen Production in Cylindrical Photobioreactors Using Different Mixing Systems

In this work, the ability of the photosynthetic purple non-sulfur bacterium Rhodopseudomonas sp. to produce H₂ was investigated in two cylindrical photobioreactors (PBRs). The PBRs used in this work had different working volumes: 0.2 L of working volume (named 0.2-PBR) and 4.0 L of working volume (named 4.0-PBR). Two mixing methods were tested in the 4.0-PBR. The first used a rotor with four paddles, and the second a spiral rotor. Additionally, light conversion efficiency (LCE) was assessed for the three conditions tested. The culture in the 0.2-PBR produced 142.15 mL of H₂ with an average H₂ production rate of 0.74 mL/h, an average productivity of 3.70 mL/L/h and an LCE = 0.59%. The culture in the 4.0-PBR produced a total of 806.05 mL and 1642 mL of H₂ with the paddle rotor and the spiral rotor, respectively. The average H₂ production rate and LCE of the two rotors were 2.29 mL/h and LCE = 0.58% in the case of the paddle rotor and 2.87 mL/h and LCE = 0.72% in the case of the spiral rotor. The more uniform and thus more efficient mixing of the cells achieved with the spiral rotor played an important role compared to the paddle rotor, resulting in a higher LCE. This study presents a scale-up from 0.2 L to 4.0 L of the photofermentation process using the purple non-sulfur bacterium Rhodopseudomonas sp. S16-VOGS3.