Continuous Photo-hydrogen Production from Acetate Using Rhodopseudomonas Palustris WP 3-5

Phototropic hydrogen production using components of dark fermentation metabolites (i.e. acetate) is an ideal biohydrogen production route since it enables the highest yield possible (i.e., 12 mol H₂/mol hexose) and could also reduce COD content in the dark fermentation effluent to mitigate the environmental burden. In this work, a purple nonsulfur photosynthetic bacterial isolate Rhodopseudomonas palustris WP3-5 was utilized to produce H₂. To assess the performance of the photo-H₂ production system, the continuous photobioreactor was operated at a hydraulic retention time (HRT) of 48 h under different acetate concentrations. The optimal H₂ production efficiency occurred when the acetate concentration was 3000 mg COD/l, leading to an overall H₂ production rate and a H₂ yield of 20.4 ml/h/l and 0.98 mol H₂/mol acetate, respectively. In all tests, the acetate conversion was nearly constant at 87.0±3.1%. Next, the preferable HRT leading to the best H₂ production performance was identified. The results show that at an acetate concentration of 3000 mg COD/L, H₂ production rate increased with a decrease in HRT, giving the best performance at HRT=12 h with an overall H₂ production rate of 42.05 ml/h/l and a H₂ yield of 1.39 mol H₂/mol acetate. Operation under the aforementioned optimal substrate concentration and HRT, the overall H₂ production rate could be enhanced two-fold higher when compared with that obtained from preliminary tests. In addition, the HRT setting was found to play a key role in influencing the performance of continuous H₂ production by R. palustris WP3-5. The proposed continuous cultures seem to be a favourable choice of bioreactor strategy, possessing the potential to achieve higher H₂ production efficiency with an excellent stability.