Controlling optimal fermentation types by pH and redox potential for bio-hydrogen production

This study is to understand the operational conditions of anaerobic process in continuous-flow acidogenic reactors and the optimal fermentation types to maximize bio-hydrogen production. Mixed cultures instead of pure cultures of microorganisms were employed in the fermentation process. The operational conditions of anaerobic process to control fermentation types, including pH value and redox potential (Eh) were investigated. Butyric acid-type fermentation occurred at pH > 6, propionic acid-type fermentation occurred at pH about 5.5 with Eh > -278mV, and ethanol-type fermentation occurred at pH < 4.5. The representative strains of butyric acid-type, ethanol-type and propionic acid-type fermentations are Clostridium, Bacteriodes and Propionibacterium respectively. By comparing the operational stability and hydrogen production capacity between ethanol type fermentation and butyric acid type fermentation, the optimal fermentation type was found to be ethanol type fermentation, the highest OLR reached 86,1kgCOD/m³d. However, in the operation of butyric acid type fermentation, OLR was only increased to 21kgCOD/m ³-d, it demonstrated that ethanol type fermentation was comparatively stable. The average biogas production rate of ethanol type fermentation was about 25.12 L/d, and the maximum rate reached to 29.97 L/d. Comparatively, the biogas production rate of butyric acid type fermentation was within a range of 16.22-20.36 L/d, the maximum hydrogen production rate was only 7.8 L/d.