Biohydrogen production from algal biomass (Anabaena sp. PCC 7120) cultivated in airlift photobioreactor

• Biomass production using different photobioreactors.
• Correlation between mixing time and configuration of airlift photobioreactors.
• Enzymatic hydrolysis is suitable for the pretreatment of algal biomass.
• H2 production from algal biomass using thermophilic mixed culture.

The present study deals with the optimization of pretreatment conditions followed by thermophilic dark fermentative hydrogen production using Anabaena PCC 7120 as substrate by mixed microflora. Different airlift photobioreactors with ratio of area of downcomer and riser (Ad/Ar) in range of 0.4–3.2 were considered. Maximum biomass concentration of 1.63 g L−1 in 9 d under light intensity of 120 μE m−2 s−1 was observed at Ad/Ar of 1.6. The mixing time of the reactors was inversely proportional to Ad/Ar. Maximal H2 production was found to be 1600 mL L−1 upon pretreatment with amylase followed by thermophilic fermentation for 24 h compared to other methods like sonication (200 mL L−1), autoclave (600 mL L−1) and HCl treatment (1230 mL L−1). The decrease of pH from 6.5 to 5.0 during fermentation was due to the accumulation of volatile fatty acids. Amylase pretreatment gave higher reducible sugar content of 7.6 g L−1 as compare to other pretreatments. Thermophilic fermentation of pretreated Anabaena biomass by mixed bacterial culture was found suitable for H2 production.