Global warming and rising air pollution which has been caused by using too much fossil fuel has led to look for a new clean source, sustainable and eco-friendly of energy like H2, which can be produced by cyanobacteria and microalgae. In this study, Anabaena sp. was used in a continuous operation to achieve biohydrogen production. To this end, an airlift photobioreactor (20 L) was considered. The effects of the gas holdup, liquid circulation velocity, and the amount of dissolved oxygen on hydrogen production were investigated. Gas holdup, liquid circulation velocity, and KLa (mass transfer coefficient) showed an upward trend by increasing the velocity of the inlet gas. Maximum biomass concentration of and maximal H2 production were observed 1.2 g L-1 d-1 and 371 mL h-1 PBR-1, respectively under light intensity of 3500 lux/m2 applying a light-dark cycle in 7 days, at Ad/Ar of 1.25 and 0.185 and 0.542 cm/s. pH, temperature (30+2 °C), light intensity, and inlet gas flow to the bioreactor (containing 98% air and 2% carbon dioxide) were remained steady. Using the airlift photobioreactor with a good mass transfer and light availability to cyanobacteria growth can be a cost-effective and environmentally technology for biological H2 production.

The rise of CO2 concentration in the Earth is a major environmental problem, which cause global warming. To solve this issue, several methods have been applied, but among these solutions using microalgae is an eco-friendly and cost-effective way of reducing carbon dioxide, as they can efficiently sequestrate CO2 and produce biomass as valuable products. In this study, hydrodynamic parameters, bubble sizes and carbon dioxide uptake were investigated in an airlift bioreactor. Experiments were studied at two different superficial gas velocities (0.185 and 0.524 cm/s) for Spirulina sp. microalgae into a 20-liter airlift bioreactor to find out the amount of carbon dioxide sequestration and cyanobacterial biomass. The highest efficiency of carbon dioxide removal and maximum dry weight of Spirulina sp. were achieved 55.48% and 0.86 g/L respectively at 5% CO2 (v/v) and superficial velocity of 0.185 cm/s. This experiment was conducted in 7 days, light intensity (2600 lux/m2), temperature (30\(\pm\)2 °C) and a light-dark cycle (12–12), which all were constant. The hydrodynamic parameters studied by Spirulina sp. demonstrated a capability of CO2 sequestration in this airlift photobioreactor.