Effect of CO2 bubbles behaviors on microalgal cells distribution and growth in bubble column photobioreactor

Indirect biophotolysis hydrogen production by microalgae is vigorously studied due to both hydrogen production and CO2 fixation. CO2 capture and fixation by microalgae as the first and most important step greatly affects microalgae growth, organics accumulating and then hydrogen production. In this process, CO2 gas is always pumped into photobioreactors in the form of bubbles whose dynamic behaviors CO2 transmission greatly affect the distribution and growth of microalgae. In this work, therefore, the dynamic behaviors of CO2 bubbles in bubble column photobioreactor were visually studied and the effect of inlet CO2 concentration, blast orifice size and gas flow rate on the microalgal distribution and growth were also analyzed. It was found that abundant microalgal cells and cells community were adsorbed on the bubble surface and rose with the rising bubble because of the bubble carrying, resulting in the enrichment of microalgae at the top of the microalgae suspension. Moreover, the non-uniform distributions of microalgal cells along the photobioreactor became more serious with the growth of microalgae. In addition, parametric study indicated that higher inlet CO2 concentration, smaller blast orifice size and larger gas flow rate gave rise to more serious bubble carrying, leading to higher microalgae cell density at the top of the microalgae suspension. The optimal operational conditions for the microalgae growth were 5% (V/V) in the inlet CO2 concentration, 20 μm in the blast orifice size and 20 ml/min in the gas flow rate. These findings will be a helpful for the optimized design and operation of photobioreactors.