New ultra-flat photobioreactor for intensive microalgal production: The effect of light irradiance

One of the main bottlenecks for the exploitation of microalgae is the low biomass concentration of the cultures: high harvesting costs and large cultivation area are always required. This bottleneck is partly due to a low light availability along the optical path of photobioreactors. An ultra-thin flat photobioreactor (UFP) (3 mm thickness) was proposed to increase both biomass concentration and productivity. The performance of the UFP was investigated: the effects of incident light intensity - from 50 and 1000 μmolPhotons m-1 s-1 - on cell growth, photosynthesis rate, and biochemical composition of Chlorella sorokiniana were characterized. The maximum microalgal concentration and the maximum areal productivity were 24 kg m−3 and 1.34 g m−2 h−1, respectively. The cell specific growth rate reached 0.1 h−1 at 1000 μmol m−2 s−1. The biochemical composition of the microalgal biomass changed with the light irradiance. Protein content increased from 35 up to 53% of DW with increasing the light intensity. The concentration of storage compounds, such as starch and lipids, decreased from 30 to 16% and from 30 to 10%, respectively, with increasing the light intensity. A limit in the maximum biomass concentration achievable was identified. Several hypotheses have been discussed. A light transfer model was applied to assess the presence of light limitation. Other hypotheses were analyzed in depth and the most feasible explanations were found to be a) the damage to the photosystem when exposed for long period to continuous and high light irradiances, b) nutrient limitation due to salt precipitation or c) gas-liquid transfer of the CO2. Finally, benefits and drawbacks of the ultra-thin culture system were discussed.