Internally illuminated photobioreactor for microalgal cultivation

An Internally Illuminated Photobioreactor (IIPBR) was developed to demonstrate its feasibility in microalgal cultivation with supplemental carbon dioxide. Biomass growth and lipid accumulation studies were conducted with two microalgal species-Scendesmus sp. and Nannochloropsis salina, under constant light intensity and varied CO2-to-air ratios to establish optimal CO2 concentrations for algal and lipid productivity. CO2-to-air ratios of 0.035%, 1%, 2%, 3%, 4% and 5% were studied with Scendesmus sp. at a gas flow rate of 800 mL min-1 while Nannochloropsis salina was studied with CO2-to-air ratios of 0.035%, 0.5%, 1% and 2% at two gas flow rates of 800 mL min-1 and 1200 mL min-1. Further, preliminary experiments were conducted with the two species to assess their lipid-accumulation potential under stressing with additional external lights and carbon dioxide-limited conditions.;Maximum productivity of 0.401 g L-1 d-1 was recorded at CO2-to-air ratio of 4% with Scenedesmus sp. while maximum productivity of 0.104 g L-1 d -1 was obtained at CO2-to-air ratio of 1% in case of Nannochloropsis salina. Maximum lipid productivities of 0.075 g L -1 d-1 with Scendesmus sp. at CO 2-to-air ratio of 4% and 0.055 g L-1 d-1 with Nannochloropsis salina at CO2-to-air ratio of 1% recorded in this study were comparable to the lipid productivities reported in the literature with similar species. The optimal productivity per unit energy input achieved with the IIPBR was 1.42 g W-1 d -1 with Scenedesmus sp. and 0.34 g W-1 d-1 with Nannochloropsis salina. This level of productivity per unit energy input of the IIPBR was shown to be better than that of several PBRs reported in the literature.;A mechanistic model was developed to predict algal growth as a function of time. This model was validated with the data from the experimental observations of the two algal species. The goodness of fit between predicted and experimental biomass concentrations in the case of Scendesmus sp. was r2 = 0.876 at a gas flow rate of 800 mL/min; that in the case of Nannochloropsis salina r2 = 0.878 at a gas flow rate of 800 mL/min and r2 = 0.942 at a gas flow rate of 1200 mL/min. Based on the above quality of predictions, the algal growth model proposed could be seen to be useful in predicting temporal biomass concentrations, optimal harvesting volumes and cycles, and scaling up the reactor.