(296c) Multiscale Modeling of Mixotrophic Algal Cultivation in Pilot-Scale Photobioreactors with Synergistic Integration of CO2 Sequestration and Wastewater Treatment

This multiscale model quantifies transport and reaction processes in mixotrophic microalgal growth at three characteristic length scales, namely, macro (photobioreactor), meso (algal cell), micro (organelles). The macro and the meso scale equations capture the temporal dynamics of the transport of CO₂, O₂, H⁺, organic carbon and nitrogen sources in the photobioreactor and the cell, respectively, while the micro scale quantifies the reaction rates of CO₂ fixation and photorespiration in the chloroplast, and mitochondrial respiration. Our model is validated using our experiments on urea, CO₂ (0.04-5%), and acetic acid-mediated mixotrophic cultivation of Chlorella sorokiniana for 138 hours using municipal wastewater (with/without media) at 11000 lx light in 25-liter pilot-scale bubble-column photobioreactors. A biomass yield of 2.74 g/L with 28.3% lipids and 35.3% proteins is obtained, while the COD, ammonium, phosphate, nickel, and H⁺ concentrations reduce by 65-89% as the algae assimilate NH₄⁺ and PO₄^3- present in wastewater into amino acids and ATP, respectively. Our simulations quantify the autotrophic and heterotrophic components of mixotrophic biomass yield to find the optimal inlet CO₂ concentration (of 3%) that synergizes autotrophic CO₂ sequestration with heterotrophic assimilation of organic carbon, thereby maximizing both autotrophic and heterotrophic growths. Super-optimal levels of inlet CO₂ acidify the stroma of the chloroplast, inhibit RuBisCo’s enzymatic activity for CO₂ fixation in the Calvin Cycle, decelerate carrier-mediated uptake of acetate, and reduce biomass yields. Microalgal cultivation at atmospheric (0.04%) CO₂ using municipal wastewater without media produces 0.65 g/L biomass in 3 days with 26% proteins and 22.8% lipids, by utilizing the COD (65.4%), ammonium (88.4%), and phosphates (89.08%) present in the wastewater while simultaneously removing the Nickel (82.6%). Our multiscale reaction-transport model provides a useful tool for further scaling up this pilot-scale technology that synergistically integrates CO₂ sequestration and wastewater treatment with rapid microalgal cultivation, and cost-effective harvesting in 30 minutes.