Multiscale modelling of mixotrophic algal growth in pilot-scale photobioreactors and its application to microalgal cultivation using wastewater

Abstract

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 (R2 = 0.96-0.99) on urea, CO2 (0.04–5%), and acetic acid-mediated mixotrophic cultivation of Chlorella sorokiniana for 138 h using municipal wastewater (with and without media) at 11,000 lx light in 25-liter pilot-scale bubble-column photobioreactors, which produces 0.47–2.74 g/L biomass with 22.8–29.6% lipids, while reducing the COD, ammonium, phosphate, nickel, and H+ concentrations by 65–89%. The alga assimilates the ammonium and the phosphates 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. Our harvesting process drastically reduces the algal harvesting time to less than 29 min. This 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 (using municipal wastewater without autoclaving) and cost-effective harvesting.