Optimization of microalgae-sourced lipids production for biodiesel in a top-lit gas-lift bioreactor using response surface methodology

Highlights: • A top-lit gas-lift bioreactor was studied as a deep tank microalgae cultivation method. • Areal biomass productivity obtained was notably higher than traditional raceways. • A two-phase factors screening-optimization method was applied. • The optimum operational parameters to maximize lipid production was obtained. • The lipid produced is suitable for conversion into biodiesel. Microalgae bioreactors that capture industrial carbon dioxide (CO₂) emissions to produce lipids for biodiesel are of significant interest. Sun-lit open raceways are generally considered the most economic method for mass cultivation, but the large physical footprint of these shallow systems can limit both industrial site availability and CO₂ transfer into the culture medium. To address these issues, a deep top-lit only, gas-lift bioreactor to culture microalgae and capture CO₂ was designed and investigated. The results show a three times increase in areal biomass and lipid productivities when compared to traditional raceways used in large-scale microalgae production. Operational factors exerting significant effects on areal biomass and lipid productivities were identified through Plackett-Burman experimental design as gas flow rate, feed gas CO₂ content, and dispersion height. By employing response surface methodology, models to predict areal biomass and lipid productivities were derived. The resulting desirability function was then applied to obtain the optimal combination of operational parameters that maximize lipid production per unit area occupied by the bioreactor, whilst keeping biomass production low to reduce downstream processing costs. The optimum operational parameters that fulfill the requirements of the optimization function resulted in areal biomass productivity of 32.1 g_dwm⁻²d⁻¹ and areal lipid production of 198.4 g_Lipidm⁻².