Context. Fossil fuels must eventually be replaced by environmentally and economically sustainable, carbon-neutral fuels. Microalgae biomass can be a viable alternative source of third-generation biofuel production. Agri-food aqueous wastes like cheese whey from the dairying industry can become important feedstocks for microalgal production.
Gap. The feasibility of microalgae cultivation under different trophic regimes has been shown at lab scale. However, process design, scaling-up and optimization can benefit from mathematical models for full understanding of the whole process.
Aim. We built a comprehensive predictive mathematical model of microalgal growth and metabolism to investigate/facilitate the production of triacylglycerol-containing microalgae biomass from cheese whey in open mixed cultures.
Methods. A white-box model type was built by (i) detailing microalgae mixotrophic metabolism from intracellular to extracellular constraints (ii) deriving stoichiometric coefficients using compound balances and formulating rate equations; (iii) designing photo-bioreactor batch/chemostat configurations and dark-famine/light-feast cycles; and (iv) implementing ODEs of model variables in Matlab®. The model integrated non-axenic regimes to capture process conditions associated with microbial competition with chemoorganoheterotrophic and chemolithoautotrophic bacteria.
Findings. Applying periodic cycles of organic feast under light (36 h) and starvation in the dark (12 h) on a 2 days basis in a continuous-flow photobioreactor fed by raw cheese whey resulted in a high-grade enrichment of microalgae outcompeting chemotrophs and in a volumetric productivity of triacylglycerol of 83 mol·d-1·m-3. This highlights the possibility of selecting for a triacylglycerol-containing mixotrophic microalgae to valorize cheese whey in a mixed-culture photobioreactor operated with light control.
Utilization. The model supports experimental designs and process optimizations toward implementation of microalgal processes for the treatment of agri-food concentrated wastewater and its low-cost valorization via non-axenic mixed-culture biotechnology.
