Analysis and modelling of microalgae growth and production of high added-value metabolites

Microalgae are a very versatile microorganism that have the ability to modify their biomass composition under controlled condition in order to accumulate products having applications in several sectors. The aim of this thesis work is the analysis and modelling of both microalgal growth and production of high added-value metabolites, focusing also on their extraction and purification. An outdoor 10 bubble column photobioreactors (PBRs) pilot plant for the cultivation of two microalgae named Tetradesmus obliquus and Graesiella emersonii, covering a 9 months cultivation period (March 2017-December 2017), has been installed and operated. All collected data (as microalgal growth rate, outdoor parameters and initial cultivation’s conditions) have been used to develop an empirical model for prediction of microalgal growth in photobioreactors at specific outdoor conditions, using Principal Component Analysis and Partial Least Squares regression method, obtaining acceptable outcomes for both responses: microalgal specific growth rate (μ) and maximum productivity (Pmax). Concerning microalgal metabolism, also a new mathematical model able to represent in a simple way the accumulation of metabolites inside microalgae, focusing on the carbon partitioning process between triacylglycerides (TAG) and starch during nitrogen starvation in phototrophy, has been developed, obtaining high R-Squared values as index of model’s goodness of fitting. A future application of these models can be found in the MEWLIFE European project, in which Bio-P has a role as partner, since this project has as aim the production of microalgal biomass in an integrated phototrophic and heterotrophic cultivation system using preconcentrated olive oil wastewaters (OMWW) as carbon source. As a completion of the microalgal process treatment, a study of the downstream processes for the extraction (using supercritical CO2) and purification of the high added value metabolites (with molecular distillation) has been carried out, developing a feasibility study also from the economical point of view. As regard the supercritical CO2 the best extraction conditions in terms of operative variables have been: T = 60°C, P = 250 bar and SSR = 5 h−1 with a daily amount of the desired products equal to 147 kg and OPEX = 561.7 k€/year and CAPEX = 2717.9 k€/year. Regarding the molecultar distillation process, the best operating conditions have been found at T = 128 °C and P = 0.33 Pa, obtaining OPEX = 498.23 k€/year and CAPEX = 2387.4 k€/year.