Abstract
Euglena gracilis is a photosynthetic, acidophilic flagellate, also classified as a microalga, growing also in mixotrophic conditions. It can accumulate high-value molecules, but its production is very expensive, especially due to the cost of the required carbon source. The present work was carried out to optimize at lab-scale (4 L photobioreactors, 1.5 L working volume) the batch cultivation of Euglena on industrial residues from a firm producing food commodities. The effluent from the anaerobic digestion of the liquid residues was used and mixed with a sugar-rich vinasse. In the first tests different working pH, acidifying agents, stirring methods and vinasse concentrations were tested. After selecting the best operation parameters among them, different kinds and intensity of light were tested, as well as different cultivation times. As the aim of Euglena production is to use its biomass in pet feed formulations, it was also important to check contamination, especially by eumycetes, which find favourable growth conditions in the presence of nutrients and sugars and at low pH. The best results were obtained in batch, at pH 5, with the mix of UASB effluent and 2.5 % of vinasse, using purple light, provided by means of LED strips at 50 µmol/m2/s. The maximum TSS concentration was 1.14 g/L, after 72 h, with a paramylon content total of 28 %. COD removal efficiency, on the contrary, reached the maximum value of 78 % at 144 h and N removal did not vary between 72 and 144 h. The content of pigments also increased between 72 and 144 h. In the selected optimal condition, no significant contamination occurred. The use of UASB effluent enriched with vinasse as substrate proved to be a good, reliable and economic option for Euglena cultivation whose performances were comparable or even better than reported for synthetic, sterilized substrates, and occurred when using the lower intensity of purple light.