Valorization of macroalgae digestate into aromatic rich bio-oil and lipid rich microalgal biomass for enhanced algal biorefinery performance

The valorization of macroalgae digestate as a secondary resource for high value chemicals and nutrients will promote the sustainability and circularity of anaerobic digestion based biorefinery. In this study, three digestates from A. nodosum C.linum and L. digitata were separated into liquid and solid fractions to investigate the production of high value added chemicals through pyrolysis using Pyrolysis Gas Chromatography Mass Spectroscopy (Py-GC/MS) while the filtered liquid fractions were tested as an alternative culture media to grow C. sorokiniana under mixotrophic conditions. The digestates showed different thermal degradation and an improvement of bio-oil profiles compared to the starter material. Pyrolyzates from raw macroalgae were characterized by a high anhydrosugar content in contrast to high aromatics observed in the case of their digestates. Toluene, benzofuran and vinylphenol, base chemicals for many industries, represented together 30–37% of the total chemicals produced during pyrolysis of the three macroalgae digestate. On the other hand, C. sorokiniana cultured on digestate-based media showed a higher lipid content with an increase in monounsaturated fatty acids and a lower polyunsaturated fatty acid content in comparison to microalgae grown in standard tris-acetate-phosphate media. Thus, the acyl composition was shifted in a direction more suitable for biodiesel production by this process. In addition, the increase of Chemical Oxygen Demand and Volatile Fatty Acids concentration in the digestate was found to reduce ammonium toxicity. Finally, 94% of Chemical Oxygen Demand and 83% of ammonium were removed by microalgae from the digestate-based media which will reduce the pollution risk of the biorefinery. Overall, the results indicate that using macroalgae solid digestates can generate improvements in the quality of products obtained by pyrolysis and the liquid digestate can positively influence microalgae growth and its products.