Regular articleAnaerobic digestion of lipid-extracted microalgae: Enhancing nutrient recovery towards a closed loop recycling

- Lipid extracted C. vulgaris is a good substrate for anaerobic digestion.
- Liquid digestate from lipid-extracted microalgae can be used to support algal growth.
- Soluble sulfate is lacking in the digestate and has to be amended.
- Phosphorus in the liquid digestate is limiting as it precipitates in the solid phase.
- Addition of sodium bicarbonate to the digestate enhances P re-solubilisation.

Nutrient recycling is essential to make microalgae cultivation sustainable at industrial scale. To this aim, lab-scale anaerobic digestion experiments of Chlorella vulgaris after lipid extraction were carried out to evaluate biogas yield and nutrients recovery in the liquid digestate. Then, this liquid fraction was used as source of nutrients for cultivation of a closely related C. vulgaris species, in order to assess the possibility of re-cultivating microalgae towards a closed-loop nutrient recycling process. The biological methane potential tests resulted in biogas production of about 347 NmL gVS−1 with 43% v/v methane content. In re-growth experiments, the liquid digestate showed an insufficient amount of soluble sulfate and phosphorus. However, by amending these two nutrients, the specific growth rate and final biomass concentration increased to about 2 d−1 and 2 g L−1, respectively, which were comparable to those obtained in a defined control medium. The low content of soluble phosphorus in liquid digestate was mainly due to precipitation and removal with the solid phase. Several techniques were hence tested to enhance phosphorus solubilisation, and highest recoveries of up to 41% were obtained when using NaHCO3. Finally, C. vulgaris was grown in such treated digestate, obtaining a final biomass production comparable to that of the control, without the need of external phosphorus supply.