Exploring a microalgal consortium-based approach for the remediation of an agrifood-derived digestate

This study investigates the potential of employing a microalgal consortium to remediate agrifood-derived digestate while simultaneously producing valuable algal biomass. Digestate, a nutrient-rich by-product of anaerobic digestion, poses environmental risks if improperly managed, yet it can serve as an economical nutrient source for microalgal cultivation. The research explored the functioning of a consortium composed of three chlorophytes, Tetradesmus obliquus, Chlamydomonas reinhardtii, and Auxenochlorella protothecoides, through a three-step approach. Initially, a screening step identified a 7% digestate dilution as the lowest concentration that did not inhibit algal growth, enabling physiological adjustments that allowed the cells to acclimate to moderate stress. In the subsequent optimisation step, acclimated cells were tested as free-living cultures or immobilised in alginate beads, with or without the addition of the plant growth-promoting bacterium Azospirillum brasilense. Immobilisation was found to accelerate the onset of exponential growth, although its benefits varied with the media, while the bacterium's effect was more pronounced on maximum cell density than on growth rate. The final remediation step evaluated the consortium's performance in reducing key pollutants under batch cultivation conditions simulating parameters relevant for scale-up. Results demonstrated efficient removal of nitrogen and phosphorus, and a complete removal of heavy metals such as cadmium (Cd), chromium (Cr), and lead (Pb) when algae were immobilised. This study demonstrates that, despite the considerable potential for further optimisation, digestate can be effectively valorised as a nutrient source for selected microalgal consortia. When properly applied, this consortium based remediating systems not only support sustainable digestate management but also enable the production of biomass with promising applications.