Agricultural digestate ultrafiltrate outperforms synthetic media for microalgal biomass production while enabling biostimulant generation

Livestock digestate ultrafiltrate (UF), obtained through membrane treatment of anaerobic digestate, is a nutrient-rich stream whose valorization remains largely unexplored. By removing suspended solids while retaining dissolved nutrients, ultrafiltration reduces turbidity and overcomes major limitations associated with using digestate for microalgal cultivation. This study evaluated UF as a nutrient source for Chlorella sorokiniana and Nannochloropsis oceanica, coupling ammonium recovery and biomass production with biomass valorization as a plant biostimulant. Given the high ammonium content of UF, ammonium tolerance was assessed in both species using ammonium bicarbonate as the nitrogen source. Increasing UF concentrations were then tested in laboratory photobioreactors and compared with synthetic media. Biomass productivity, nutrient removal, biomass composition, and biostimulant activity were evaluated. Both species tolerated ammonium-rich conditions and grew efficiently in UF-based media. UF significantly outperformed synthetic media, increasing biomass accumulation by up to 104% in N. oceanica and 133% in C. sorokiniana, while ammonium removal exceeded 99% and 98%, respectively. C. sorokiniana maintained high productivity when UF was diluted exclusively with water, indicating that UF supplied most nutrients required for growth. The enhanced productivity could not be explained solely by ammonium availability, suggesting a contribution from the complex nutrient composition retained during ultrafiltration. UF-derived biomass exhibited biostimulant activity, promoting tomato root development and increasing root and shoot biomass under reduced nitrogen availability. Digestate-derived UF therefore acts as both an efficient nutrient-recovery stream and a growth-promoting microalgal medium, enabling the conversion of membrane-treated digestate into biostimulant biomass within a circular biorefinery framework.