Treating liquid anaerobic digestate using natural zeolite and Arthrospira platensis cyanobacteria: From laboratory to pilot-scale

• Zeolite treatment effectively reduced digestate toxicity, prior to cyanobacteria cultivation. • 95 % of the initial NH 4 + concentration was eliminated using 0.5 kg zeolite .L -1 . • A. platensis was successfully cultivated in zeolite-pretreated digestate without dilution. • Experimental results were scaled up from 450 mL to 15.5 L, maintaining similar performance. • A novel digestate management solution was proposed. Over the past decade, using liquid digestate as a nutrient source for microalgae cultivation has gained considerable attention. However, its high ammonium concentration and turbidity often inhibit algal growth. To address this, natural zeolite was explored as a novel approach to reduce digestate toxicity before microalgae cultivation at both laboratory and pilot-scales. Clinoptilolite, a type of natural zeolite, was applied in adsorption columns at a ratio of 0.5 kg zeolite .L -1 to treat 0.45 L of liquid digestate. After 24 h of treatment, ammonium levels decreased significantly from 2273 to 115 mgN.L -1 , corresponding to a 95 % removal efficiency and an adsorption capacity of 4.31 mg.g zeolite -1 . Arthrospira platensis demonstrated strong growth in the treated digestate with minimal dilution (≤5x), in contrast to the high dilution (≥20x) required for the untreated digestate. Laboratory-scale results were effectively scaled up to pilot scale, detoxifying 15.5 L of digestate with similar performances. The pretreated digestate was subsequently used as a culture medium for Arthrospira platensis in flat panel photobioreactors without further dilution, achieving a final concentration of 0.82 g DW .L -1 and a biomass productivity of 33 mg.L -1 .d -1 . These findings underscore the potential of natural zeolite in enhancing microalgae-based processes for digestate detoxification and CO 2 mitigation.