Integrated microalgal process for biogas upgrading, digestate bioremediation and astaxanthin production

The integration of photosynthetic biogas upgrading, anaerobic digestate bioremediation and astaxanthin biosynthesis by microalgae entails environmental and economic benefits. However, the toxicity of the H 2 S present in biogas and of the NH 3 in digestate challenges microalgal growth and the biosynthesis of added-value compounds. This study evaluated the combined effects of headspace composition and digestate dilution on the performance of Coelastrella sp. during biogas upgrading coupled to nutrient removal from digestate. In Test Series I, cultivation under biogas with different H 2 S concentrations (0, 600, and 5000 ppm v ) with digestate pre-treatments (untreated, pH-adjusted, and activated sludge-bioaugmented) showed that moderate H 2 S levels (600 ppm v ) supported high biomass growth (maximum concentration of 6.5 ± 0.1 g TSS L −1 ) and efficient CO 2 and nutrient removal. High H 2 S levels (5000 ppm v ) inhibited culture growth, although the inhibitory effect was treatment-dependent and was substantially mitigated by activated sludge bioaugmentation. In Test Series II, the influence of digestate dilutions (10, 50, and 100%) was investigated under 600 ppm v H 2 S. Diluted digestate (10-50%) enhanced Coelastrella sp. growth, CO 2 capture, and nitrogen assimilation compared to synthetic digestate and undiluted real digestate. While operation with 50% digestate mediated the highest biomass concentrations, 10-fold diluted digestate provided the most balanced performance, including total nitrogen and phosphate removal. A two-stage cultivation strategy was applied to foster carotenoid production under high-light stress. Lutein and β-carotene decreased after high light exposure, while astaxanthin showed a treatment-dependent response, with the highest accumulation (0.39 ± 0.07 mg g −1 ) under stress conditions, particularly at high digestate concentration. Optimization of digestate dilution and cultivation strategy is essential to balance process performance and metabolite production.