Trisynergy of photosynthetic biogas upgrading, anaerobic digestate bioremediation, and pigment biosynthesis

Integrating environmental biotechnologies with valuable product biosynthesis can enhance the economic feasibility of environmental protection while reducing the costs of bioactive compound production. This study presents the first proof of concept for photosynthetic biogas upgrading and nutrient removal from anaerobic digestate, coupled with pigment biosynthesis in algae and cyanobacteria cultures. Pre-cultivation with aerobic activated sludge mitigated the toxicity of NH₄⁺ from the liquid fraction of anaerobic digestate and H₂S from biogas, protecting pigment-producing strains. Partial pre-nitrification of NH₄⁺ followed by complete H₂S pre-oxidation with aerobic activated sludge supported the growth of Chlorella sp. and Coelastrella sp., while Arthrospira platensis required complete pre-nitrification to grow with both clean and H₂S-laden biogases. In the liquid fraction of anaerobic digestate, sulfur supplementation via H₂S-laden biogas stimulated the propagation of all cultures. Under these conditions, the complete removal of H₂S by aerobic activated sludge was recorded, while final CO₂ levels of 6.0 %, 2.4 %, and 0.07 % were observed in Chlorella sp., Coelastrella sp., and A. platensis tests, correspondingly. Chlorella sp. and Coelastrella sp., respectively, removed 79 and 70 % nitrogen and 64 and 65 % of phosphorus under clean biogas, while A. platensis achieved 82 % nitrogen and 73 % phosphorus removals under pre-desulfurized H₂S-laden biogas. These findings support the development of efficient cultivation strategies for biogas upgrading and the remediation of the liquid fraction of anaerobic digestate using pigment-producing strains.