H₂S-resilient biogas upgrading and ammonium removal by mixed-culture microalgae-bacteria consortium without pH...

Anaerobic digestion (AD) is a well-established technology that simultaneously stabilizes organic pollutants and produces renewable biogas. Biogas upgrading enhances its fuel quality by removing gaseous impurities, enabling broader and higher-value applications across various sectors. This study investigated an environmentally enriched microalgae-bacteria consortium for photosynthetic biogas upgrading using real AD effluent as nutrient source, with a focus on microbial structural and functional responses to a wide range of H₂S concentrations (0-10,000 ppmv in biogas). The experimental cultures were operated in a semi-continuous, mixed-culture system without pH control to reflect the practical constraints of realistic operational conditions. At H₂S levels up to 5,000 ppmv, the cultures maintained efficient biogas upgrading with a CO₂ removal rate of 4.75 mmol/L·d, over 10 % higher than the H₂S-free control (4.28 mmol/L·d), and comparable ammonium removal (>65 %) even under acidified conditions (pH < 4) resulting from ammonium assimilation. At 10,000 ppmv H₂S, both CO₂ fixation and ammonium removal were significantly suppressed initially but recovered substantially, though not fully, following an adaptation lag. Microbial community profiling revealed the overwhelming dominance of stress-tolerant Parachlorella in the microalgal communities (>94 % of 18S rRNA gene reads) across all conditions, which likely accounts for the observed resilience. Although H₂S had a significant influence on the bacterial community structure, the persistent prevalence of sulfide-oxidizing Fuscovulum and microalgae-associated Mucilaginibacter, Paraburkholderia, and Sphingomonas also likely supported the ecological adaptability and functional resilience of the cultures. These findings offer advanced insights into the potential of environmental microalgae-bacteria consortia for sustainable biogas upgrading and digestate valorization.