Integrated bioprocess for single-cell protein production: Converting waste-derived biogas into sustainable feed protein

The increasing demand for sustainable protein sources and carbon–neutral bioprocesses necessitates innovative strategies for carbon utilization. This study presents an integrated bioprocess that couples anaerobic digestion (AD) and methanotrophic fermentation to convert waste-derived methane (CH₄) and CO₂ into single cell protein (SCP). In the first phase, co-digestion of vegetable waste (VW) and sewage sludge (SS) was optimized, with the 75:25 VW:SS ratio yielding the highest biogas yield (20.68 ± 2.46 mL/gVS; cumulative 258 mL/gVS) and producing an ammonium-rich digestate (422 ± 14.1 mg/L NH₄⁺-N). The second phase focused on methanotrophic SCP production, where a co-culture of Methylosinus sporium and Methylophilus methylotrophus demonstrated the highest biomass yield (880 ± 51 mg/L) with a 51 % protein content, further increasing to 976 ± 48 mg/L under an optimized CH₄ to air ratio of 40:60. Replacing CH₄ with raw biogas and synthetic mineral salt with digestate reduced biomass yield (610 ± 61 mg/L), highlighting the need for biogas purification and provision of essential trace elements in digestate for optimal CH₄ oxidation. Semi-continuous fermentation over 60 days achieved steady-state SCP production (965 mg/L) with an amino acid profile rich in essential nutrients, underscoring its potential as an alternative feed protein. This study establishes a waste to protein biorefinery concept, leveraging biogas as a circular carbon source, contributing to greenhouse gas reduction and climate change mitigation and offers a scalable approach for sustainable SCP production. Future optimizations in biogas purification and nutrient supplementation could further enhance yields, advancing the role of biological carbon utilization in climate-resilient food systems.