From by-products to resources: Bioconversion of agricultural digestate, carbon dioxide and syngas into value-added microbial protein

This is the authors' abstract. We don't add key points for this paper.

Anaerobic digestion (AD) for biomethane production generates nutrient-rich-digestate, carbon dioxide (CO 2 ), and carbon-rich off-gases from thermochemical conversion of solid digestate. Their integrated bioconversion could valorise AD-derived nutrients and carbon while producing biomass with potential feed applications. However, residual microbial contaminants limit the direct use of AD by-products for protein production, a challenge potentially overcome by combining thermophilic cultivation, digestate pretreatment and gaseous substrates. This study evaluated microbial protein production using the thermophile Hydrogenibacillus schlegelii , the mesophile Cupriavidus necator and an anaerobic consortium. Pre-treated agricultural liquid-digestate supplied nutrients, while two synthetic-gas mixtures provided carbon and energy: (i) green hydrogen (H 2 ) combined with CO 2 from AD-biogas upgrading, and (ii) syngas representing solid digestate gasification, containing H 2 /CO 2 , and carbon monoxide (CO). Initial screening in synthetic media compared growth kinetics and biomass protein contents under H 2 /CO 2 and syngas conditions, identifying the two pure cultures as the most suitable candidates for digestate-based cultivation. During subsequent semi-continuous cultivation with pre-treated digestate, both strains maintained stable growth, with maximum protein contents of 75 ± 3% w /w for C. necator grown on H 2 /CO 2 ; and 72 ± 6% for H. schlegelii grown on syngas, with amino acid profiles comparable to soybean meal and fishmeal. Digestate pre-treatment reduced solids and microbial load, while Hydrogenibacillus and Cupriavidus accounted for 100% and 94% of bacterial reads in thermophilic and mesophilic cultures. This work demonstrates that new AD-upcycling routes based on thermophilic syngas fermentation could enable the conversion of AD-derived nutrients and gases into microbial protein for feed applications, promoting AD plant circularity.