How acetate alleviates ammonium inhibition and enhances methane-fed single-cell protein production

Anaerobic digestion produces methane-rich biogas and ammonium-rich digestate, but their joint conversion into single-cell protein (SCP) requires coordinated carbon and nitrogen utilisation. Here, a methane-fed mixed consortium was operated in a membrane biofilm reactor to determine how acetate regulates carbon-nitrogen coordination under ammonium stress. Ammonium addition impaired nitrogen conversion and protein enrichment without eliminating methylotrophs. Acetate restored these functions, increasing protein content to 61.25 % and the apparent N-based SCP yield from 0.34 to 1.46 g SCP/g N. Nitrogen balances showed greater incorporation of inorganic nitrogen into cellular organic nitrogen with acetate, consistent with the increased metagenomic potential for the glyoxylate shunt and ammonium assimilation. Excess acetate, however, promoted soluble microbial products and extracellular polymeric substances, lowering the apparent carbon-based SCP yield. These findings suggest that acetate improves nitrogen incorporation and protein production in a methane-fed mixed consortium while methane utilisation remains active. Controlled acetate retention in anaerobic digestate may therefore support the joint recovery of waste-derived carbon and nitrogen as microbial protein.