Microbial response to varying ammonia conditions in mesophilic anaerobic digestion mediated by mixed bioaugmentation consortia

Bioaugmentation demonstrates significant potential for enhancing anaerobic digestion (AD) efficiency; however, bioaugmentation consortia composition, regulatory mechanisms under different ammonia levels inhibition and high organic loading rate (OLR) require further elucidation. This study investigated the effects of mixed consortia on ammonia-inhibited mesophilic AD of food waste (OLR 6.0 gVS L⁻¹ d⁻¹, ammonia 3700–8500 mg L⁻¹). Bioaugmented reactors produced 1.46–4.45 times methane than that of controls at 4500–8500 mg L⁻¹ ammonia concentrations, with stable process parameters (total volatile fatty acid/total alkalinity (TVFA/TA) 0.02–0.40). Control reactors exhibited progressive VFA metabolism inhibition and suppressed acetoclastic and hydrogenotrophic methanogenesis as ammonia increased, whereas bioaugmented systems resisted the 8500 mg L⁻¹ ammonia shock and sustained methane production efficiency under ammonia-acid co-inhibition. Low/medium ammonia (4500 and 6000 mg L⁻¹) inhibition was mitigated via reinforced acetoclastic methanogenesis and facultative methanogenesis by Methanosarcina, respectively. High ammonia (8000 mg L⁻¹) triggered a strategic shift toward syntrophic partnerships between VFA-oxidizing bacteria and methanogens, additionally, enriched hydrogen-dependent methylotrophs Ca. Methanomethylophilus and Ca. Methanofastidiosum help sustain low H₂ partial pressure essential for thermodynamically favorable VFA metabolism. Concurrently, enhanced Methanosarcina exhibited greater contribution to methylotrophic methanogenesis than to hydrogenotrophic/acetoclastic pathways. Methylotrophic methanogenesis proved crucial for stabilizing methanogenic performance, especially under high-ammonia stress. This study elucidates the bioaugmentation mechanisms for mitigating inhibition across varying ammonia conditions, providing a theoretical basis for practical applications of high ammonia AD.