Mitigating ammonia inhibition in in situ biomethanation using anaerobic moving bed biofilm reactor

In situ biomethanation is an efficient technology that converts carbon dioxide (CO2) in biogas into methane (CH4) using hydrogen (H2). However, high ammonia concentrations can negatively impact process efficiency. This study investigates the inhibitory effect of ammonia on in situ biomethanation, microbial community dynamics and potential mitigation mechanism.Ammonia is one of the key process inhibitors in anaerobic digestion and biomethanation. Its high concentration in the liquid phase makes it a major limiting factor in in situ biomethanation. This study was conducted under thermophilic condition (51 °C) using degassed cow manure as inoculum, with H2 and biogas (CH4 and CO2) as substrate gases and NH4Cl as a model ammonia source.Initially, batch assays were conducted to quickly determine the ammonia concentration threshold for inhibition. The result showed significant inhibition of biomethanation observed above 3 g/L. To further explore this effect, a fed-batch experiment was performed using an anaerobic moving bed biofilm reactor (AnMBBR). The AnMBBR system showed a reduction in ammonia inhibition by 45% and 43% at 2.5 and 5 g/L ammonia nitrogen, respectively, highlighting the resilience of AnMBBR against ammonia inhibition.Microbial community analysis (16S rRNA sequencing) revealed that Methanothermobacter, a thermophilic hydrogenotrophic methanogen, increased up to 59% of the recovered archaeal community under ammonia stress, compared to 13% in the bioreactor with only H2/CO2 feeding. These finding demonstrated the potential of AnMBBR to mitigate ammonia inhibition in in situ biomethanation.