Mitigating ammonia inhibition in <i>in situ</i> biomethanation using anaerobic moving bed biofilm reactor
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<i>In situ</i> biomethanation is an efficient technology that converts carbon dioxide (CO<sub>2</sub>) in biogas into methane (CH<sub>4</sub>) using hydrogen (H<sub>2</sub>). However, high ammonia concentrations can negatively impact process efficiency. This study investigates the inhibitory effect of ammonia on <i>in situ </i>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 <i>in situ </i>biomethanation. This study was conducted under thermophilic condition (51 <sup>°</sup>C) using degassed cow manure as inoculum, with H<sub>2</sub> and biogas (CH<sub>4</sub> and CO<sub>2</sub>) as substrate gases and NH<sub>4</sub>Cl 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 <i>Methanothermobacter, </i>a thermophilic hydrogenotrophic methanogen, increased up to 59% of the recovered archaeal community under ammonia stress, compared to 13% in the bioreactor with only H<sub>2</sub>/CO<sub>2</sub> feeding. These finding demonstrated the potential of AnMBBR to mitigate ammonia inhibition in <i>in situ</i> biomethanation.
