In-situ methane enrichment in continuous anaerobic digestion of pig slurry by zero-valent iron nanoparticles addition under mesophilic and thermophilic conditions

The effect of zero-valent iron nanoparticles (nZVI) addition on methane production during anaerobic digestion of pig slurry was assessed. Experiments were conducted using two experimental set-ups: batch and long-term continuous operation at a fixed nZVI dosage. Two different temperature operation ranges (mesophilic and thermophilic) were assessed. Biogas production and methane content were monitored, and the specific methanogenic activity of the biomass and nZVI oxidation state were evaluated at different times. The results of batch experiments at mesophilic temperature operation showed an inhibition of methane production at all tested dosages (42, 84, 168 and 254 mgnZVI g−1 VSS concentrations), while methane production was boosted with the lowest dosage in thermophilic temperature operation. In continuous operation, nZVI addition produced an increase in methane content of biogas, achieving values between 80 and 85% in both temperature ranges. The average methane production rate increased 165% and 94% with respect to the control in thermophilic and mesophilic temperature range, respectively. The oxidation state of nZVI showed a value of +3 almost immediately after contact with substrate and a slower progressive oxidation during the reactors operation. The obtained results indicate that nZVI addition in anaerobic digestion is an interesting strategy for in situ biogas upgrading.