Methane enrichment in psychrophilic anaerobic digestion of pig slurry by using iron zero-valent nanoparticles

Anaerobic digestion at ambient temperature is an alternative for low-cost anaerobic digesters to be implemented at rural areas, although lower methane production rates are expected compared to mesophilic or thermophilic temperature operations. Thus, strategies to enhance methane production at psychrophilic temperature are needed. The aim of this study is to examine the effects of nano zero-valent iron (nZVI) dosage onto anaerobic digestion of pig slurry in psychrophilic temperature operation in lab-scale batch and continuous assays. First, different doses of nZVI (0, 42, 84, and 168 mg gVS-1) were tested in batch assays, establishing that the dose of 84 mg gVS-1 achieved the highest enhancement in methane production compared to the control (71%). The 84 mg gVS-1 dose of nZVI was tested in a 6 L anaerobic digester operated in continuous mode for 320 days, in 5 different phases with different nZVI addition strategies. Methane content in biogas achieved a maximum of 81% when adding a weekly pulse of nZVI of 84 mg gVS-1, compared to 69% with no nZVI addition. Although biogas production decreased 24% when nZVI were added to the reactor, it still can be a valuable method to upgrade biogas in psychrophilic anaerobic digestion.