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Anaerobic digestion (AD) is a biochemical process that converts waste organic matter into energy-rich biogas with methane as the main component. Addition of electric electro-conductive, such as that nanoparticles (NP), has been shown to improve biogas generation. Interspecies electron transfer and direct interspecies electron transfer (DIET) using conductive materials is one of the mechanisms responsible for observed increases in CH 4 . This article discusses the effect of the type and size of electro-conductive NPs on improving microbial degradation within AD systems, as well as the effect of electro-conductive NPs on microbial community shifts and syntrophic metabolism. Limitations and future perspectives of using NPs in an AD system is also discussed., (Copyright © 2021 Elsevier Ltd. All rights reserved.)

Graphical abstract Highlights • Metal nanoparticles (NPs) were added to anaerobic digestion (AD) of poultry litter. • Adding Ni NPs (12 mg/L) increased CH 4 production by 38.4% (368 mL CH 4 /g VS). • Fe, Co and Fe 3 O 4 NPs added individually also increased CH 4 with no effect on H 2 S. • Higher concentrations of NP mixtures greatly reduced H 2 S with no effect on CH 4. • A method for extraction of NPs from digestate for SEM detection was developed. Abstract The effects of metal nanoparticle (NP) addition during anaerobic digestion (AD) of poultry litter was tested using two sequential experiments: Exp. A) four NPs (Fe, Ni, Co, and Fe 3 O 4) at three concentrations; and Exp. B) NP combinations (Fe, Ni, and Co) at four concentrations. Scanning electronic microscopy (SEM) and elemental analysis were used to confirm NP inclusion after dispersion (before AD) and track nanoparticles post-AD, and new technique for NP extraction post-AD was developed. Before AD, NPs ranged from 30.0 to 80.9 nm for Fe, Ni, and Co, and 94.3 to 400 nm for Fe 3 O 4. Methane production increased with NPs addition compared to poultry litter-only, with the highest increases observed with NPs concentrations (in mg/L) of 12 Ni (38.4% increase), 5.4 Co (29.7% increase), 100 Fe (29.1% increase), and 15 Fe 3 O 4 (27.5% increase). Nanoparticle mixtures greatly decreased H 2 S production. The SEM post-AD detected Fe, Ni, and Fe 3 O 4 at concentrations ≥100 mg/L. [ABSTRACT FROM AUTHOR]