Nanoparticle-Induced Inhibition of Anaerobic Digestion and Its Alleviation

The increasing global application of nanoparticles (NPs) inevitably results in the release and accumulation of NPs in environmental media such as wastewater and waste sludge. To date, diverse NPs including zero valent metallic NPs, metal oxide NPs, carbonaceous nanomaterials, and multi-compound NPs have exhibited inhibitory effects on anaerobic digestion, which mainly depend on their elementary composition, size, concentration, and surface properties. Once introduced into anaerobic digesters, NPs can be absorbed onto sludge and change sludge morphology. Subsequently, NPs can inhibit the activities of key enzymes involved in hydrolysis, acidogenesis, and methanogenesis, thereby affecting the efficiencies of these critical steps of anaerobic digestion. In addition, NPs exposure can shift the microbial communities, reducing the abundances of key methanogens. In general, the toxicity of NPs to anaerobic microorganisms is attributed to the production of reactive oxygen species and the release of toxic substances. Hence, strategies targeting these two aspects such as the addition of other NPs, biochar, cationic polyacrylamide, and sulfide can efficiently recover methane yields of anaerobic digesters exposed to NPs.