Nanosilver impact on methanogenesis and biogas production from municipal solid waste

Silver nanoparticles (AgNPs, nanosilver) released from industrial activities and consumer products may be disposed directly or indirectly in sanitary landfills. To determine the impact of AgNPs on anaerobic digestion of landfill waste, municipal solid waste (MSW) was loaded in identical landfill bioreactors (9L volume each) and exposed to AgNPs (average particle size=21nm) at the final concentrations of 0, 1, and 10mgAg/kg solids. The landfill anaerobic digestion was carried out for more than 250 days, during which time the cumulative biogas production was recorded automatically and the chemical property changes of leachates were analyzed. There were no significant differences in the cumulative biogas volume or gas production rate between the groups of control and 1mgAg/kg. However, landfill solids exposed to AgNPs at 10mg/kg resulted in the reduced biogas production, the accumulation of volatile fatty acids (including acetic acid), and the prolonged period of low leachate pH (between 5 and 6). Quantitative PCR results after day 100 indicated that the total copy numbers of 16S rRNA gene of methanogens in the groups of control and 1mgAgNPs/kg were 1.97±0.21×10(7) and 0.90±0.03×10(7), respectively. These numbers were significantly reduced to 5.79±2.83×10(5)(copies/mL) in the bioreactor treated with 10mgAgNPs/kg. The results suggest that AgNPs at the concentration of 1mg/kg solids have minimal impact on landfill anaerobic digestion, but a concentration at 10mg/kg or higher inhibit methanogenesis and biogas production from MSW.