The Influence of ZnO Nanoparticles on Producing Bioenergy in Municipal Wastewater Treatment Plants

Zinc Oxide Nanoparticles (ZnO NPs) have a significant role in sustainable growth in our society due to their unique applications in medicine, cosmetics, agriculture, and commercial products. These NPs are released into wastewater treatment plants (WWTPs) and accumulate in activated sludge. Sludge containing NPs enters anaerobic digestion processes and can affect inoculum ability to produce energy from the waste stream. Anaerobic digestion is widely used as a promising solution to not only provide required energy in WWTPs but also achieve zero-waste policy. So, it is crucial to understand the influences of ZnO NPs on anaerobic digestion procedures. In this study, we designed an anaerobic digestion process and measured ZnO NPs impacts on methane production during the anaerobic digestion process. The substrate was sludge from primary and secondary clarifiers of Jackson Pike WWTP in Columbus, Ohio. The inoculum was also collected from anaerobic digesters in Jackson Pike. ZnO NPs were introduced to the designed anaerobic digestion process at different concentrations (0, 1, 10, 20, 40, 60, 80, and 100 mg/g TSS) in triplicates. Generated methane was measured continuously for 30 days, and its amount was found to be dependent on the concentration of ZnO NPs and time. In the short term (24 h), ZnO NPs at the lowest concentration (1 mg NPs/g sludge) increased methane production yields while higher concentrations decreased the yields. In the long term (30 days), there was no impact observed at the lowest concentration, while higher concentrations showed adverse effects on methane production.