Effect of size, coating and concentration of zinc oxide nanoparticles on anaerobic digestion of municipal wastewater sludge

The effect of average particle size (30 nm and 100 nm) and type (coated and uncoated) of ZnO nanoparticles (NPs) was studied on anaerobic digestion treatment process utilizing municipal wastewater sludge under mesophilic (35±2ºC) and thermophilic (55±2ºC) conditions. The effect was investigated in two stages with different digester feeding regime: (1) batch biochemical methane potential (BMP) assays, and (2) semi-continuously fed reactors. In the first stage with BMP assays, the inhibition of biogas (methane) producing cultures due to presence of ZnO NPs was investigated. A total number of 72, 160 mL serum bottles containing anaerobic inocula acclimatized to mixed sludge from Kelowna’s municipal wastewater treatment plant (WWTP) were operated for a period of about 90 days. Three different ZnO dosages were used leading to concentrations of 6 (low), 75 (medium) and 150 (high) mg NP/g mixed sludge total solids (TS). In the second stage, four mesophilic and four thermophilic semi-continuously fed digesters were set-up with an effective volume of 500 mL. The effect of NPs on the performance of biogas/methane production, organic solids removal, and biogas composition in terms of presence of odorous volatile sulfur compounds was assessed at the low dosing concentration. The eight digesters were operated during 60 days with a solid retention time of 20 days. The results showed that ZnO NPs do not inhibit methane forming bacteria at the lowest concentration regardless of their average particle size, but they are inhibitory at higher levels. Thermophilic bacteria were more sensitive to ZnO compared to mesophilic bacteria, as at elevated digester temperatures. Coated NPs created less inhibition than non-coated NPs, possibly due to decreased purity (Zn) of the inhibitor per mass in coated NPs. Batch reactors dosed with medium and high concentrations of coated NPs partially recovered after 25 days of digestion. For the non-coated ZnO NPs, only the mesophilic batch assays were able to recover at the medium concentration and the thermophilic reactors presented chronic inhibition and could not recover. As a beneficial outcome, coated ZnO NPs significantly reduced odour causing volatile sulfur compounds in digester headspace in comparison to the non-coated NPs by chemical precipitation.Applied Science, Faculty ofEngineering, School of (Okanagan)Graduat