Effect of Fe3O4 nanoparticles on anaerobic digestion of municipal wastewater sludge

Anaerobic digestion of waste activated sludge often achieves less than 50% destruction of volatile suspended solids and stabilization of chemical oxygen demand to methane gas. Although the impact of Fe3O4 nanoparticles on anaerobic digestion performance was investigated in past studies, this study focused directly on the impacts of the size and concentration of the Fe3O4 nanoparticles. Fe3O4 nanoparticles with two different ranges -- (12–18)-nm and (50–100)-nm -- and different concentrations were evaluated for their impact on anaerobic digestion of combined wastewater sludges (waste activated sludge + primary sludge). The volume of methane produced and the concentrations of volatile suspended solids, total chemical oxygen demand, and Fe2+ were measured during Biochemical Methane Potential (BMP) experiments. Adding Fe3O4 nanoparticles enhanced solids digestion and methanogenesis, and the impact was greater for methanogenesis. Adding 120 mg/L of the smaller nanoparticles had the greatest impact: a 1.7-fold increase in the methane yield, 26% increase in volatile suspended solids destruction, and 35% increase in total chemical oxygen demand removal, compared with the control. The smaller nanoparticles, which had higher impacts on anaerobic-digestion performance than the larger nanoparticles, led to greater release of soluble Fe2+, which may have decreased sulfide inhibition of methanogenesis, increased the rate of solids hydrolysis, or increased inter-species electron transport. A modified Gompertz model quantified how adding 120 mg/L of the (12–18)-nm nanoparticles increased the rate and extent of methanogenesis. [Display omitted] •Adding Fe3O4 nanoparticles ((12–18)-nm or (50–100)-nm) improved anaerobic digestion.•Smaller nanoparticles led to a bigger increase in methane production than larger nanoparticles.•120 mg/L of the smaller nanoparticles had the strongest positive impact on methanogenesis.•Fe3O4 was reduced to soluble Fe2+, which may have enhanced hydrolysis and methanogenesis.