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.