| Summary: | Supplementation with iron oxide nanoparticles has been suggested as a potential method for improving energy generation through anaerobic digestion, specifically by enhancing the rate of methane production. This investigation examined the effects of iron oxide (Fe<sub>3</sub>O<sub>4</sub>) nanoparticles (NPs) on anaerobic co-digestion of cow manure (CM) and sewage sludge (SS) through batch testing conducted under mesophilic conditions (35 °C) using a RESPIROMETRIC Sensor System 6 Maxi—BMP (RSS-BMP). The use of Fe<sub>3</sub>O<sub>4</sub> nanoparticles at doses of 40, 80, 120, and 160 mg/L (batches M1, M2, M3, and M5) was studied. The use of 160 mg/L Fe<sub>3</sub>O<sub>4</sub> nanoparticles in combination with mixtures of different ratios (M4, M5, and M6) was further investigated. The findings indicate that the addition of Fe<sub>3</sub>O<sub>4</sub> nanoparticles at a concentration of 40 mg/L to anaerobic batches did not significantly impact the hydrolysis process and subsequent methane production. Exposing the samples to Fe<sub>3</sub>O<sub>4</sub> NPs at concentrations of 80, 120, and 160 mg/L resulted in a similar positive effect, as evidenced by hydrolysis percentages of approximately 94%, compared to 60% for the control (C2). Furthermore, methane production also increased. The use of Fe<sub>3</sub>O<sub>4</sub> nanoparticles at a concentration of 160 mg/L resulted in biodegradability of 97.3%, compared to 51.4% for the control incubation (C2). Moreover, the findings demonstrate that supplementing anaerobic batches with 160 mg/L Fe<sub>3</sub>O<sub>4</sub> NPs at varying mixture ratios (M4, M5, and M6) had a significant impact on both hydrolysis and methane production. Specifically, hydrolysis percentages of 94.24, 98.74, and 96.78% were achieved for M4, M5, and M6, respectively, whereas the percentages for the control incubation (C1, C2, and C3) were only 56.78, 60.21, and 58.74%. Additionally, the use of 160 mg/L Fe<sub>3</sub>O<sub>4</sub> NPs in mixtures M4, M5, and M6 resulted in biodegradability percentages of 78.4, 97.3, and 88.3%, respectively. In contrast, for the control incubation (C1, C2, and C3) biodegradability was only 44.24, 51.4, and 49.1%.
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