Improper disposal of waste engine oil (WEO) poses significant environmental risks, while conventional treatment methods are often energy-intensive and unsustainable. Anaerobic co-digestion (ACoD) with sewage sludge (SS) offers a promising alternative; however, the recalcitrant nature of hydrocarbons in WEO limits its biodegradability. This study aimed to investigate the ACoD of WEO with SS and evaluate the Fenton’s reagent (viz., hydrogen peroxide and ferrous sulfate [H2O2/Fe (II)]) pretreatment on biogas production rates at a total solids (TS) concentration of 3% and an inoculum-to-substrate ratio (ISR) of 0.5. For this purpose, single-stage digesters with an effective volume of 700 mL were employed under mesophilic conditions (35°C) for 110 days. Fenton’s reagent pretreatment was applied by dissolving ferrous sulfate (FeSo4.7H2O) in H2O2 at Fenton molar ratios (FMRs) of 100, 1000, and 10,000, with FeSo4.7H2O stabilized at a concentration of 278 mg/L. The results showed that Fenton pretreatment at FMRs of 100 and 1000 increased biogas production rates by 55.4% and 37.8%, respectively, compared to the control sample (0.120 m3/kg-VSadded), which involved co-digestion without chemical pretreatment. Moreover, pretreatment at FMR = 100 reduced TS and volatile solids (VS) percentages by 47% and 59%, respectively, relative to the control sample. Soluble chemical oxygen demand (COD) and biochemical oxygen demand (BOD) were also reduced by 17.7% and 32.3%, respectively. These findings highlight the effectiveness of Fenton’s reagent in enhancing biodegradability and biogas production. Kinetic modeling of ACoD demonstrated that the Logistic Function (LF) and Modified Gompertz (MG) models provided excellent fits to the experimental data, whereas the First Order model failed to accurately predict the biogas production rate.

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