**Background:** Anaerobic digestion (AD) is a widely used technology for treating sewage sludge, but its performance is often limited by low organic matter degradation rates and slow hydrolysis. Biochar additives can enhance AD by facilitating direct interspecies electron transfer (DIET), buffering pH, and promoting microbial growth. Magnetic biochar (MBC), produced by magnetizing biochar with iron compounds, may further improve electron transfer and biogas production. Camellia oleifera shell (COS) is an abundant agroforestry waste that can be used as a biochar feedstock. This study aimed to evaluate the effects of different dosages of MBC derived from COS on mesophilic AD performance, iron ion dynamics, and kinetic parameters.
**Methods:** MBC was prepared by immersing COS biomass in KOH solution, then FeCl₃ solution, followed by pyrolysis. Batch AD experiments were conducted in 3 L glass reactors at 35.0 ± 1 °C for 41 days. Five MBC dosages were tested: 5, 10, 20, 40, and 80 mg/g TS (labeled MB_1 through MB_5), plus a control without MBC. Seed sludge and dewatered sludge were collected from a municipal wastewater treatment plant in Changsha, China. Biogas volume was measured by the saturated salt water-replace method. TS, VS, sCOD, soluble protein, soluble polysaccharide, VFAs, and soluble Fe²⁺/Fe³⁺ concentrations were measured using standard analytical methods. Kinetic analysis was performed using the Modified Gompertz Model and Cone Model.
**Key Results:** MBC characterization by SEM, EDS, FTIR, and XRD confirmed successful magnetization with magnetite and Fe crystalline phases. Cumulative biogas production after 41 days was 309.20 mL g⁻¹ VSadded (CK), 353.58 (MB_1), 372.28 (MB_2), 400.16 (MB_3), 439.71 (MB_4), and 418.54 mL g⁻¹ VSadded (MB_5), representing increases of 14.35% to 42.21% over control. The highest enhancement was at 40 mg/g TS (MB_4). TS removal efficiency ranged from 26.86% (CK) to 46.13% (MB_4), VS removal from 48.99% (CK) to 63.81% (MB_4), and sCOD removal from 83.54% (CK) to 86.71% (MB_4). Soluble polysaccharide removal ranged from 77.22% (CK) to 83.43% (MB_4), and soluble protein removal from 59.12% (CK) to 70.36% (MB_4). VFA concentrations peaked around day 5 and then declined; acetic and propionic acids were the main metabolites. pH remained stable between 6.93 and 7.46 across all reactors. The Modified Gompertz Model showed higher R² values (0.9988–0.9997) than the Cone Model (0.9941–0.9971). The maximum methane production rate (Rₘ) was highest in MB_4 at 39.18 mL g⁻¹ VS d⁻¹, 21.56% higher than CK. Lag phase (λ) was shortest in MB_4 (1.02 d) compared to CK (2.01 d), a reduction of 49.25%. The hydrolysis rate constant (K) increased with MBC dosage up to 40 mg/g TS (0.1522 d⁻¹) but decreased at 80 mg/g TS (0.1227 d⁻¹). Soluble Fe³⁺ and Fe²⁺ concentrations fluctuated during AD, with Fe³⁺ decreasing during acidogenesis and increasing again around day 23, corresponding to increased biogas production.
**Clinical Implications:** This study demonstrates that MBC derived from Camellia oleifera shells can significantly enhance mesophilic anaerobic digestion of sewage sludge, increasing biogas yield by up to 42.21% and improving organic matter removal. The optimal dosage was 40 mg/g TS, while excessive addition (80 mg/g TS) caused inhibition. The mechanism involves continuous release of iron ions, promotion of DIET, and stimulation of the acetic acid methanogenic pathway. These findings support the use of MBC as a cost-effective additive for improving AD performance in wastewater treatment plants, while also providing a valorization pathway for agroforestry waste. However, the study was limited to batch experiments; long-term semi-continuous and scale-up studies are needed to confirm practical applicability.