Performance and Stability of Anaerobic Co-Digestion of Food Waste Filtrate and Landfill Leachate at Different Mixing Ratios

Food waste filtrate (FW) and landfill leachate (LL) are high-strength organic wastewaters with complex compositions that pose significant challenges for conventional biological treatment. Anaerobic co-digestion is considered an effective strategy to improve process stability and methane recovery through substrate complementarity. In this study, an internal circulation (IC) anaerobic reactor was used to evaluate the co-digestion performance of FW and LL at different volumetric mixing ratios (3:7, 5:5, and 7:3). Methane production, COD removal, pH, volatile fatty acids (VFA), alkalinity, extracellular polymeric substances (EPS), enzyme activities, sludge morphology, and sludge structural and spectroscopic characteristics were analyzed to evaluate process performance and explore stability-related responses under different mixing ratios. The results showed that the 5:5 mixing ratio achieved the best overall performance. Under this condition, methane content remained at 78.79–81.60%, the volumetric methane production rate reached 893.38–1080.43 L CH 4 /(m 3 ·d), and methane yield was 0.219–0.265 L CH 4 /g COD. COD removal efficiency was maintained at 86.93–88.35%. Meanwhile, the reactor operated within a relatively stable window, with pH of 6.98–7.80, VFA of 485.6–521.6 mg/L, alkalinity of 2000–3100 mg CaCO 3 /L, and a VFA/TA ratio of 0.167–0.261. Compared with the other ratios, the 5:5 condition was associated with higher EPS levels, more favorable enzyme activity patterns, and a more compact sludge structure. Overall, FW-LL co-digestion exhibited clear ratio dependence, and the 5:5 mixing ratio provided the best balance between methane production, organic matter removal, and process stability. These findings offer quantitative support for substrate-ratio optimization and stable operation of anaerobic treatment systems for high-strength organic wastewaters.