Investigating the Potential Role of Biochar Amended Anaerobic Co-digestion of Agricultural Crop Residue with Cattle Dung

Abstract

Anaerobic co-digestion offers a practical route for recovering renewable energy from agricultural residues and livestock waste, but the recalcitrant structure of wheat stalk can restrict substrate degradation and methane production. This study evaluated the combined effects of substrate ratio and wheat-stalk-derived biochar dosage on the mesophilic co-digestion of cattle dung and wheat stalk. Biochar was produced by pyrolysis at 500 °C and applied at 0, 2 and 4 g/L to cattle dung–wheat stalk mixtures prepared at 50:50 and 60:40 ratios on a volatile-solids basis. Digestion was conducted at 37 ± 1 °C for 25 days using an Automatic Methane Potential Test System. Scanning electron microscopy showed that pyrolysis converted the compact fibrous surface of wheat stalk into a porous, honeycomb-like structure. The substrate mixtures had a mean pH of 7.15 ± 0.25 and a mean C/N ratio of 21.92 ± 1.47. Biochar supplementation increased cumulative gas production by 43.11–67.76% relative to the corresponding controls. Treatment T2, comprising the 50:50 substrate ratio and 2 g/L biochar, produced the highest peak daily gas volume (705.10 NmL), cumulative gas volume (4167.70 NmL), biochemical methane potential (371.8 mL CH₄ g⁻¹ VS added) and volatile-solids reduction (67.81%). More than 80% of the cumulative gas was recovered within the first 20 days. Under the tested conditions, moderate biochar supplementation improved digestion performance, with 2 g/L biochar at the 50:50 ratio providing the most favourable response.