In-situ biogas upgrading during anaerobic digestion of food waste amended with walnut shell biochar at bench scale

Abstract

A modified version of an in-situ CO₂ removal process was applied during anaerobic digestion of food waste with two types of walnut shell biochar at bench scale under batch operating mode. Compared with the coarse walnut shell biochar, the fine walnut shell biochar has a higher ash content (43 vs. 36 wt%) and higher concentrations of calcium (31 vs. 19 wt% of ash), magnesium (8.4 vs. 5.6 wt% of ash) and sodium (23.4 vs. 0.3 wt% of ash), but a lower potassium concentration (0.2 vs. 40% wt% of ash). The 0.96–3.83 g biochar (g VS_added)^-1 fine walnut shell biochar amended digesters produced biogas with 77.5%–98.1% CH₄ content by removing 40%–96% of the CO₂ compared with the control digesters at mesophilic and thermophilic temperature conditions. In a direct comparison at 1.83 g biochar (g VS_added)^-1, the fine walnut shell biochar amended digesters (85.7% CH₄ content and 61% CO₂ removal) outperformed the coarse walnut shell biochar amended digesters (78.9% CH₄ content and 51% CO₂ removal). Biochar addition also increased alkalinity as CaCO₃ from 2800 mg L^-1 in the control digesters to 4800–6800 mg L^-1, providing process stability for food waste anaerobic digestion.