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

Authors

Abstract

A modified version of an in-situ CO2 removal process was applied during AD of food waste with two types of walnut shell biochar (WSB) at bench-scale under batch operating mode. Compared to the coarse WSB, the fine WSB 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 VSadded)-1 fine WSB amended digesters produced biogas with 77.5-98.1% CH4 content by removing 40-96% of the CO2 compared to the control digesters at mesophilic and thermophilic temperature conditions. In a direct comparison at 1.83 g biochar (g VSadded)-1, the fine WSB amended digesters (85.7% CH4 content and 61% CO2 removal) outperformed the coarse WSB amended digesters (78.9% CH4 content and 51% CO2 removal). Biochar addition also increased alkalinity as CaCO3 from 2,800 mg L-1 in the control digesters to 4,800-6,800 mg L-1 providing process stability for food waste AD.