Characteristics of the digestates from OFMSW methane production at psychrophilic, mesophilic, and thermophilic conditions under different organic loading rates

Abstract

ABSTRACT The anaerobic digestion of the organic fraction of municipal solid waste (OFMSW) has shown to be a viable alternative since it allows energy recovery in the form of methane and generates a residue (digestate) that can be applied effectively as a soil improver or fertiliser. The potential for methane production and the digestates’ characteristics depend on the substrate characteristics and the process variables such as temperature, solids retention time, and organic load. This study dealt with OFMSW anaerobic digestion under different organic loading rates and temperatures and the characteristics of the resulting digestates. Three semi-continuous reactors were operated at 20, 35, and 55°C and fed daily with ground, fresh OFMSW from Mexico City. The inoculum was temperature-adapted UASB granular sludge. The main results indicate that the anaerobic digestion was adequate, as the pH values were slightly alkaline, which is sufficient for methanization, and the alkalinity was not a limiting factor. Potassium and PO4-P increased with the organic load, and Kjeldahl nitrogen decreased. At 20°C, total organic carbon (TOC) increased substantially with the organic load; at 35°C, it remained without significant changes; and at 55°C, TOC slightly decreased with the organic load. The C/N ratio changed accordingly to TOC variations. At 20°C, the residual biogas potential increased with the organic load; at 35 and 55°C, it decreased with increasing organic load; the residual biogas potential increased with residual fatty acids concentrations. To comply with international standards for agricultural use, the digestates need only dewatering and supplementing with PO4-P.