Utilization of green waste from vegetable market for biomethane production: influences of feedstock to inoculum ratios and alkalinity

In this study, biogas yields and methane contents of vegetable wastes from Thailand's largest agricultural market were investigated. Granular sludge from an up-flow anaerobic sludge blanket (UASB) reactor was used as inoculum. To determine the influences of the feedstock to inoculum (F/I) ratio and alkalinity, batch experiments were conducted at three different F/I ratios (i.e., 0.5, 1.0, and 2.0 g VSfeedstock/g VSinoculum), and five NaHCO3 concentrations (i.e., 0, 300, 500, 600, and 900 mg/g VSadded). The reuse of digestate from previous anaerobic reactors, as sources of alkalinity and inoculum, was also investigated. After 53 days of the experiment, without adding NaHCO3, an F/I of 0.5 gave the highest biogas yield of 851 mL/g VS and a CH4 content of 52.3%. Adding external alkalinity significantly helped to improve the biogas production and methane contents at F/I ratios of 1.0 and 2.0. The results from using the recycled digestate as inoculum were comparable with reactors using UASB sludge and helped to decrease the amount of NaHCO3 to 150 mg/g VS. The high anaerobic digestion performance in the study demonstrated the potential of utilizing the organic waste from a vegetable market as a renewable energy source.