Long-Term Anaerobic Digestion of Seasonal Fruit and Vegetable Waste Using a Leach-Bed Reactor Coupled to an Upflow Anaerobic Sludge Bed Reactor

Fruit and vegetable waste (FVW) generated locally in open (public or wholesale) markets is a valuable resource and should not be considered as waste. The anaerobic digestion (AD) of FVW can minimize landfill disposal and generate renewable energy, thus decreasing greenhouse gas emissions. Moreover, the digestate after the AD of FVW, devoid of antibiotics and animal fats in manure and food waste, may have a high fertilizing value. In this study, FVW mixtures were composed to mimic the real FVW generated in Mediterranean open markets annually. The first goal was to evaluate the biochemical methane potential (BMP) of different size fractions resulting from FVW grinding. Indeed, the FVW was ground and separated into two size fractions, 0-4 mm and 4-10 mm, respectively. The 0-4 mm fraction exhibited a lower BMP but a higher rate constant than the 4-10 mm fraction. The second goal was to first evaluate the BMP of the lumped fraction of FVW after grinding (0-10 mm) via BMP assays and then feed it to a mesophilic two-stage leaching-bed reactor (LBR)-upflow anaerobic sludge bed (UASB) system for almost one year. The BMP of the FVW ranged between 406 and 429 L kg-1 of volatile solids (VS) independently of the FVW production season. The system received an average organic loading rate (OLR) of 3.1 +/- 0.7 g VS L-1 d-1. During operation, the LBR gradually transited from acidogenic to methanogenic, and the overall methane yield of the system increased from 265-278 to 360-375 L kg-1 VS, respectively. The proposed technology does not require water addition or liquid digestate removal. Compared to the continuous stirred tank reactor (CSTR) digester technology, the LBR/UASB system is suitable for the anaerobic digestion of FVW. The results of this study can be further used to upscale the proposed technology and contribute to the societal need for affordable and clean energy included in the Sustainable Development Goals (SDGs).