Comparative analysis of energy recovery and digestate quality from anaerobic co-digestion of fruit and vegetable wastes under ambient and mesophilic conditions

Abstract

Large quantities of fruits and vegetables are lost along the supply chain due to their perishable nature, with factors such as inadequate refrigerated transport, poor road conditions, and limited cold storage facilities contributing to this loss. Anaerobic digestion offers a promising solution for mitigating the negative impacts of fruits and vegetable wastes (FVW). This study investigated the anaerobic co-digestion of FVW under ambient and mesophilic conditions, focusing on biogas yield, digestate quality, and energy efficiency. A 50:50 mixture of FVW and cow dung at mesophilic temperatures resulted in the highest biogas yield of 1,328.4 mLg−1VS, with a specific methane yield of 258.4 mL CH4 g− 1 VS. The digestate from this mixture contained 64.56, 15.34, and 20.32 gkg− 1 of nitrogen (N), phosphorus (P), and potassium (K), respectively. The heavy metal concentrations of copper (Cu), zinc (Zn), lead (Pb), iron (Fe), and manganese (Mn) were 0.02, 0.51, non-detectable, 88.60, and 3.67 mgkg− 1, respectively, all falling within the acceptable. limits for digestate/compost quality according to national and international standards. However, additional regulated metals such as As, Cr, Hg, Se, and Cd were not analyzed in this study. Therefore, compliance with international biofertilizer standards should be limited to the analyzed heavy metals. Although the average H2S levels remained below 22 ppm, routine monitoring is advisable, as elevated levels can compromise biogas quality, necessitating additional costs for biogas purification. From an energy recovery perspective, integrating insulation and correctly sizing the tank was found to improve both energy yield and operational efficiency, thereby enhancing the overall sustainability of the anaerobic digestion system. Further research should focus on temperature adaptability experiments in different climate zones, co-digestion with other organic wastes, long-term operational stability testing at the pilot scale, heavy metals and microbial analysis of the solid digestate.