Fruit and vegetable wastes as co-substrates in anaerobic co-digestion: Effect of storage temperature on physicochemical properties and biogas production

Global population growth has led to a significant increase in food waste, including Fruit and Vegetable Waste (FVW). Anaerobic co-digestion offers a sustainable way to valorise FVW, especially when combined with Municipal Sewage Sludge (MSS) to mitigate imbalances in their mono-digestion. This study investigates the effects of storage temperatures (10 °C and 25 °C, which represent Mediterranean climates with an Atlantic influence like Portugal) on the degradation of apple, carrot, and banana peels. Changes in physicochemical properties were assessed and anaerobic co-digestion batch assays with purées of fresh and stored FVW alongside MSS were performed. Results indicated that apple peels purées, at a 1:2 peel-to-water ratio, achieved over the double of volatile solids concentration compared to MSS, with each FVW type having a C/N ratio above 40. Storage at 10 °C significantly reduced the degradation of total and volatile solids, as well as chemical oxygen demand, with apple peels retaining the highest carbohydrate concentrations. Anaerobic co-digestion with fresh FVW boosted biogas and CH₄ production by 19.5% and 15.2%, respectively. FVW storage at 10 °C further enhanced CH₄ yield and decreased H₂S content by 71% relative to MSS mono-digestion. These findings demonstrate that mild storage temperatures can improve biogas quality and yield by facilitating controlled FVW degradation.