Anaerobic Digestion of the Liquid Fraction of Fruit and Vegetable Waste: Two-Stage versus Single-Stage Process

This study evaluated the anaerobic digestion of the liquid fraction of fruit and vegetable waste to optimize energy recovery through sequential hydrogen and methane production. Two configurations were tested: a single-stage (SS) system using an upflow anaerobic sludge blanket (UASB) reactor and a two-stage (TS) system combining an anaerobic structured bed reactor (AnStBR) with a UASB reactor. The objective was to identify the most efficient configuration under various organic loading rates (OLR). In the AnStBR, OLR ranged from 40 to 80 g COD/L_reactor·day, while in UASB reactors, the OLR range was from 1.5 to 22.0 g COD/L_reactor·day. The TS outperformed the SS in methane production, achieving 3.6 NL CH₄/L_reactor·day at an OLR of 22.0 g of COD/L_reactor·day. It also demonstrated a higher energy potential, generating a total of 1554.9 KJ/day35.7% more efficient than the SS. Additionally, the UASB-TS reactor maintained stability throughout the study, with minimal acid accumulation and an intermediate alkalinity/partial alkalinity ratio (IA:PA) consistently below 0.4, even at elevated OLR. In contrast, the UASB-SS reactor showed signs of acidification at higher OLR. These findings suggest that TS anaerobic digestion offers a more robust and efficient solution for treating high-strength organic waste, enhancing both energy recovery and process stability.