Revalorization of organic waste through psychrophilic anaerobic co-digestion: Insights from fluorescence spectroscopy and kinetic evaluation

The dissolved organic matter of digestate during the psychrophilic anaerobic co-digestion of sewage sludge and food waste was analyzed through fluorescence spectroscopy. Batch-fed digesters were operated at 20.2 ± 2.0 °C for 275 days, at 40-120 days of hydraulic retention time. Fluorescence 3D excitation-emission matrices revealed key transformations of protein-like and humic-like substances in substrates and soluble microbial products. Kinetic modeling (Grau second-order and modified Stover-Kincannon) yielded a substrate removal rate k_S = 0.20 d^-1, and maximum utilization rate U_max = 5.22 g-VS L^-1 d^-1. Despite lower kinetics than mesophilic digestion, the process achieved >60 % volatile solids removal and a SMY of 0.41 L-CH₄ g-VS_rem^-1, demonstrating its potential for cost-effective operation in cold climates. Additionally, the nutrient-rich digestate presents opportunities for valorization as a biofertilizer, contributing to organic waste revalorization and circular economy goals. This study underscores the value of fluorescence spectroscopy for understanding biochemical pathways and optimizing anaerobic digestion performance.