Case-specific modelling and experimental validation of a two-stage anaerobic digestion system for food waste using a modified ADM1 framework

This study presents a case-specific adaptation and experimental validation of a modified Anaerobic Digestion Model No. 1 (ADM1) for a laboratory-scale two-stage anaerobic digestion system treating food waste. The system separates acidogenic and methanogenic phases to improve process understanding under controlled conditions. Comprehensive substrate characterization, including total solids (TS), volatile solids (VS), chemical oxygen demand (COD), and biochemical composition, was incorporated into the model framework to ensure realistic simulation inputs. A dynamically coupled ADM1 model was developed in MATLAB/Simulink and calibrated using experimental data obtained from a custom-designed two-stage reactor. Model performance was evaluated using Root Mean Square Error (RMSE), R2, and Normalized Mean Bias Error (NMBE), demonstrating good agreement within the calibration domain (R2 ≈ 0.97). The effects of key operational parameters, organic loading rate (OLR), hydraulic retention time (HRT), temperature, and reactor volume ratio were analysed. Results indicate that stable operation occurs within a moderate OLR range, while excessive loading leads to volatile fatty acid accumulation and reduced methane productivity. The optimal conditions identified (OLR ≈ 4–4.5 kg VS/m3·day, HRT ≈ 15–20 days) are specific to the studied system and should not be generalized without further validation. The study highlights limitations related to model simplifications and short validation periods. The findings contribute to improved understanding of two-stage digestion behaviour and provide a basis for future model refinement, extended validation, and scale-up studies.