Simulation-based optimization of liquid digestate recirculation and urea addition in co-digestion of wheat straw with cattle manure

Anaerobic co-digestion of wheat straw and cattle manure offers a route to valorize agricultural residues, but the management of the resulting digestate remains a challenge. This study investigates the combined influence of liquid digestate recirculation (LDR) and urea addition on process performance and profitability through a simulation-based approach using a simplified ADM1 framework implemented in SuperPro Designer. Response surface methodology was applied to evaluate the effects of urea dosage (50–300 t a −1 ) and recirculation ratio (0–100 %) on methane yield, fresh water demand, and gross profit for a biogas plant with an electrical capacity of 2.5 MW. The model predicted a maximum methane yield of 5500 t a −1 (0.316 Nm 3 kg −1 VS_Input; VS = volatile solids) at 100 % LDR and 251 t a −1 urea addition, and an optimal gross-profit increase of 124 % (2.4 million € a −1 ) at 239 t a −1 urea dosage. Both cases achieved up to 95 % reduction in fresh water demand, demonstrating the potential of nutrient recycling within the biogas process and controlled nitrogen supplementation to enhance efficiency and profitability in agricultural biogas systems. The model captures the key stoichiometric conversions and nutrient-interaction effects governing methane formation, but represents steady-state conditions without dynamic phenomena such as nitrogen accumulation or microbial adaptation. Further experimental and modeling studies under different substrates and process conditions are needed to validate these mechanisms and extend the applicability of LDR optimization to a wider range of biogas processes.