Co-Composting Liquid Digestate and Green Waste: Nitrogen Retention and Process Enhancement

The sustainable management of biogas production digestate and lignocellulosic green waste remains a major challenge in circular bioeconomy systems. This study evaluated the feasibility and effectiveness of co-composting liquid digestate with woody pruning residues as an integrated strategy for nitrogen conservation and enhanced composting performance. Experiments were conducted at both laboratory and intermediate scales using shredded residues from several tree species soaked in digestate or nitrogen solutions prior to aerobic incubation or open-pile composting. Nitrogen mass balance analysis based on laboratory incubations demonstrated that plant residues, which retained substantial amounts of liquid (173% of dry weight on average), enabled negligible nitrogen losses, even under high ammonium loading conditions. Temporal monitoring of inorganic nitrogen forms indicated an initial phase of microbial immobilization followed by gradual remineralization, but also strong nitrogen retention within the lignocellulosic matrix. Intermediate-scale composting trials confirmed the operational feasibility of the approach. Digestate-amended piles rapidly entered the thermophilic phase, reaching 58.8 °C within five days, whereas control piles treated only with water remained slightly above ambient levels. The results suggested that co-composting of digestate with green waste improves moisture conditions, enhances decomposition of recalcitrant biomass, and mitigates ammonia-related nitrogen losses. The proposed soaking-based co-composting strategy represents a promising and scalable solution for sustainable management of both digestate and urban green waste streams.