Feasibility of Co-Composting Sewage Sludge with Vine Pruning Residues for Sustainable Waste Management

The increasing generation of sewage sludge (SS) and the accumulation of agricultural residues such as vine pruning residues (VPR) represent significant environmental challenges, including pollution risks and resource underutilization. Composting offers a sustainable strategy to transform these wastes into high-quality compost, facilitating resource recovery within a circular framework. This study aimed to evaluate the feasibility of co-composting SS with VPR as a strategy for organic waste valorization. Two mixtures were prepared using cattle slurry solid fraction (CS) as inoculum, M1 (50% SS + 30% VPR + 20% CS) and M2 (30% SS + 50% VPR + 20% CS). The results showed that both mixtures exhibited typical composting temperature profiles, with the thermophilic phase effectively eliminating Salmonella and E. coli, confirming adequate hygienization. Macronutrient concentrations were consistently higher in M1 (N: 43.44, P: 12.41, and K: 25.27 g kg−1 DW) than in M2 (N: 32.14, P: 8.57, and K: 23.66 g kg−1 DW). All potentially toxic elements remained below the regulatory thresholds established by Ordinance No. 185/2022. Compost maturity was confirmed in both treatments by final C/N ratios below 20, holocellulose/lignin ratios below 1.8, N-NH4+ concentration below 300 mg kg−1 FW (only M2 satisfied this criterion), and N-NH4+/N-NO3− below 3. However, M2 achieved a very stable respiration rate of 1.95 mg CO2 g−1 OM d−1 and a germination index of 90.22%, while M1 recorded a respiration rate of 2.38 mg CO2 g−1 OM d−1 and a lower germination index of 74%. The findings suggest that while M1 is a superior source of macronutrients, M2 provides a more stable and less phytotoxic organic amendment, with both being safe for agricultural soil application.