Valorization of Winery Residues: Optimizing Mesophilic Co-composting of Grape Stalks and Winery Waste Activated Sludge for High-Quality Soil Amendment Production

This study evaluates the valorization of grape stalks (GS) and winery waste activated sludge (WWAS) via mesophilic co-composting to produce high-quality soil amendments. The primary objective was to resolve uncertainties regarding the necessity of a thermophilic phase for these residues and to identify the optimal operational parameters for effective stabilization. Composting performance was assessed through laboratory-scale trials (12–42 °C) and a nine-week pilot-scale experiment conducted under ambient conditions. The results demonstrate that effective stabilization and organic matter degradation are achievable without a thermophilic phase, provided moisture levels are maintained between 55 and 75%. Microbial activity was found to be highly moisture-dependent, with significant performance declines observed when moisture dropped below 40%. The final composts exhibited superior agronomic quality, characterized by low phytotoxicity (Germination Index > 90%), rapid odor neutralization, and enhanced carbon retention (C:N ratio of 23.8 in pilot trials), suggesting lower greenhouse gas emissions compared to traditional thermophilic processes. These findings establish mesophilic co-composting as a technically viable, low-energy, and scalable strategy. By maximizing carbon fixation and simplifying infrastructure requirements, this approach offers a cost-effective solution for transforming underutilized winery by-products into safe, stable organic fertilizers, directly supporting circular economy goals in viticulture.