Thermophilic composting of dewatered sewage sludge under subzero temperatures using a full-scale pre-fermentation reactor: A 10-day valorisation strategy for year-round operation

Abstract

Thermophilic composting of sewage sludge under subzero temperatures remains a significant operational barrier to sustainable waste management. In this study, we report a full-scale composting process that treats 180 t of dewatered sewage sludge per day, achieving complete compost maturation within 10 days, even during freezing winter conditions. The system integrates a pre-fermentation reactor equipped with internal heating and aeration, enabling rapid thermophilic activation and maintaining aerobic stability despite ambient temperatures falling below −1.8 °C. Microbial community profiling revealed structured thermophilic succession even during the winter process, with Ureibacillus, Planifilum, and Mycolicibacterium enriched during peak activity. Short-chain fatty acid analysis indicated rapid acetate degradation during the reactor phase and a transient accumulation of butyrate following raw sludge remixing in winter, suggesting brief anaerobic microenvironments that were quickly resolved under resumed aerobic conditions. All major short-chain fatty acids were depleted by the maturation stage, reflecting a stable and sustained aerobic environment throughout the later composting phases. The final compost satisfied agronomic and safety criteria, including organic matter stabilisation, neutral pH, pathogen elimination, and heavy metal concentrations. These findings demonstrate the feasibility of year-round, thermophilic, and aerobically stable composting of dewatered sewage sludge at industrial scale. Furthermore, by minimizing anaerobic conditions during early decomposition, the system presents a strong potential for methane avoidance. We highlight its applicability as a scalable composting strategy for cold-climate regions, contributing to the development of sustainable sludge valorisation infrastructure within circular bioresource and climate-aligned waste management frameworks.