Resource Recovery–Oriented Pilot-Scale Acidogenic Fermentation of Municipal Sludge: Engineering Control, Process Stability, and Circular Economy Implications

The shift toward a circular economy has redefined wastewater treatment plants (WWTPs) as resource recovery facilities. A key pathway is the conversion of organic wastes into volatile fatty acids (VFAs), which are valuable precursors for bioplastics and industrial chemicals. Compared to conventional biogas production, VFAs offer greater economic potential. However, scaling acidogenic fermentation from laboratory to industrial systems remains challenging, particularly in maintaining stable operation and effective mixing. This study examined the scalability of a thermal-only acidogenic fermentation process using an 800 L continuous pilot-scale reactor treating real primary sludge at the Büsnau WWTP in Stuttgart, Germany. The goal was to suppress methanogenesis and enhance acidification without chemical buffers or additives. Long-term monitoring of total suspended solids, total organic carbon, and VFAs composition showed that thermophilic temperatures above 45°C combined with strong hydraulic mixing were essential for process stability. After resolving initial engineering limitations related to heating and mixing, steady-state VFAs concentrations of 3–6 g L −1 were achieved, with peak values approaching 11 g L −1 . Acetic and propionic acids dominated the product profile, representing roughly 80% of total VFAs. Compared with chemical-intensive pretreatment approaches reported in recent studies, the thermal-only method produced similar or superior results while offering a simple operational strategy. The elimination of chemical additives reduced overall costs by more than 50% avoided additional carbon emissions from chemical production. These findings demonstrate that acidogenic fermentation can be effectively scaled without costly pretreatments, providing a practical and sustainable blueprint for transforming WWTPs into efficient biorefineries within circular economy frameworks. • Novel pilot-scale dark fermentation reactor enabling real industrial application. • Pilot-scale tests (800 L) showed heating and mixing control performance for scalable design. • Thermal-only fermentation produced 3–6 g/L VFAs from primary sludge without chemicals. • Acetic and propionic acids dominated (80%), reaching up to 11 g/L under optimal conditions. • The thermal method cut costs by 50% and avoided chemicals, sodium buildup, and CO 2 footprint.