Abstract Novel thermal technologies for phosphorus recovery from sewage sludge require a pulverized fuel – dried sludge with particle sizes below 500 µm. However, dewatered sludge typically exhibits particle sizes ranging from 10 µm to 800 µm, while state-of-the-art drying systems can produce dried sludge particles of 100 µm to 6 mm. This study investigates a thin-film dryer specifically engineered to combine sludge drying and grinding in a single step. Pulverized sludge can be produced by enabling prolonged attrition after the formation of dried sludge granules. Sludge samples from Germany were evaluated for moisture, organic content, chemical composition, and rheology to elucidate the agglomeration mechanisms under agitated drying conditions. Results indicate that particle interactions, driven by the complex sludge matrix, enhance granule formation and limit comminution. To overcome this intrinsic effect, this study employed a 2 3 factorial design to investigate the performance of a pilot drying-grinding system in producing pulverized sludge. The effects of varying sludge feed rate, rotor speed, and heating temperature on particle size were examined. Rotor speed was identified as a dominant factor affecting the dried sludge particle size, reducing the marginal Dv90 mean by 22 %. Independent of the operating conditions, the pilot dryer consistently achieved sludge dryness above 90 % dry matter. An optimal condition, achieving Dv90 values around 1 mm, was obtained under low feed rate, high rotor speed, and lower heating temperature. This work demonstrates the potential of a thin-film dryer for pulverized sludge production, providing insights into the upscaling of the drying-grinding system.
Schober et al. (Thu,) studied this question.