A new methodology for measurement of sludge residence time distribution in a paddle dryer using X-ray fluorescence analysis

• A new analytical technique is validated for sludge RTD measurements in a continuous paddle dryer.
• Any mineral tracer heavier than oxygen and present in low concentration in sludge can be used.
• Tracer solubility has no influence on RTD measurements: sludge behaves as a homogeneous material.
• Sludge mean residence time is almost doubled when storage duration increases from 24 to 48 h.

Drying is a necessary step before sewage sludge energetic valorization. Paddle dryers allow working with such a complex material. However, little is known about sludge flow in this kind of processes. This study intends to set up an original methodology for sludge residence time distribution (RTD) measurement in a continuous paddle dryer, based on the detection of mineral tracers by X-ray fluorescence. This accurate analytical technique offers a linear response to tracer concentration in dry sludge; the protocol leads to a good repeatability of RTD measurements. Its equivalence to RTD measurement by NaCl conductivity in sludge leachates is assessed. Moreover, it is shown that tracer solubility has no influence on RTD: liquid and solid phases have the same flow pattern. The application of this technique on sludge with different storage duration at 4 °C emphasizes the influence of this parameter on sludge RTD, and thus on paddle dryer performances: the mean residence time in a paddle dryer is almost doubled between 24 and 48 h of storage for identical operating conditions.