Influence of Operating Parameters on Sewage Sludge Drying in a Paddle Dryer: Design of Experiments for the Determination of Hold-Up and Water Content Profiles

Abstract

In previous works, solids hold-up proved to be the most influential parameter of a stochastic flow model of a continuous paddle dryer; we thus aimed at correlating it with operating parameters. A fractional factorial Design of Experiments (DoE) was implemented in which dryer slope, stirrer speed, overflow level, and sludge flow rate were initially selected as factors. The DoE was then completed after reducing the number of influent parameters and their interactions, using surface response methodology (RSM), to get a predictive relationship for solids hold-up determination. The nonlinear influence of overflow height and dryer slope was confirmed by the different water content profiles obtained during the experiments. Finally, the key role of dry solids residence time on final sludge water content was highlighted, with an almost linear relationship. These results emphasize the importance of hold-up for modeling purposes; this parameter has an effect on sludge flow but also on the contact area between sludge and dryer walls.

Keywords:

• Design of experiments
• Empirical correlation
• Paddle dryer
• Solids hold-up
• Water content profile