Analysis of shrinkage and cracking behavior in open-air drying processes of sewage sludge

Abstract

This study focuses on the open-air drying of sewage sludge and highlights the phenomena of shrinkage and cracking. It aims to determine the effective diffusion coefficient of five samples, with and without taking shrinkage into account, using mathematical models describing the behavior of sewage sludge. The results show that the SWeibull model, proposed in this work, proved to be the most accurate. This model displayed high coefficients of determination (R² = 0.9901) and low errors (SSE = 0.0596, RMSE = 0.0237, MAPE = 3.6670 %). A comparative analysis of diffusion coefficients revealed that diffusion without shrinkage varied between 3.54 × 10⁻¹¹ and 4.25 × 10⁻¹¹ m².s⁻¹, while accounting for shrinkage decreased the range to 2.27 × 10⁻¹¹ and 3.00 × 10⁻¹¹ m².s⁻¹. Statistical analysis of the numerical and experimental results confirms that incorporating shrinkage into the diffusion coefficient calculation accurately describes the moisture diffusion phenomenon as a function of drying time. The study of cracking phenomena reveals that sample thickness plays a decisive role: thicker samples develop deeper, more complex cracks, while thinner samples exhibit lighter cracks. The phenomenon of cracking has a direct and proportional impact on drying speed. The results show that integrating shrinkage and cracking phenomena into drying kinetics modeling enables more accurate estimation of kinetic parameters, thus optimizing the sewage sludge drying process for more efficient management.