Modelling the drying characteristics of human faeces using thin-layer drying models and calculation of mass transfer properties at ambient conditions

Abstract

Convective drying was used to investigate the drying characteristics of human faeces. Thin-layer drying semi-empirical models were used to describe the drying characteristics of human faeces under varying conditions of temperature, air velocity and relative humidity. The drying experiments were conducted at moderate near-ambient conditions, with temperatures of 20 °C, 25 °C, 30 °C, 35 °C and 40 °C, and air flows of 0.1 m/s, 0.2 m/s and 0.5 m/s, at relative humidity of 5%, 10%, 20% and 40%, and initial moisture content of 82.2%, 78.1%, 71.7%, 71.1% and 64.2%. The experimental data curves for faeces for varying temperatures were fitted to the Newton, the Henderson and Pabis, the Page, and the Midilli-Kucuk models, while other experimental conditions were kept constant. Page model was the best fitting model, with R2 values ranging from 0.9950 to 09,977. Effective moisture diffusivity and convective mass transfer coefficient were found to vary from 8.81 × 10−8 m2/s and 1.31 × 10−5 m/s at 20 °C to 9.57 × 10−7 m2/s and 3.49 × 10−4 m/s at 40 °C, respectively. The activation energy was found to be 42.38 kJ/mol. The results presented in this study confirm that human faeces can be dried under moderate ambient conditions by using solar and wind energy. The modelled kinetics can inform the development and design of a dry sanitation system that does not require electricity or any other fuels for heating and air convection.