Modelisation and Simulation of Heat and Mass Transfers during Solar Drying of Sewage Sludge with Introduction of Real Climatic Conditions

Abstract

Sewage sludge presents a real problem with the urban and industrial expanding. So, the drying technique is indispensable in the sludge treatment process to minimize its volume and its revalorization. For cost and environmental reasons, the solar drying is becoming increasingly attractive for small and medium wastewater treatment plants. Therefore, the aim of this work is the modelisation of solar dryer of residual sludge. The model studied is a rectangular agricultural greenhouse. In the lower part, the sludge (assimilated to a porous medium), acts as an absorber. It is subjected to a forced laminar flow. The transfers in the greenhouse and the porous medium are described respectively by the classical equations of forced convection and the DarcyBrinkman-Forchheimer model. The implicit finite difference method is used to discretize the governing differential equation. The algebraic systems obtained are solved using the Gauss, Thomas and Gauss-Seidel algorithms. In order to complete the model and to determine the drying rate we associate a model of the sewage sludge drying kinetics. This work is realized with the meteorological data of the Tataouine region in the south of Tunisia. This data have undergone statistical treatment using the Liu and Jordan method. In order to show the advantages of solar drying, we especially studied the various transfer modes, the drying kinetics and the dryer performance.

One-Dimensional Mathematical Model for Solar Drying of Beds of Sludge

“…For higher gas velocities drying times are smaller than 2,000 hours. The simulated results are in agreement to previous works on sludge drying and other products drying (Ilic and Turner, 1986;Bennamoun, Belhamri, 2003, Font, Gomez-Rico andFullana, 2011;Oueslati, Mabrouk and Mami, 2012;Hassine, Chesneau and Laatar, 2017;Younis, Abdelkarim and El-Abdein, 2017). Figures 12 and 13 show the influence of the height of the sludge bed on the sludge moisture at the drying process.…”

“…Reduction of the moisture content of fresh sewage sludge is an important part of sludge processing, as it increases transport and storage efficiency as well as caloric value. How moisture contents achievable through industrial dewatering techniques are not sufficient for most following appliances, drying has become a necessary step for valorization of the biomass (Ficza 2010, Mujumdar 2014, Hassine, Chesneau and Laatar, 2017, Bennamoun, 2012.…”

Facilitating the natural semi-drying of oily sludge by changing the form of water

“…The water content of the oily sludge decreased significantly as the temperature increased. The evaporation and diffusion of water in oily sludge is considered a one-dimensional heat transfer process in porous media [ 21 , 22 ]. When the elevated temperature reached the oily sludge surface, this increase in the surface temperature generated an increase in the mass fraction of free water at the surface and an increase of the evaporation velocity.…”