The drying of residual sludge is a current environmental problem not sufficiently described in the literature, hence research investigations on this drying process are required. Sun drying sludge whilst covered is becoming increasingly attractive for small and medium sized wastewater treatment plants. The control of the quality of the dry product is becoming more and more necessary in order to ensure effectivity. Then new mathematical models should integer these requirements and consider the phenomena of shrinkage. This work gives a mathematical formulation of the different transfer phenomena (heat, mass), which describe the process. The model tries to be relatively simple but sufficiently complete in order to predict and analyze the distribution of temperature, and the moisture during the process. The developed nonlinear differential equations were solved by the classical fourth-order Runge-Kutta method. A validation of these results is achieved by the comparison of the numerical and experimental data.
The sun-drying method is one of the methods used to dry kilishi in Niger and throughout Sub-Saharan Africa. It is a purely traditional process, which only requires the knowledge of the butcher. The objective of our work consists of, on the one hand, the evaluation study of the drying in the sun in a thin layer of slices of meat in the form of kilishi, in typically Sahelian climatic conditions and, on the other hand, the study of mathematical modeling of the physical process which takes place during this drying. The methodological approach consists in establishing a mathematical model, which predicts the thermal and mass balance of the product during the drying process and validates this model with an experimental test. The equations of the mathematical model governing the drying system are solved by the numerical method of Runge-Kutta in the 4th order. The results of the sun-drying evaluation study made it possible to determine the temperature evolution of the upper and lower side of the meat slices in kilish form, during the entire drying process. The monitoring of the loss of masses of the slices of meat allowed us to determine experimentally the water content as a function of time and the drying rate as a function of the water content. The comparison of the results of the numerical simulation of the mathematical model and those recorded experimentally are consistent. The relative errors between the simulation results and the experimental results are given by the statistical parameters R 2 , MSE and RMSE. We have obtained for the set of values of R 2 close to one (1) and MSE, RMSE close to zero (0). This allowed us to conclude that the model is satisfactory.
The treatment of sludge by solar drying under a greenhouse is a technique increasingly used for small and medium sized wastewater treatment plants. In order to contribute to the improvement of dimensioning and the effectiveness of the drying of sludge, we propose using the DVS system (Dynamic System of Vapor Sorption), to determine experimentally the isotherms of desorption at 30 suggested by Oswin. Consequently, experimental results of drying by the forced convection of sludge were obtained, using well controlled thermal conditions of air (30 and 50°C for the temperatures 40 and 60% for relative humidity 1.0 and 2.0 m/s for air velocity). A characteristic curve of drying is built for the different aerothermic conditions. This simplified method makes it possible to simulate the experimental kinetics satisfactorily.
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