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Drying sewage sludge with coal fly ash for producing a soil amendment
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The high quantities of sewage sludge (SS) and coal fly ash (CFA) produced require an adequate management for protecting human health and the environment. This study aims to investigate the possibility of using CFA as SS drying adjuvant for producing a soil amendment. The study involved the characterization of SS and CFA at a physical and chemical level. The drying process of small cylinders, for several combinations of SS:CFA (100:0-control; 95:5; 90:10 and 85:15) at different temperatures (40, 70, 100 and 130 ºC) was investigated. The dynamic model of the drying process was based on the Fick law and an infinite length was assumed for the small cylinders. The kinetics analysis showed that as the quantity of CFA increases the rate of drying is faster. The effective diffusion coefficient and other relevant parameters were determined for each thermal drying condition. The results showed that the drying process of SS with small quantities of CFA may be a route for valorizing these wastes in agricultural soil.
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The knowledge of moisture diffusion, Deff and convective mass transfer, hm coefficients during drying of food product is necessary in modeling and optimization of solar dryers. In the present paper, a methodology is proposed for the determination of these coefficients using variable drying parameters (lag factor, k0 and drying coefficient, k). Mixed-mode solar dryer with potato cylinders and slices was used to obtain variable (temperature dependent) k0 and k from drying kinetics. Results of investigation indicate the increasing trend of Deff and hm with temperature and their values are found to be higher for cylinders compared to slices. Temperature rise from 33 to 48 °C for cylinder during drying results in the increase of 85.6% and 159% of Deff and hm, respectively, whereas these figures are 72% and 89% for slices. The statistical error analysis reveals that for each of the sample geometry, the proposed prediction models can better simulate experiments when variable parameters instead of constant ones are considered.
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