Drying characteristics of sludge were examined for average moisture content from 2 to approximately 0.7-0.08 kg water/kg dry matter using hot air of the range of 20-40-80 degrees C and simulating the interval that can be provided by an air solar heater and by air velocities of 1 m/s at a laboratory scale dryer. Different mathematical models were used for the simulation of the sludge drying curves, such as Lewis, Page, Modified Page, Henderson and Pabis, Wang and Singh, Logarithmic, Two term, Two Term exponential, Modified Henderson and Pabis, Midilli, Approximation of diffusion, Verma et al. and Simplified Fick's diffusion models. The performance of these models was investigated by comparing four statistical parameters: the square of the coefficient of determination (r(2)), reduced chi-square (chi(2)), root mean square error (RMSE) and sum of residuals between the observed and predicted moisture ratio. The Midilli model represented the drying characteristics better than the others. The effective diffusivity coefficient of moisture transfer varied from 2.224 x 10(-10) to 6.993 x 10(-10) m(2)/s over the temperature range. The temperature dependence of the effective diffusivity coefficient was expressed by Arrhenius type relationship. The activation energy for the moisture diffusion was found to be 15.77 kJ/mol. (c) 2006 Elsevier Ltd. All rights reserved.