In this study, the optimal use of renewable solar energy and evaluation of the function of a solar dryer system with phase change material (PCM) for drying of wastewater treatment plant sludge is considered. Drying efficiency can he improved by providing specific internal conditions. The thermal efficiency of the drying system was investigated in the autumn when the intensity of the solar radiation decreased. In this research, in September. October and November, respectively, by providing 2429, 2277 and 2223Wh/m(2) internal cumulative solar radiation waste sludge water easily removed. In this season, the mean external solar radiation was 422 +/- 24.2 Wh/m(2) and maximum radiation was 623 Wh/m(2), as well as temperature, humidity and time factors are affected the system performance by transferring heat to sludge. The sludge moisture content decreased from 80% to 8.8%, 18.7% and 27.9% respectively. The porous surface created in the sludge after drying was imaged by scanning electron microscopy (SEM). In fact, solar energy is a convenient economic method by reducing sludge waste volume.