Using free solar energy for wastewater sludge drying can be benefit in point of view of energy consumption and in consequence on the cost of the drying system. This review has shown that in whole presented cases, the wastewater sludge is spread in thick layers, in the floor of a greenhouse chamber, which is the main part of the presented drying systems. Fans and ventilations can be added, in order to have homogeneous distribution of the inlet air at the surface layers of the product. Also, ventilation is used to evacuate the saturated air and replace it by fresh air. Generally mixing the product manually or using a robot, once or several times in a day was effectuated which permits to have harmonized distribution of the dried product. Several, materials were added in order to increase the efficiency of the drying systems, such as heat pump, infrared lamps, heating the floor of the greenhouse using solar water heater or adding rock-bed as thermal energy storage system. According to the obtained results and the cost of the operation, it was not benefit, in all cases, to use a second source of energy beside to free solar energy. The performances of the drying systems change through the period of use, the geographical situation of the place where the experimental work was done, what is more it depends on the origin of the wastewater sludge.