One of the common water desalination apparatuses in freshwater regimes is Single Slope Solar Still (SSSS). This paper numerically investigated the heat transfer processes that took place inside the still. Also, implementing an optimization process to explore the best sustainable feed water preheating tank design. Comsol Software package (v 5.4) was utilized to develop and validate the 3D Mathematical model. The investigation included comparing four different tank designs namely are sphere, half-sphere, cone, and flat plate. In addition, the influence of altering the volume of the tank has been considered. The study revealed that the half-sphere design with a volume of 2.5 times the productivity of Conventional SSSS accompanied the highest recorded water temperature and productivity. The efficiency of the SSSS was enhanced by 27% in contrast to that without preheating.