Hollow fiber (HF) membranes with circular geometry, are used in many separation processes such as water and wastewater treatment. Since optimization of energy efficiency is important for wastewater treatment, the aim of this study was to investigate the effect of non-circular geometry of the inner surface of the HF on the separation performance. To this purpose, the HF bundle has been assumed as a double porous media having two porosities and permeabilities. Since these two parameters are defined by the geometry of the porous medium, any change in the geometry affects their values and the media performance. Therefore, in this study a mathematical modeling has been divided into five categories, including circular, oval, square, rectangular and triangular geometries, and their geometric properties have been calculated based on three different strategies. The results have been compared with the data obtained from literature and showed that the membrane inner surface to cross-section area ratio (a), axial permeability, and porosity in the inner region for the non-circular HF are larger than that of the circular HF and a increased 16%, 27%, 35% and 65% in ellipse, square, rectangle and triangle geometry, respectively, in comparison with the circle. Axial permeability increased 98%, 68%, 63%, and 26% for a triangle, rectangle, ellipse, and square respectively in the third strategy when compared to the circle. Due to 50% feed flow rate reduction, maximum transmembrane pressure (TMP) reduction was 85% related to the rectangular geometry in the first strategy and minimum was 55% corresponding to the triangle in the third strategy. As a increased up to 65%, TMP reduced by up to 200% and consequently energy consumption and operating costs of the system are decreased.