Abstarct: Wastewater treatment plants are essential infrastructures for any community and are crucial for development in various economic, environmental and social aspects. Primary sedimentation tanks are one of the simplest, economic, oldest and fundamental physical treatment units in a sewage treatment plant. At present, the design of these physical treatment units is baxsed on conventional scientific known parameters such as surface overflow rate, average hydraulic retention time and weir loading rate. Computational fluid dynamics (CFD) models can be used as a powerful tool for design and operation of these treatment units and simulate the mechanism of settling suspended particulate matters in the tank for any design and operation scenario. In this research, using CFD tools in well-known and widely used FLOW 3D software, the modeling of these tanks with different design scenarios has been investigated and the mechanism and efficiency of the settling tank have been evaluated. The results showed that by changing the horizontal flow velocity in the rectangular sedimentation tank from 0.1 m/s to 0.3 m/s, in addition to increasing the surface overflow rate from 57.6 m/day to 172.8 m/day, its hydraulic retention time is reduced from 2250 seconds to 750 seconds and the efficiency of the tank in removing suspended particles reduces from 30% to 4%. Changing the inlet and outlet layout in opposite arrangement in the tank, with a surface overflow rate of 57.6 m/day and a detention time of 2250 seconds, increases the removal efficiency of the particulate matter to 40%. Increasing the length of the tank has a significant effect on its removal efficiency, so that for the tank length of 20,15 and 40 m, surface overflow rate is 21.6, 43.2 and 57.6 m/day respectively and residence time is 2250 , 3000 And 6000 seconds, and ultimately, the efficiency of the tank increases to 40, 55 and 65 percent. Also, by increasing the depth of the tank from 1.5 m to 3 m, the surface overflow rate was 57.6 constant, the retention time increased from 2250 seconds to 4500 seconds and the tank removal efficiency increased from 40% to 50%. Increasing the number of outlets results in a significant increase in the efficiency of suspended particulate removal, so that the tank efficiency is increased to 60% when the tank has one inlet and two outlets at the same surface overflow rate and retention time.