In this master’s thesis, numerical simulation of desander with rectangular cross section is conducted with a goal of determining optimal parameters for the chosen geometry. First stage of filtration is carried out in desanders by removing gravel, sand and simmilar particles from the water. Flow velocities within desander are kept in a range starting with 0,01 up to 0,3 m/s so gravity can have time to affect suspended solids which deposits at the bottom of sedimentation tank. To make the sedimentation process efficient, flow must be achieved with least as possible recirculation zones. A baffle is built in near entrance and exit of rectangular desanders in order to increase efficiency of sedimentation. Equations for describing a free surface and sediment transport are to establishing mathematical model. For the modeling of free surface, the following methods are used: Volume of Fluid and Ghost Fluid method. At the bottom of desander, suspended particles were deposited and depth of the desander was altered in simulation. Change of depth of the desander is described by Exner equation which is discretized with finite area method. In order for computational mesh to be adjusted to domain, automatic moving mesh method was used which is based on nodes and finite element method. Numerical simulation is conducted in foam-extend. For the chosen geometry, simulations were carried out with variations in the size of suspended particles and velocities of water flow. With the development of computational fluid dynamics, better understanding of sedimentation dynamics within desanders, as well as optimization of their geometry is enabled.