On a model of a circular secondary settling tank the dissertation deals with typical flow structures that establish at twelve different combinations of hydraulic and solids loading. Another variable parameter beside the already mentioned were also solid particles that served as suspended matter. The first group of used particles was fine ground hazelnut shell that was available in two different compositions of grain size; the second group were glass microbeads in three different granulations. Three control experiments were conducted at neutral density conditions by introducing a dye. All experiments were filmed with a digital camera. By processing video clips of experiments an indicator of concentration in the inflow and in the vicinity of the effluent was captured by photogrammetric approach and data on flow field was obtained by computer-aided flow visualization. A repetition of the first series was carried out in order to collect samples of inflow and outflow suspension. A wide set of experiments with carefully chosen parameters enabled a systematic study of hydrodynamic phenomena in the sedimentation chamber. Flow field and clarification efficiency were found out to be closely related to the value of the densimetric Froude number Frd. All experiments exhibited trends of growing efficiency when values of the densimetric Froude number Frd were decreasing. A three-layer flow structure never occurred when the biggest particle granulation from glass microspheres and when coarse ground hazelnut shell was used. By the rest of the model particles a low value of Frd reflected in a formation of favourable three-layer flow structure, whereas the threshold value of Frd (when the value is exceeded, the three-layer flow does not develop) was independent from the type or size of the particles.