The performance of secondary clarifiers is strongly related to density effects in the sedimentation vessel. The upper level of the clarifier's chamber - once considered to be inactive at the sedimentation process now appears to play an important role in solids removal. A fully developed three-layer flow phenomenon that develops in some cases is proved to promote settling. Therefore, investigation of flow patterns at different operative conditions was carried out. Experiments were conducted in a model of a circular, centre-feed settling tank with continuous operation. Ground hazelnut shells served as the settling matter: in one set of experiments only dye was used. All laboratory runs were filmed and analysed, qualitatively and quantitatively, by means of computer-aided visualisation. The results of the study showed that a determining factor of the conditions favourable for formation of a fully developed three-layer flow is the theoretical densimetric Froude number. If the hydraulic and solid loadings are soundly chosen in accordance with these findings, the clarifier performance can be evidently improved. (c) 2013 Elsevier B.V. All rights reserved.