The theoretical densimetric Froude number values with favourable effect on the clarifier performance

• Published in: Chemical engineering and processing 2013-12, Vol.74, p.97-105
• Main Authors: Prešeren, Tanja, Steinman, Franci, Širok, Brane, Bajcar, Tom
• Format: Article
• Language: English
• Subjects: Clarifier; Densimetric Froude number; Fluid mechanics; Separation; Visualisation; Wastewater treatment
• ISSN: 0255-2701; 1873-3204
• DOI: 10.1016/j.cep.2013.09.001

•A physical model of a circular, centre-feed settling tank was set.•Investigation of flow patterns at different operative conditions was carried out.•Three-layer flow phenomenon in clarifier is related to densimetric Froude number.•Formation of the upper current improves the quality of the effluent.•Low densimetric Froude number indicates formation of a three-layer flow. 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.