The paper presents the results of a long-term research program on the improvement of existing shallow circular secondary settling tanks (SSTs) based on the computational fluid dynamics (CFD) investigation of their inner hydrodynamic processes. The results provide insight into the flow and concentration pattern within such SSTs and highlight some important details that largely determine their performance. As a novel detail of the research of SSTs, a direct dependence was found between the inlet height and the length of the radial density jet induced by the entering water-sludge suspension. A hydrodynamically optimized (low positioned) inlet facility decreases the kinetic energy and the length of the density jet, as a result of which it vanishes before hitting outer boundary of the SST. This results in an enhanced performance indicators even of such shallow SSTs, under both dry weather and peak load conditions as well. (C) 2016 Institution of Chemical Engineers. Published by Elsevier B.V, All rights reserved.