In process and wastewater treatment applications, lamella settlers (LS) are well-known for their relatively high space-time-yield compared to ordinary settling basins. However, they are still characterised by relatively poor performance mainly due to poor hydrodynamic design and the application of incomprehensive models for the overall separation efficiency. These have both led to non-uniform feed distribution in multi-channel separators and poor capacity sizing causing some discontent in practical applications. In this work, a comprehensive description of the critical fluid-suspension-sediment interactions is proposed and used to interpret the results from secondary sludge separation using a CFD-flow optimised multi-channel counter-flow LS unit. Unlike contemporary models that are limited to particle-fluid interactions derived from single channel experiments, the suggested approach considers the effects of feed distribution and the consequent evolving hydrodynamic conditions on the particle population dynamics and deposit flow in a multi-channel separator. The overall separation efficiencies based on suspension mass balance show unique tendencies that are dependent on the feed rate, suspension concentration and the angle of inclination. For further clarity, a comparison is drawn between the LS and a hypothetical idealised batch sedimentation results to illuminate the effect of hydrodynamics and sediment flow on the average deposit flux and hence the overall system separation efficiency. (c) 2007 Elsevier B.V. All rights reserved.