This paper describes two semi-empirical formulas for the hindered settling velocity at high concentration. The first formula is based on the Richardson & Zaki formula (1954) with the inclusion of the effect of the maximum volume concentration of matter cmax ≈ 0.65. The second formula is based on the solid-fluid mixture theory. Both expressions leads the best results among the studied formulas in case of non-cohesive particles. The formula based on the fluid-mixture theory tends however to be sensitive to the estimation of the viscosity of the mixture. In case of cohesive particles, as larger uncertainties exist due to unknown parameters as the density and the size of the flocs, no conclusion could be done. The Winterwerp formula (1999) as well as the two proposed formulas presents similar results. The estimation of the gelling concentration is also an important challenge, and appeared to occur for a volume concentration of the flocs phi = phimax ≈ 0.8.