Gravity belt thickening is commonly used for dewatering activated sludge. However, only a few results concerning this process, and the mathematical description of the drainage of residual sludges, have been published. Laboratory drainage experiments were carried out on several activated sludges to compare different published modelling approaches. Data showed a linear relationship between the infinite mass of the cake (M-C) and the total mass of the dry solids in the initial sludge sample (DS): M-C = a(1) x DS + a(2) so, an empirical equation was modified to predict the evolution of the cake dry solids content-over time: C(t) = DS/(a(1) x DS + a(2)) + (c(1) x exp(c(2) x DS) x t + 1/M-0 -(a(1) x DS + a(2)))(-1) Two sets of empirical coefficients were derived according to the range of the initial dry solids content of the sludge (C-b < 2% or C-b > 2%). These coefficients were not significantly influenced by the origin of the activated sludge and thus can be considered as constants. Simulation results using the chosen model accord will with experiments carried out on an industrial gravity belt thickener working under field conditions. (C) 2004 Society of Chemical Industry.