The tailing effect in the displacement washing operation of lime mud in the pulp and paper industry was investigated using two different model approaches. Both approaches utilise the dispersion model, the first approach adding micropore diffusion and particle size distribution, the second adding a disturbance in the inlet boundary condition. To investigate the models, washing tests were performed in an apparatus in which a filter cake was formed and subsequently washed. The parameters in the washing experiments were type of lime mud, type of salt as solute, flow velocity and bed length. It was found that a good fit could be obtained with the inlet disturbance model, while the micropore diffusion model could not explain the tailing in a satisfactory way. Furthermore, it was found that the salt used had some influence on the dispersion, and that both the salt and the type of lime mud influenced the adsorption constant.