A mathematical simulation model is developed that calculates the evolution of particle removal and headloss in a packed bed filter, as functions of the morphology of the colloidal particle deposits in the filter. The expressions derived are based on the postulate that particles form deposits that grow as mass fractals. The model is calibrated and validated with a set of laboratory data previously obtained. The calibrated value of fractal dimension was found to coincide with measurements obtained in an earlier study using light scattering. The effect of the filtration velocity and particle size on the fractal dimension of the deposits is evaluated. The hypothesis that higher filtration velocities create denser deposits is supported. However, a trend of increasing fractal dimension with decreasing particle size runs counter to theoretical expectations. The model is extended to include the effect of variable conditions in the influent.