A model that predicts the progression of clogging of leachate collection systems for solid waste landfills is outlined. The model represents a leachate flow path as a fixed-film reactor. A transport model is used to track acetate, propionate, butyrate, calcium, and suspended biomass. Within each element along the flow path, the model simulates methanogenesis of acetate, acetogenesis of propionate and butyrate, the synthesis of fixed biomass, and the precipitation of calcium carbonate, which is proportional to the loss of the acids. Detachment, decay, and attachment of fixed biomass also are included in the model. Characterizing the porous medium as spheres, and assuming that films are of uniform thickness, the model estimates the changes in porosity and specific surface from geometric computations. An example application of the model shows that smaller stone size results in much faster and greater loss of porosity, and the accumulated clog material is mainly inorganic precipitates.