For upflow anaerobic fixed film reactors with high hydraulic retention time, biofilm sloughing or attrition may result in rather large and active microbial aggregate fragments that are retained in the reactor. Detailed modelling and validation of this system has been previously published. The present paper is focused on a simplified semi-empirical mathematical model developed for this system, based on Contois kinetics and the definition of some parameters related to the capability of the reactor for biomass retention and its substrate distribution at a given time. The model is represented by two ordinary differential equations corresponding to substrate and biomass evolution on time. These equations have been solved analytically using simplification assumptions, obtaining a simple equation depending on a one lumped parameter only. This solution has been applied. for prediction of some variables representing the average reactor performance on periods of approximately a week, such as the average efficiency on substrate removal or total methane production during the period, under transient loading rates of a low degradable substrate -swine wastes-, with satisfactory results.