Infiltration percolation is an extensive treatment process aimed at eliminating organic pollution, oxidizing ammonia and removing pathogens. The main purpose of this work was to improve, by means of a numerical model, the understanding of oxidation processes and biomass development in an intermittent sand filter. Hydrodynamic, dispersion and biologic parameters were calibrated and validated after data from the monitoring of an infiltration basin set in the Dissa area (south-east Tunisia). The predicted values were in good agreement with the observed data. Simulations confirm the influence of the filter depth and the fractionation of the daily hydraulic load on organic matter elimination and nitrogen oxidation. Results showed that increasing the fractionation improves the oxidation performances of the infiltration percolation process and leads to accumulated biomass in the upper layers of the sand filter. The model allowed simulation of biomass growth and its impact on the hydrodynamic characteristics of the filtering bed.