Non-Saturated Flow Drying Beds permit the effective treatment of septic tank discharge sludge. However, they are vulnerable to clogging. To ensure the sustainability of this technology, through the choice of a suitable filter bed, an unsaturated flow model coupled with the variation of the biofilm has been developed. Calibration, validation and comparison of model simulation results and experimental results were performed on the basis of the NASH criterion. The simulations covered three types of sand: fine sand, medium sand and coarse sand. The results show that the maximum organic load applicable is 24.7 mgO2 / cm2 / day at the fine sand, 43.3 mgO2 / cm2 / day at the medium sand and 31.6 mgO2 / cm2 / day at the level of the sand coarse sand. Then, the Chemical Oxygen Demand removal yield is 92 % at the medium sand level versus 73 % for the coarse sand. Finally, medium sand is less vulnerable to clogging (91.3 %) than fine sand (95.45 %). So, of the three types of sand, it is the average sand that is best suited for the treatment of sludge septic tanks by Non-Saturated Flow Drying Beds. However, when the average sand is not available, the developed model makes it possible to fix the operating conditions and the purification yields of the available sand.