This research evaluated the hydraulic behaviour of a biological filter (BF) that used marine shells as support material for the post-treatment of municipal wastewater (MWW). The BF was theoretically designed with ascendantdescendent flow to describe a piston flow. The hydraulic evaluation was carried out in the liquid phase and in operation using Li + (LiCl) as liner applied instantly in the affluent in a 6.43 hours theoretical hydraulic retention time (THRt). The analysis of the distribution of residence times allowed to determine that the flow pattern in the filter has a piston flow tendency, which was corroborated with a 0.145 and a 0.08 dispersion, a piston fraction of 0.916 and 0.839 with tap water and in operation respectively. The support material increased the piston flow's behavior, as a consequence of the reduction of the mixture there was a small fraction of stagnant zone. (0.012).