A filtration tank intended to be buried and for containing wastewater in particular, the tank (100) comprising a casing (110) having an upper part (200) and a lower part (300), the tank being provided with voussoirs (610, 610′) for reinforcing the structure of the casing (110) so that it can withstand the pressure exerted by the surrounding material when the tank (100) is buried and is not completely filled. The voussoirs (610, 610′) bearing against the upper part (200) and in that the tank (100) incorporates means (650d, 670) for absorbing the supporting forces of the voussoirs (610, 610′) so as to transfer said forces onto the bottom (310) of said tank. A tank is thus obtained which withstands being buried without deforming even though it may be only partially filled.