The invention relates to a filtration cell that comprises a tank with a bottom wall (2) and four side walls extending upwards from the bottom wall as well as an upward opening, the four side walls including two opposite longitudinal walls (3, 5) as well as a front facing wall (4) and a rear facing wall connecting them, a filtration bed (7) supported in the tank, an outlet opening for discharging the filtrate, and a covering flap (24) protruding outwards from the top of one of said longitudinal walls, the longitudinal wall (5) opposed to that fitted with the flap including a lower portion (25) extending upwards from the bottom wall (2) and an upper portion (26) extending slantedly upwards and inside the tank from the top of said lower portion (25), and/or the front facing wall (4) comprises a bottom portion (27) extending upwards from the bottom wall (2) and a top portion (28) extending slantedly upwards and inside the tank from the top of said bottom portion (27).