This tank has a first vertical cylinder (10) having a sewage inlet (11) at the lower end and a sewage outlet (12) at the upper end, a second cylinder (20) concentrically around the first cylinder at a distance therefrom, a contaminant guiding tube (60) connected to the upper end of the second cylinder and having an upper narrow-diameter part at least partly over the upper end of the second cylinder, and a tube (50) guiding foam mixed with water connected to the lower part of the first cylinder (10).
