Abstract: The device comprises a back module 2, consisting of a housing 4 provided with an opening 10 and with a window 11, and of a front element 3, consisting of a container 20 having a tub shape and turned upside-down, designed to be partially inserted into the window 11 of the housing 4, with the lower edge of the latter below the water level of a water basin and with the opening 10 totally sunk.A blowing unit 12 feeds air into a diffusing element 33, inside the housing 4, so that the air released in water, when going up again to the surface, is collected into said container together with the resulting froth which bears, in suspension, the products of oxidizing reactions of phosphorus compounds contained in water. The froth is then collected by collecting means 35.