as shown in the drawings is an apparatus for the cleaning of water such arrangement having a separating space 1 disposed above an activating space 2 in a single container having an outer wall or mantle 3 the interconnection of the activating space 2 with the separating space 1 is accomplished by a by pass channel 4 formed by an inclined partition wall 5 which in the apparatus shown is of conical shape wall 5 separating the separating space 1 from the activating space 2 and by a lower inclined wall 6 also of conical shape disposed below the inclined partition wall 5 and having an apex angle somewhat smaller than that of partition wall 5 the lower inclined wall 6 passing over in its lower part into a return channel 7 formed by a cylindrical conduit or mantle 8 an inlet passage 9 for the separating space 1 is provided in its lower end the passage being formed by the lower edge 10 of the inclined partition wall 5 the by pass channel 4 communicates in its upper part with the activating space 2 through inlet openings 11 in the lower inclined wall 6 a catch trap 12 for sludge separated by flotation is disposed within the separating space 1 trap 12 being formed by an upwardly converging frusto conical wall 13 which joins at its top with a vertically disposed circular cylindrical extension 19 a vessel.