Backflow preventer for water distribution installations comprising a body in which are obtained at least one inlet conduit arranged along an inlet axis, at least one outlet conduit arranged along an outlet axis, and at least one discharge conduit arranged along a discharge axis of the body, check valve means (25,56) being provided that prevents a backflow of fluid from the outlet to the inlet in the event of a low pressure upstream of the backflow preventer and/or high pressure downstream of the backflow preventer, discharge valve means being provided arranged along the discharge axis of the body of the backflow preventer for discharging fluid in the aforesaid low and/or high pressure conditions, wherein the check valve means comprises a first check valve at the inlet of the body of the backflow preventer and a second check valve at the outlet of the body of the backflow preventer, and the discharge valve means comprises a discharge valve having a hollow shutter cylinder that is slidable on a piston and the discharge valve being arranged coaxially with respect to the first check valve, wherein the first check valve is arranged inside the discharge valve, wherein the first check valve and the discharge valve are incorporated into a valve cartridge arranged removably inside the body of the backflow preventer, a discharge pipe being further provided that is connected to said discharge valve means to discharge the fluid outside the backflow preventer along an end discharging axis of the fluid, characterised in the fact that: - coupling means (50,51,52) are provided for varying the angular position of the discharge pipe with respect to the body of the backflow preventer, the discharge pipe being formed by a first tube portion arranged along the discharge axis and connected to the body via a snap coupling and by a second tube portion arranged along the end discharge axis, the snap coupling retaining the first tube portion on the body and permits rotation of the first tube portion with respect to the body, - the valve cartridge comprises a cage inside which the first check valve and the discharge valve are arranged, wherein the cage is fixed inside the discharge conduit by means of a threaded bush screwed in an open threaded part of the body of the backflow preventer at the discharge conduit, wherein the bush locks the cage against an annular seat inside the body of the backflow preventer, wherein the piston is fixed at one end to the cage of the cartridge by a nut, wherein one end of the shutter cylinder interacts with an annular seat of the cage to open or close a passage to the inside of the bush according to the position of the shutter cylinder, wherein to the opposite end of the shutter cylinder an annular diaphragm is fixed by means of a ring and a ring nut between which the inner edge of the diaphragm is interposed, and wherein the ring is screwed onto the end of the shutter cylinder and the ring nut is screwed onto the ring, and wherein the outer edge of the diaphragm is locked between the cage and the annular seat of the body of the backflow preventer. - coupling means (50,51,52) are provided for varying the angular position of the discharge pipe with respect to the body of the backflow preventer, the discharge pipe being formed by a first tube portion arranged along the discharge axis and connected to the body via a snap coupling and by a second tube portion arranged along the end discharge axis, the snap coupling retaining the first tube portion on the body and permits rotation of the first tube portion with respect to the body, - the valve cartridge comprises a cage inside which the first check valve and the discharge valve are arranged, wherein the cage is fixed inside the discharge conduit by means of a threaded bush screwed in an open threaded part of the body of the backflow preventer at the discharge conduit, wherein the bush locks the cage against an annular seat inside the body of the backflow preventer, wherein the piston is fixed at one end to the cage of the cartridge by a nut, wherein one end of the shutter cylinder interacts with an annular seat of the cage to open or close a passage to the inside of the bush according to the position of the shutter cylinder, wherein to the opposite end of the shutter cylinder an annular diaphragm is fixed by means of a ring and a ring nut between which the inner edge of the diaphragm is interposed, and wherein the ring is screwed onto the end of the shutter cylinder and the ring nut is screwed onto the ring, and wherein the outer edge of the diaphragm is locked between the cage and the annular seat of the body of the backflow preventer.