A water-processing electrochemical reactor that comprises a cylindrical inner anode (73), an outer tubular cathode (74), an intermediate chamber between the anode (73) and the cathode (74) and being crossed by the water, an outer shell (77) surrounding the cathode (74), a water inlet (71) and a water outlet (78), and a gas inlet (80) and gas outlet (79) connected to the outer shell (77) and to the gas chamber. The cathode surrounds the inner anode (73) and is porous to gas. A gas chamber is defined between the cathode (74) and the outer shell (77). The gas chamber contains a gas comprising oxygen and is at an overpressure that forces the gas through the porous cathode (74).