Apparatus for purifying water to be purified, said apparatus comprising: - a tank for the water to be purified, which develops around a main axis of development and which comprises a bottom, conformed as a hopper, to receive sediment separated from the water to be purified, and an inlet connected to said bottom; - at least one pump mean, outside said tank, and connected to said inlet, the pump mean being able to feed under pressure a flow of water to be purified in said tank; - at least one conduit disposed axially inside said tank and diverging upward with respect to said main axis of development, said conduit being fluidically connected to said inlet to receive pressurized water fed by said at least one pump mean; - a hydraulic accelerator/mixer assembly disposed in said tank and which connects said inlet to said conduit, wherein said hydraulic accelerator/mixer assembly is configured to increase the speed of the flow of water to be purified introduced into the tank and determine a recirculation flow at entry to said hydraulic accelerator/mixer assembly obtaining a mixture of the flow of water to be purified with at least part of the sediment present in correspondence with said bottom and/or at least part of substances to be removed being deposited on the bottom; characterized in that said hydraulic accelerator/mixer assembly is of the multi-stage type and comprises at least two accelerator/mixer devices (23a, 23b) disposed connected in sequence along said main axis of development which comprise a first accelerator/mixer device disposed connected directly downstream of said inlet and able to increase the speed of the flow of water to be purified introduced into the tank and a second accelerator/mixer device disposed downstream of said first accelerator/mixer device and able to increase the speed of the flow of water to be purified received from said first accelerator/mixer device, wherein said at least two accelerator/mixer devices (23a, 23b) are also configured to determine respective recirculation sub-flows (R1, R2) of said recirculation flow at entry respectively to said first accelerator/mixer device and said second accelerator/mixer device and each comprises a respective recirculation chamber and a nozzle, of the Venturi type, disposed inside said recirculation chamber and configured to generate said respective recirculation sub-flow (R1, R2) from a recirculation zone to the recirculation chamber, and wherein the recirculation chamber and the respective nozzle of said first accelerator/mixer device have smaller sizes than respectively the recirculation chamber and the respective nozzle of said second accelerator/mixer device. - a tank for the water to be purified, which develops around a main axis of development and which comprises a bottom, conformed as a hopper, to receive sediment separated from the water to be purified, and an inlet connected to said bottom; - at least one pump mean, outside said tank, and connected to said inlet, the pump mean being able to feed under pressure a flow of water to be purified in said tank; - at least one conduit disposed axially inside said tank and diverging upward with respect to said main axis of development, said conduit being fluidically connected to said inlet to receive pressurized water fed by said at least one pump mean; - a hydraulic accelerator/mixer assembly disposed in said tank and which connects said inlet to said conduit, wherein said hydraulic accelerator/mixer assembly is configured to increase the speed of the flow of water to be purified introduced into the tank and determine a recirculation flow at entry to said hydraulic accelerator/mixer assembly obtaining a mixture of the flow of water to be purified with at least part of the sediment present in correspondence with said bottom and/or at least part of substances to be removed being deposited on the bottom; characterized in that said hydraulic accelerator/mixer assembly is of the multi-stage type and comprises at least two accelerator/mixer devices (23a, 23b) disposed connected in sequence along said main axis of development which comprise a first accelerator/mixer device disposed connected directly downstream of said inlet and able to increase the speed of the flow of water to be purified introduced into the tank and a second accelerator/mixer device disposed downstream of said first accelerator/mixer device and able to increase the speed of the flow of water to be purified received from said first accelerator/mixer device, wherein said at least two accelerator/mixer devices (23a, 23b) are also configured to determine respective recirculation sub-flows (R1, R2) of said recirculation flow at entry respectively to said first accelerator/mixer device and said second accelerator/mixer device and each comprises a respective recirculation chamber and a nozzle, of the Venturi type, disposed inside said recirculation chamber and configured to generate said respective recirculation sub-flow (R1, R2) from a recirculation zone to the recirculation chamber, and wherein the recirculation chamber and the respective nozzle of said first accelerator/mixer device have smaller sizes than respectively the recirculation chamber and the respective nozzle of said second accelerator/mixer device.