Waste water coming from the work process (1), are conveyed to the collecting well (2), then sent to the clarifier (3) through a submerged pump (2); the presence of the polyelectrolyte into the clarifier makes the suspended substances to precipitate. The polyelectrolyte coming from the preparation and dosing station (4) is injected into the waste water-lifting pipe. After the clarification processing, the water comes out to the stocking tank (10) to be recycled inside the plant while the solid substances (mud) settle into the clarifier cone are automatically discharghed through the pneumatic valve (5) into the mud collecting tank (6). A special pump (7) sends the mud to the filter-press (8) where solids and liquids now finally split. The pressed panels of mud are automatically discharged by the filterpress (8) while the liquids are recycled. The whole system is automatically controlled from the control panel (9), which is managed by a self-diagnostic PLC.