An exhaust system provided with exhaust gas after-treatment for a combustion engine comprising an exhaust pipe and a pumping device, which is buried inside the tank, from which it draws so as to feed a water solution of urea under pressure to an electromagnetic injector; the tank is supplied with urea powder and water, which are mixed on the inside so as to obtain a water solution of urea with a variable concentration; and a water supply circuit to supply water to the tank, which is provided with a first pipe, which draws the water from a basin and is regulated by a first valve, which allows water to be introduced into the tank, if necessary, or to be drained to the surrounding environment when the water quantity already contained in the tank is sufficient.