Volumetric pressure exchanger comprising at least one unit for pressure exchange with a hollow cylindrical body, a piston sliding in said body, said piston comprising a piston crown separating the inside of said cylindrical body into a downstream chamber and an upstream chamber, said piston comprising a rod arranged on the downstream chamber side and passing through said downstream chamber, said downstream chamber being equipped with a device for admitting and delivering water for treatment, said upstream chamber being equipped with a three-way rotary-slide valve, said rotary slide being rotated by motorized drive means, said valve comprising a pressurized-liquid supply orifice, a discharge orifice for said liquid, and an opening communicating with said upstream chamber.