The invention relates to a method for two-step separation of water, salt and particles from a hydraulic fluid by the use of a control unit. The hydraulic fluid is carried into a return oil tank with an electrostatic coalescer provided with a high-voltage transformer with direct current to two electric grids whose electrostatic field gathers water into drops which settle into a collecting tank. The hydraulic fluid is circulated to a filter module for removing any remaining water, salt and particles in filter elements connected in parallel, having hygroscopic cellulose fibers which swell until saturation, the swelling being measured by a sensor measuring the degree of saturation, alternatively conductivity.