A method of cleaning a semipermeable membrane, the semipermeable membrane being configured for carrying a fiber web, includes the steps of providing a cleaning fluid and applying the cleaning fluid on the semipermeable membrane. The cover has at a center thereof a circular aperture, a circular protrusion or shelf along the aperture, a filter, and a brush dust removing filter having a connection portion. The final precharge voltage can be set by appropriately selecting the size of a capacitor in the precharge circuit.