An aerator for aerating water comprises a distribution system for distributing air below the water surface and a compressor with an air inlet and at least one air outlet. The compressor is configured to be placed below the water surface. The infiltration of water at the air inlet is prevented by an intake pipe with one open end above the water surface and the other open end has a watertight connection to the compressor. The distribution system has a watertight connection to the air outlet and, due to its nature or by at least one valve, can hold back water. The housing is in thermal contact with the water to cool the compressor. The compressor is a centrifugal compressor that comprises an impeller that is driven by a shaft that it is mounted on air bearings, magnetic bearings or both.
