Abstract: The present invention is for method and means for purification of water using reversed osmosis. Water is forced through a membrane element (10) under pressure, which is obtained by a high pressure pump (2), where the water which is supplied to the element (10) is both from the outside supplied, unpurified water and internally circulated water that has passed by the element at its inlet side. The high pressure pump operates intermittently and the quantity of water that is circulated internally is at least as large as the quantity of water that is supplied from the outside. After that the pressure has been reduced there is a time delayed flush cleaning of the element (10) by flushing with water that is supplied from outside.