A system comprising a blood circuit including a membrane arranged in a blood filter (7) in which blood is in contact with a first side of said membrane; and a dialysis circuit in which a dialysis fluid is in contact with the other side of the membrane for exchange of substances and ions through the membrane as well as water. The dialysis fluid is circulated from a dialysis compartment and through a RO filter (9) . The permeate fluid, i.e. water is returned to the dialysis compartment. The retentate fluid from the RO filter (9) passes through a separation filter (13) . The retentate fluid is returned to the dialysis compartment and the permeate fluid is removed from the dialysis circuit as a waste fluid to a receptacle (19) . The retentate fluid is concentrated in the RO filter in a ration of at least 3:1.