The present invention relates to a method and an apparatus for treatment of whole blood comprising two steps, firstly, a step of extracorporeal preseparation whereby the whole blood is separated into a blood plasma rich component and a blood cell rich component and secondly, a step of collecting and/or treating the plasma rich component, e.g. performing dialysis, plasma donation or plasma-pheresis. In one embodiment of the invention, the blood plasma rich component is achieved after particle separation using an ultrasound separator comprising micro-channels formed in a plate structure.