Membrane filtration assembly

A membrane filtration assembly comprising:

• a feed inlet for a fluid to be filtered, said feed inlet opening to a retentate side of a membrane filter;
• a retentate outlet for a retentate fluid;
• first recirculation means for recirculating at least a part of the retentate fluid to the feed inlet
• a permeate outlet for filtered permeate fluid,
• second recirculation means for recirculating at least a part of the permeate fluid to a permeate inlet,
• said inlets, outlets and first and second recirculation means being arranged to maintain a retentate fluid stream on the retentate side of the membrane filter and a permeate fluid stream on a permeate side of the membrane filter in such a manner that the permeate fluid and retentate fluid streams are concurrently passed along the membrane filter with approximately uniform drop in pressure across the membrane,
• several membrane units in form of tubular supporting structures disposed in the membrane filter, each supporting structure comprising one or more filter membranes said membrane units being mounted in parallel inside a common housing in such a manner that one of the permeate fluid and retentate fluid streams flows inside the membrane units while the other of the permeate fluid and retentate fluid streams flows in a space surrounding said membrane units and in a manner wherein parallel fluid flow between units occurs, each membrane unit being provided with a separate passageway which is defined on the inner side by the membrane unit and on the outer side by a channel tube having an inner cross section which is 4 to 200% larger than the outer cross section of the membrane unit.