Membrane filtration is often used to supplement conventional centrifugal separation processes based on gravity differences.     Alfa Laval reverse osmosis spiral membranes use membranes with pores so minute that only small fractions of salts can pass through, along with the water that is the prime component of the permeate. Certain organic compounds with low molecular weights can also pass through - but only to a limited extent. However, it is impossible for any other components suspended or dissolved within the liquid flow (salts, sugars, etc.) to do so. Alfa Laval reverse osmosis spiral membranes are used for the high-flux concentration of either the direct feed stream, or permeates from ultrafiltration and nanofiltration.