brane interface of species with a diameter larger than the We have investigated the spatial variation of local protein conmembrane cutoff. They are retained on the membrane surcentration and filtration flux by a scintigraphic technique in the face mainly by the transmembrane pressure forces. They ultrafiltration of bovine albumin solutions and blood. The feed partially return to the bulk fluid by action of the wall shear was mixed with 99m Tc albumin macroaggregates and circulated rate, which tends to erode the polarization layer, and by through a polysulfone 30,000 MWCO hollow fiber filter placed in diffusion. Inversely, species with a size smaller than the the field of a gamma-camera. Concentration profiles c b (x) were membrane cutoff cross the membrane by action of transreconstructed from scintigraphic images and the local ultrafiltramembrane pressure from the bulk fluid toward the filtrate tion flux was calculated by differentiating c b (x) and using mass conservation. Tests were run at various inlet shear rates from 472 compartment. The polarization layer is the main factor limto 1415 s 01 and under two different filtration regimes: no net iting the ultrafiltration flux, because it reduces fluid transfer filtration (permeate valve closed) and large filtration (below the in two ways ( ). First, it acts like a second membrane put pressure independent plateau). The data confirm the filtration in series with the main membrane; therefore, the apparent decay from the filter inlet to outlet but an unexpected result is the membrane resistance is greater than the resistance due solely presence of high retrofiltration in the downstream part of the filter to the UF membrane. Second, it increases considerably the length in the case of large filtration. This retrofiltration can be osmotic pressure near the interface, because the osmotic explained by a high osmotic pressure at the membrane created pressure grows rapidly with the protein concentration c w of by the protein polarization concentration. Assuming a constant the polarization layer, which is much higher than the bulk pressure gradient along the fibers, it is possible to estimate the concentration c b . So, the driving force due to hydraulic preslocal osmotic pressure at the onset of retrofiltration and to infer from it the protein concentration at the membrane, which is found sure tmp is partially compensated for by that due to osmotic to vary from 170 to 250 g/liter when g w increases. Similar experipressure Dp. Above a pressure threshold, i.e., when the ments were run with blood and a microfiltration membrane (0.55filtration flux reaches a maximum independent of transmemmm pores). In that case no retrofiltration was obtained, which brane pressure (plateau), the polarization of the membrane confirms our explanation since in this case the polarization layer is complete. is composed of red cells which exert no osmotic pressure. α§ 1996 No theoretical model completely describes the concentra-Academic Press, Inc. tion polarization phenomenon. Depending on the system and