The purpose of this research was to show that controlled centrifugal instabilities-Dean vortices-produced by solutions and suspensions from typical biotechnology applications flowing through curved tubes can be used to reduce concentration polarization and/or fouling in pressure-driven ultrafiltration (UF) and microfiltration (MF) processes. Experiments were conducted to (i) evaluate the ultrafiltration performance of hollow fiber membranes in linear and helical configurations with dextran (low fouling) and bovine serum albumin (high fouling) solutions and (ii) compare the performance of linear and helical coiled UF hollow fiber modules with that of similar MF modules using baker's and beer yeast (Saccharomyces cerevisiae) suspensions as feed. Both constant transmembrane pressure (TMP) and constant permeation flux (J) experiments were utilized here. The membrane material was polyether sulfone. For the ultrafiltration experiments, the helical module performed consistently better than the linear module with dextran T500 and BSA solutions, resulting in performance improvements (helical versus linear) from 20 to 200% and up to 85%, respectively. For the comparative experiments between UF and MF, the helical module again performed better than the linear module for low concentration baker's yeast suspensions (0.5-1% dry wt). At constant TMP, the flux improvements for UF were 30-120%, while at constant J, the capacity or loading was 4.5 times higher for the UF as compared to the MF membrane. At high beer yeast concentrations (5.1-6.8% dry wt), although flux improvements were not observed between the linear and helical modules for UF, the UF fluxes were 72% higher than that obtained with MF. Also, for MF, with the same high beer yeast concentrations, the helical module exhibited 30-90% higher fluxes than that obtained with the linear module. At constant flux (117-137 L m-2 h-1) and intermediate baker's yeast concentrations (0.65-2.7% dry wt), 10-20 times the capacity was obtained for the helical over the linear module. Yeast cells were the dominant foulant. For constant UF flux (70 L m-2 h-1) experiments at high beer yeast concentrations ((4.3-7.7) x 10(7) cells/mL or 5.1-6.8% dry wt), the capacity (loading) for the helical module was 10 times that of the linear module. Again, the yeast cells were the dominant foulant. A new mass-transfer correlation for ultrafiltration of dextran T500 solutions for laminar flow in a helical hollow fiber module was obtained, viz. Sh = 0.173Re0.55Sc0.33(a/Rc)0.07.