Mass transfer in ultrafiltration for water and wastewater treatment is improved by Dean vortices. With this secondary flow, which appears in a coiled hollow-fibre module, the shear stress is higher than in a straight module and is a maximum near the external wall of the coiled tube. As a consequence, concentration polarisation and cake deposition are reduced and the limiting flux in ultrafiltration of model fluids (bentonite and yeast suspensions) and activated sludge is improved by up! to 5 times. The effect of the hydrodynamic conditions and feed concentration is tested. An energy analysis shows that an improvement in permeate flux is found at a given energy consumption.