Capillary nanofiltration offers the possibility to treat raw water to high quality (process) water in just one step. Surface water can be treated for drinking water production and effluent for water recycling. In capillary membranes no feed-side spacers are present and a high concentration polarization is thus observed, limiting the overall system performance. In this study, the effect of air sparging to limit concentration polarization is investigated. Experiments are carried out with synthetical water at different TMPs and superficial water and air velocities. As expected, air sparging decreases concentration polarization, resulting in an increase in permeate flux and retention. Moreover, air sparging uses less energy, so the industrial relevance is high. (c) 2006 Elsevier B.V. All rights reserved.