During ultrafiltration (UF), the driving force along the membrane is diminished due to concentration increase and frictional pressure drop. In most applications of ultrafiltration the influence of the frictional pressure drop is negligible. However, when treating concentrated, viscous liquids the frictional pressure drop can reduce the transmembrane pressure (TMP) significantly. The influence of cross-flow velocity and inlet pressure on flux and frictional pressure drop during concentration of retentate from the Stora. Enso Nymolla pulp and paper mill ultrafiltration plant treating hardwood bleach plant effluent was studied in this work. The experimental data were used in a calculation tool to investigate the membrane area and energy requirement of two module design alternatives during concentration of the retentate in an additional stage. The first alternative is the same module design as in the existing plant on the hardwood line at Nymolla, where the tubular membranes in a module are connected in series. In the second alternative, the membrane tubes in a module were instead connected in parallel. The experimental flux increased markedly with increasing pressure and cross-flow velocity at all concentrations. However, the flux was significantly reduced when the chemical oxygen demand (COD) was increased. The flux was 120 l/m(2) h at 180 g/l and 23 l/m(2) h at 330 g/l (at 1.0 MPa and 4 m/s). The concentration was 180 and 330 g/l at volume reduction factor (VRF) 1 and 2. The calculated average flux was significantly higher in the design with membrane tubes in parallel. In addition, the flux was not only higher, but it also increased with increasing cross-flow velocity, whereas the flux in the design with membrane tubes in series reached a maximum flux at about 3 m/s. (C) 2007 Elsevier B.V. All rights reserved.