The influence of module configuration on the ultrafiltration of highly concentrated fluids in tubular membrane plants was investigated. Previous work has shown that frictional pressure drop in the feed flow channel can have a significant influence on the performance when treating viscous fluids. In the present study, three module alternatives were compared: polymeric membranes connected either in series or parallel, and a ceramic membrane element with all flow channels in parallel. An economic evaluation was performed of an additional stage in the ultrafiltration plant at Stora Enso Nymolla pulp and paper mill treating bleach plant effluent. A parametric study in a laboratory equipment provided information on how the flux and frictional pressure drop vary with transmembrane pressure and cross-flow velocity along the membrane. A full-size stage was scaled-up from the experimental data obtained from the laboratory experiments. All three alternatives were shown to be economically viable. The cost of the polymeric membrane in parallel configuration ((sic) 70 200 per year) was lowest, making it the most attractive alternative. It was shown that the cross-flow velocity had a significant effect on the performance and hence on the economics of the plant. For all three alternatives, the lowest cross-flow velocity employed (2 m/s) gave the best performance. (C) 2010 The Institution of Chemical Engineers. Published by Elsevier B.V. All rights reserved.