Bleach plant effluent is treated in the ultrafiltration plant at Stora Enso Nymblla pulp and paper mill. The plant is the largest of its kind in the world, with a total membrane area of 4800 m(2) (2900 m(2) on the softwood line and 1900 m(2) on the hardwood line) and a processing capacity of 400 m(3) bleach plant effluent per hour. The ultrafiltration plant has been in operation since 1995. The operating conditions have been altered with time. In this work the present performance of the ultrafiltration plant has been evaluated and ways to improve the performance further have been investigated. The largest changes are found at the ultrafiltration plant on the hardwood line. On the hardwood line the ultrafiltration plant consists of six stages in series, with a volume reduction of about 98%. A drastic decrease of flux with axial position in the modules in the last stage was revealed. The flux was found to be zero in the last part of the modules because of the high frictional pressure drop caused by the high viscosity (7.7 cP) of the liquid in the last stage. Using a calculation tool, it is shown that the flux in the last stage can be increased and the energy requirement decreased if a higher inlet transmembrane pressure is used or if the cross-flow velocity in the last stage is decreased.