A study was performed. to evaluate the cleaning and operation of the ultrafiltration plant at the Nymolla paper mill in southern Sweden Laboratory tests were carried out in order to find out what the fouling layer consists of and to find the most appropriate cleaning procedure Production data were analysed to establish if the membranes are cleaned unnecessarily often The fouling in the plant is mainly of organic origin and is seen as brown precipitation on the surface of the membranes Analysis of the membrane surface with energy dispersive X-rays (EDX) showed that there is also some inorganic fouling, mostly silicon, iron and magnesium Cleaning tests were carried out on a small laboratory test unit from PCI using fouled membranes from the mill Both acid and alkaline chemicals were used to clean the membranes It was shown that the current cleaning procedure was not optimal and some improvements are suggested Production data showed that the membranes are often cleaned when there is no decline in flux This leads to high wear on the membranes and shortens their operational lifetime Less frequent cleaning, especially during the first stages, of the UF plant is therefore recommended It was found that many of the membrane tubes were clogged with a clay-like precipitation consisting of fibres, organic material and precipitated magnesium hydroxide This clay completely stopped the flow through the tube, thereby reducing the active membrane area of the UF plant.