The clean environment concept and the demand of decreasing the use of fresh water make it important to reuse wastewaters from industry. One of the industries producing great amounts of wastewater containing fairly low amounts of COD is the pulp and paper industry. These produced effluent waters can be cleaned by membrane filtration so that they can be reused on site. By using ultrafiltration, a water clean enough for most places in the mill can be produced. But for the more demanding places nanofiltration has to be used in order to get a micro-particle free water. Since the volumes of water to be filtered can be in the order of cubic meters per second a high flux is important. The wastewaters in the pulp and paper industry, like surface waters, contain organic degradation products from trees that are of a hydrophobic nature. These molecules easily aggregate and foul the membranes if the membranes are hydrophobic or if the process conditions are favourable to fouling. In the studies reported here the process conditions, the membrane materials and the interactions of hydrophobic molecules in the wastewater or surface water with different kinds of nanofiltration membranes were investigated. The influence of high shear on critical flux was investigated. Adsorption was to be minimised by choosing the least fouling hydrophilic membranes (characterisation by contact angle). Different measurements were made on the waste-waters like TOC, UV-absorption, conductivity, hardness, SUVA index, etc. The results showed that the best nanofiltration fluxes for long periods of time were achieved using very high shear cross-flow modules. Besides on high shear, the study focused on finding hydrophilic membranes and on trying either to use pretreatment to remove the foulants before filtration or on using some kind of adsorbing filtration aid during the filtration. In some cases fibers naturally formed the filtration aid.