During the recent decades an increase in natural organic matter (NOM) has been noted in Nordic countries. At Pitkäkoski water treatment plant this has spurred on research about the effect of NOM increase to water treatment process. Simultaneously the water treatment plant is looking to increase its production capacity. Low pressure membranes (LPM), i.e. micro- (MF) and ultrafiltration (UF) membranes have emerged as a viable alternative to conventional treatment, as they separate particles effectively and can be combined with pretreatments such as coagulation to achieve high NOM removals. However, simultaneously NOM is a major component causing membrane fouling. The diversity of ways to retrofit LPMs to the existing water treatment plants proves the robustness of MF and UF membranes. Several retrofits were studied and the findings show that most often LPMs are used as a final polishing step before water is delivered to customers or to replace sedimentation and sand filtration. If raw water is of good quality membranes can be used for direct filtration, but in over half of the cases studied LPMs were combined with pretreatments such as coagulation, adsorption, or prefiltration. By studying existing retrofits two possibilities to retrofit LPMs to Pitkäkoski water treatment plant emerged. These two possibilities were studied intensely by using a MF membrane by Pall, and the performance, both operational data and water quality, were compared to conventional treatment currently in operation. When treating clarified water after sedimentation the MF membrane experienced a smaller initial drop in permeability compared to flocculated water, but the fouling rate after initial drop was faster. After two days the permeability with both feed waters was the same. MF membrane was able to compete with conventional treatment, consisting of sedimentation and sand filtration, regarding water quality. Membranes are very effective in removing particles and microorganisms which were shown by low turbidity, low microbial growth, and residual iron in permeate. NOM removal was similar to conventional treatment. Membranes were shown to effectively remove aromatic and hydrophobic fraction of NOM, however, conventional treatment was more effective in removing hydrophilic fraction. Quality of permeate from MF was constant throughout the pilot and very little differences were found between two different feed waters.