The trend in the Danish society is toward disconnection of stormwater from the combined sewers and, where needed, local treatment using the best available technologies (BAT). The aim here was to assess a fast filtration technology for removal of particulate matter in stormwater with an emphasis on colloidal and nanosized particles. During the project period it rained 8.5 % of the time and the average daily rainfall was 2.9 mm/day. Based on three individual storm events it was found that 95 % of the particles were <10 µm. The nanosized particles (0.01-1.2 µm) were found to be anionic charged and in the size-range of 100 nm. The physical treatment of particle filtration at 10 µm was inadequate to remove the small particles identified in this project. Coagulation with a cationic coagulant and subsequently flocculation is suggested as process improvements technologies.