In the Netherlands interest in advanced treatment is increasing now that almost all waste-water treatment plants apply full biological treatment and nutrient removal. Membrane filtration, especially ultrafiltration, of waste-water treatment plant effluent appears to be a very promising method. However, many experiments show a rapid decrease in flux, which means that frequent and intensive cleaning is required. Various methods for the development, as this article shows. Further research will lead to a new understanding of the filtration and fouling processes, and will finally result in highly efficient and cost-effective ultrafiltration plants.