Filter technology has proved to be successful with regard to removal of pollutants from varying wastewater streams. A large number of filter materials have been investigated with promising results, among them several steel and iron slag materials. However, despite significant scientific evidence over the past 15 years on the efficiency of steel slag in P removal in particular from a variety of wastewaters around the world, currently there are very few full-scale steel slag filters in operation. In this paper we will present research efforts and experiences in using blast furnace slag in Sweden and electric arc furnace steel slag in Vermont, USA and hindrances faced in obtaining regulatory approvals for this innovative technology.