Research on Phosphorus (P) removal capacity by blast furnace slags (BFS) has been undertaken in Sweden for the last decade. Both laboratory experiments and field trials have been carried out. While laboratory investigations revealed that BFS has a high P-sorption capacity (95–100%), P removal in field trials was much lower, ranging from 40 to 53%. In addition, a number of problems have been observed in BFS field testing including clogging, sulfuric odor and environmental (regulatory) concerns about possible leaching of heavy metals from the slag. In spite of these problems, and questioning by the environmental regulatory authorities, research continues to provide evidence that BFS can be regarded as a suitable filter media, and attempts have also been undertaken in order to further improve the P-removal capacity of this adsorbing material.