Filter substrates that efficiently remove phosphorus (P) from wastewater can be used to optimise the nutrient removal by on-site wastewater treatment systems. A number of filter substrates have been investigated and the industrial by-product blast furnace slag has attracted attention as a promising substrate. To further evaluate the sorption and attenuation of PO(4)(3-), a column experiment was carried out under conditions of intermittent P loading. Two types of slag were used; crystalline slag (CS) and amorphous slag (AS). The slags were available in two particle sizes; 0-0.125 mm and 0.25-4 mm, respectively. The slags were mixed with inert sand to improve the hydraulic conductivity. In addition to the column experiment, a P fractionation experiment was conducted. The sorption experiment indicated that the removal of P by the CS (0.25-4 mm), the AS (0.25-4 mm) and the AS (0-0.125 mm) columns at the end of the experiment exceeded 95%. In the other columns, the P sorption had decreased remarkably indicating a beginning saturation of the substrates. The P fractionation experiment showed that the P sorption was largest in the uppermost layers. Clear gradients of the P sorption in the downward direction could also be observed. It was concluded that the P sorption by the coarser slags was high at the end of the experiment indicating that saturation had not been achieved.