Sorption of phosphorous to Filtralite-PTM—The effect of different scales

Sorption of P to the filter material Filtralite-PTM was examined at a small, medium and large scale. In the small- and meso-scale laboratory models, the sorbed amount of total phosphorus (P) was heterogeneously distributed with more P sorbed in the inlet zone and the bottom layers. The full-scale system had, on the other hand, the highest sorbed concentration in the outlet region. The overall P sorption capacity of the material was 8030, 4990 and 521 mg P kg−1 Filtralite-PTM for Box 1, Box 2 and meso scale, respectively. This equals 4.4, 2.8 and 0.29 kg P m−3 material, respectively. However, the maximum sorption capacities found were 2500, 3887 and 4500 mg P kg−1 Filtralite-PTM for the two small-scale box systems and the meso-scale container, respectively. In the full-scale system the overall P sorption capacity of the material was 52 mg P kg−1 Filtralite-PTM (0.029 kg P m−3 Filtralite-PTM) with a maximum sorbed amount of P of 249 mg P kg−1. Results from both the small- and meso-scale system show that when a constructed wetland (CW) is saturated, i.e. when the outlet concentration has reached its maximum allowed concentration of 1.0 mg P l−1, only parts of the filter material will have reached the sorption capacity. Sequential extractions of Filtralite-PTM showed that the loosely bound P, Ca–P and Al–P were the primary P sorption pools both in the small-scale models and in the full-scale CW. However, the proportion of these three fractions varied with time and change in pH. A white product precipitated in the outlet zone of both the small-scale box models as well as the onsite CW. The surface of these precipitation particles was identified by X-ray diffraction and SEM method as CaCO3 and precipitated Ca- and Mg-phosphates.