Sorption of phosphorous to Filtralite-P--the effect of different scales.

Sorption of P to the filter material Filtralite-P 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-P 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-P 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-P (0.029 kg P m(-3) Filtralite-P 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-P 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.

Phosphorous sorption by Filtralite P--small scale box experiment.

Phosphorus (P) sorption of light weight aggregate, Filtralite P has been examined through a box experiment which imitates a horizontal subsurface flow wetland system. The results showed that after the P breakthrough, the outlet P concentration increased with time according to the amount of P applied. Small scale boxes with a high inlet P concentration (15 ppm) and high loading rate (5-2.5 L d(-1)) reached 90% saturation level relatively quickly (after about 150 days of operation), while the boxes with low hydraulic loading rate (1.25 L day(-1)) were 70-90% saturated after 18 months of operation. The total P removal was dependent on pH, Ca, and the inlet P concentrations, but was independent of the hydraulic loading rate. Extraction of total P from the saturated filter material showed that the sorbed P accumulated within the inlet section of the box and decreased gradually towards the outlet as well as towards the bottom layer. Even after large amounts of Ca had leached out of the system, Filtralite P still had a very high P removal capacity. After resting periods the P sorption capacity of the material was regenerated, the P concentration in the effluent decreased by 22-53%.