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%.