Modelling of phosphate retention by Ca- and Fe-rich filter materials under flow-through conditions

Subsurface transport of orthophosphate (Pi) from fertilized agricultural fields to freshwaters may lead to eutrophication, reduced biodiversity and fish kills in inland waters. Reduction of Pi transport by means of filters in drains with Pi sorbing materials (PSMs) may be one way to improve water quality. The aims of this study were to investigate and model the effect of retention time (RT) and inlet Pi concentration on Pi sorption and desorption by two PSMs, CaO-based Filtralite (R) P and Fe oxide-based CFH-12, in a flow-through setting. Four inlet Pi concentrations (1.6, 3.2, 16 and 32 mu M) were all tested with 6 different retention times (0.5-9 min). Both materials' Pi sorption capacity and affinity is highly dependent on the Pi inlet concentration. CFH had a P sorption capacity and affinity 10 times higher than Filtralite (R) P. CFH released less than 10% of previously sorbed P. Filtralite (R) P, on the other hand, released >= 35% during desorption. Furthermore, Pi sorption by Filtralite (R) P was positively correlated to RT. This was not the case for CFH indicating that CFH will be capable of removing Pi even at high flow rates (i.e. low RT). Therefore, CFHwould be the preferred material for a P removal structure with fluctuating inlet Pi concentrations and RT. A PSM specific prediction model was successfully developed using flow-through sorption data. This model may be used to show how much Pi can be removed and how long a specific material will last until Pi saturation, as exemplified. (C) 2014 Elsevier B.V. All rights reserved.