Removing phosphorus with Ca-Fe oxide granules – a possible wetlands filter material

Phosphorus (P) is one of the nutrients causing eutrophication in many of our waterways. In the present study, we investigated Sachtofer PR Ca-Fe oxide granules as a potential P sorption material (PSM) for constructed wetlands. We found the P sorption with various experiments as follows: the 24 h batch experiment with the highest initial concentration of 50 mgP L−1 yielded 0.48 mgP g−1 P removal per mass unit, the kinetic P removal batch experiment of 600 h duration yielded 1.25 mgP g−1, the maximum phosphorus adsorption capacity estimated from the Langmuir equation yielded 23.78 mgP g−1, and the long term flow-through experiment with drainage ditch water yielded indicative saturated sorption of 1.4 mgP g−1. Flow-through experiments revealed that phosphate removal was rapid and the efficiency was 10–70%, depending on the retention time and age of granules. Possible weaknesses of this material for sorption filter systems were found to include the loss of mass caused by the rapid dissolution of gypsum, increased sulfate and calcium concentrations in the water, and rapidly changing hydraulic conductivity. Considering hydraulic and chemical properties, further pilot experiments are necessary to develop technical solutions for optimal use of Ca-Fe oxide granules in sorption filter systems at constructed wetlands.