Amendment of constructed wetland substrates with iron (hydr)oxide coatings for enhanced removal of phosphorus

Abstract

Several small-scale laboratory studies have been performed on phosphorus (P) removal from solution by oxides and hydroxides ofiron. The capacity ofthese oxides and hydroxides to bond phosphate has been recognized for a long time. In recent years, investigations concerning possible use ofthese compounds in constructed systems designed to lower the levels of phosphorus in domestic wastewater have been performed. The focus has been mainly directed towards the use ofhydrous ferric oxides in constructed wetlands and advanced wastewater treatment systems. The objectives ofthis study were to qualitatively and quantitatively characterize phosphorus bonding by hydrous ferric oxides and to simulate a constructed system for removal of orthophosphates. Two approaches were used in the simulated system:

1. Deposition of ferric hydroxide on media (sand or gravel)

2. A composite system formed by mixing of an iron containing material (mill scale) with gravel.

Different methods to coat the media were also investigated, and isotherm experiments characterizing the adsorption of phosphorus by ferric hydroxide and mill scale were performed. A synthetic water containing orthophosphates was used as an influent for the constructed system (laboratory column) and the levels ofphosphorus were determined in the effiuent over an extended time period. Both systems (coated media and composite media) showed a phosphorus removal capacity of 50 to 75% from a 5 - 10 mg/L P feed solution. Difficulties in realizing a large simulated coated system have limited exhaustive conclusions concerning application of such a system in a constructed wetland. Leaching of zinc and iron from the gravel mill scale composite system may impose limitations on the use of such a system for the removal of phosphorus. However. based on the results of this study, both coated and composite systems appear promising and warrant further investigation.