Phosphorus Removal in Constructed Wetlands Using Gravel and Industrial Waste Substrata

Abstract

The phosphorus removal efficiency of three gravel based constructed wetland systems (CWSs) has been investigated in a two year study in which secondary sewage effluent was treated. The constructed wetlands systems, 100m × 4m × 0.5m with an impervious liner, comprised an unplanted gravel “control” trench and gravel trenches planted with monoculture stands of either Typhaorientalis or Schoenoplectusvalidus. Inlet and outlet phosphorus concentrations and vertical and linear profiles of phosphorus were determined to characterise immobilisation/translocation through the systems. Laboratory phosphorus adsorption experiments were conducted with regional gravels and alternative adsorptive media including industrial slag and ash by-products. Phosphorus adsorption in the large scale gravel systems was variable and ranged from −40% to 40%. Laboratory adsorption capacity studies conducted with the gravel substratum indicated that field adsorption potential could be successfully simulated and modelled. Ion exchange experiments have been used to evaluate gravel and industrial conglomerates, with a view to improving phosphorus immobilisation through substratum selection and effluent flow management. Langmuir and Freundlich isotherms characterised phosphorus adsorption and the maximum adsorption capacity of regional gravels ranged from 25.8 to 47.5 μg P/g compared to blast furnace slag 160 to 420 μg P/g and fly ash 260 μg P/g. These results indicate that further investigations into the inclusion of industrial waste substrata in a CWS are warranted.