Phosphorus Removal from Wastewater in a Constructed Wetland

In a constructed wetland using a high hydraulic conductivity matrix such as gravel, the wastewater will flow subsurface in the root and rhizome region of the aquatic macrophyte where bacterial growth on the surfaces can absorb pollutants and break them down. Bacteria can assimilate phosphorus in their cell structures; but once a steady state of biomass is reached within the bed, no net phosphorus removal will result thereafter. From the foregoing, the conversion of the soluble phosphorus to an insoluble particulate form which would be contained within the bed, by chemical means, would be an efficient method of removal. Two experimental gravel matrix constructed wetlands, each 30 m long × 5 m wide, one planted with Phragmites australis and the other an unplanted control, were selected for the dosing experiments. The use of lime plus alum is shown to be an economic and efficient dosing mixture for the removal of phosphorus as the insoluble phosphate within the bed itself.