Physico-chemical screening of phosphate-removing substrates for use in constructed wetland systems

The objective was to provide selection criteria for substrates that would enhance phosphate removal from waste water in a constructed wetland system (subsurface horizontal flow). Measured properties of seven substrates (bauxite, shale, burnt oil shale, limestone, zeolite, light expanded clay aggregates (LECA) and fly ash) were: pH, cation exchange capacity (CEC), hydraulic conductivity, porosity, specific surface area, particle size distribution and phosphate (P) adsorption capacity. Fly ash and shale had the highest P adsorption values, followed by bauxite, limestone and LECA. Longer-term experiments in which synthetic waste water was passed over shale and bauxite gave maximum P uptake values of 730 and 355 mg P kg^-1, respectively. X-ray fluorescence measurements showed that substantial precipitation of P had occurred on the shale surfaces. On the basis of these measurements it was concluded that, of the seven materials examined, shale had the best combination of properties as a substrate for constructed wetland systems (CWS).