Phosphorus removal by constructed wetlands : substratum adsorption

The phosphorus removal characteristics of several gravel-based constructed wetland systems (CWSs) in the treatment of secondary sewage effluent was studied.Investigations were conducted on water quality parameters (redox potential, pH, dissolved oxygen and temperature) which affect phosphorus adsorption to substrata.Laboratory phosphorus adsorption experiments on Richmond CWS gravel substrata, a gravel used in Griffith CWS trials and a locally available soil, Hawkesbury sandstone, involved ion-exchange experiments and calculation of Langmuir and Freundlich adsorption isotherms and column adsorption/desorption trials.Six steelworks by-products were investigated in laboratory studies, to determine their potential for use as phosphorus adsorbers in a CWS: granulated blast furnace slag(GBF), blast furnace slag(BF), steel slag(SS), fly ash(FA), bottom ash(BA) and coal wash(CW).The ability to adsorb phosphorus was then correlated to the chemical attributes of each substratum.Of the six steelworks by-products screened in laboratory-based studies as substrata for P removal in a CWS, BF and SS slags showed the most potential due to their high phosphorus adsorption capacity and useable matrix size.Further research is recommended to evaluate the sustainability of using slags for P removal (as well as other contaminants present in wastewater), using full scale CWSs, which should include an evaluation of any likely environmental impacts using leachability and toxicity studies