Use of blast furnace granulated slag as a substrate in vertical flow reed beds: Field application

Research was conducted at Middle East Technical University (METU), Ankara, Turkey in 2000 to determine whether a reed bed filled with an economical Turkish fill media that has high phosphorus (P) sorption capacity, could be implemented and operated successfully under field conditions. In batch-scale P-sorption experiments, the P-sorption capacity of the blast furnace granulated slag (BFGS) of KARDEMİR Iron and Steel Ltd., Co., Turkey, was found to be higher compared to other candidate filter materials due to its higher Ca content and porous structure. In this regard, a vertical subsurface flow constructed wetland (CW) (30 m²), planted with Phragmites australis was implemented at METU to treat primarily treated domestic wastewater, at a hydraulic rate of 100 mm d^-1, intermittently. The layers of the filtration media constituted of sand, BFGS, and gravel. According to the first year monitoring study, average influent and effluent total phosphorus (TP) concentrations were 6.61 ± 1.78 mg L^-1 and 3.18 ± 1.82 mg L^-1; respectively. After 12 months, slag samples were taken from the reed bed and P-extraction experiments were performed to elucidate the dominant P-retention mechanisms. Main pools for P-retention were the loosely-bounded and Ca-bounded P due to the material's basic conditions (average pH > 7.7) and higher Ca content. This study indicated the potential use of the slag reed bed with higher P-removal capacity for secondary and tertiary treatment under the field conditions. However, the P-sorption isotherms obtained under the laboratory conditions could not be used favorably to determine the longevity of the reed bed in terms of P-retention.