Few studies have characterized reactive media for phosphorus (P) removal in passive treatment systems in terms of both batch and continuous flow experiments. This study uses basic oxygen steel slag (BOS) from a U.K. feedstock. Batch experiments demonstrated the effective removal of phosphorus with varying initial pH, initial P concentration, clast size, and ionic strength to represent environmental conditions. Continuous flow column experiments, operated for 406 days, with an influent P concentration of 1-50 mg/L (typical of domestic and dairy parlour waste) achieved removal of up to 62%; a second set of column experiments running for 306 days with an influent P concentration of 100-300 mg/L achieved a maximum effective removal of 8.39 mg/g. This figure is higher than that for other slags reviewed in this study (e.g., EAF Slag 3.93 mg/g and NZ melter slag 1.23 mg/g). XRD, E-SEM, and EDX data provide evidence for a sequential series of increasingly less soluble P mineral phases forming on the BOS surface (octa-calcium phosphate, brushite, and hydroxylapatite),which suggests that BOS may be a suitable substrate in passive treatment systems, providing a long-term P removal mechanism.
Pilot scale reactors have been installed at a mine water pumping station in County Durham, UK, to investigate the potential of Surface Catalysed Oxidation Of Ferrous Iron (SCOOFI) for rapid removal of iron, using BOS as the treatment medium. The water is circum-neutral, with [Fe] in the range 2-5 mg/L, [Mn] 0.60-0.70 mg/L, and [SO 4 ] approximately 300 mg/L. Saturated flow reactors with residence times of approximately 20 minutes, and a range of BOS particle sizes (20-100 mm) have been used to establish the influence of surface area on removal efficiency and system longevity. Iron removal rates are in the order of 80% for all reactors. Differences between reactors, and the potential application of this type of passive treatment, are described in detail in the paper
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