Steel slag filters to upgrade phosphorus removal in small wastewater treatment plants: Removal mechanisms and performance

Electric arc furnace steel slag (EAF-slag) and basic oxygen furnace steel slag (BOF-slag) were used as filter substrate in horizontal subsurface flow laboratory-scale filters designed to remove phosphorus (P) from a synthetic solution (similar to 10 mg P/L). The main objective of this study was to evaluate the influence of various parameters, including slag type, slag size, and slag composition, on P removal performance. Also, a series of chemical and mineralogical analyses was performed to determine the mechanisms of P removal achieved by steel slag in the filters. Over a period of 52 weeks of filter operation, small-size EAF-slag (5-16 mm) and small-size BOF-slag (6-12 mm) removed 98% and >99% of the inlet total phosphorus (TP), whereas big-size EAF-slag (20-40 mm) and big-size BOF-slag (20-50 mm) removed 88% and 95% of the influent TP, respectively. The main mechanism of P removal was related to CaO dissolution from slag followed by Ca phosphate precipitation and accumulation of the precipitates into the filters. P removal performance improved with increasing the CaO-slag content and with decreasing slag size, most probably because the specific surface available for CaO dissolution was increased. Also, the experimental results suggested that small-size slag was more efficient than big-size slag for the self-filtration of P precipitates. Chemical and mineralogical analyses indicated that, after precipitation, Ca phosphates may crystallise into the most stable form of hydroxyapatite. (C) 2014 Elsevier B.V. All rights reserved.