This thesis aimed at developing the use of electric arc furnace steel slag (EAF-slag) and basic oxygen furnace steel slag (BOF-slag) in filters designed to upgrade phosphorus (P) removal in small wastewater treatment plants. An integrated approach was followed, with investigation at different scales: (i) Batch experiments were performed to establish an overview of the P removal capacities of steel slag produced in Europe, and then to select the most suitable samples for P removal; (ii)Continuous flow column experiments were performed to investigate the effect of various parameters including slag size and composition, and column design on treatment and hydraulic performances of lab-scale slag filters; (iii)Finally, field experiments were performed to investigate hydraulic and treatment performances of demonstration scale slag filters designed to remove P from the effluent of a constructed wetland. The experimental results indicated that the major mechanism of P removal was related toCaO-slag dissolution followed by precipitation of Caphosphate and recrystallisation into hydroxyapatite (HAP).Over 100 weeks of continuous feeding of a synthetic Psolution (mean inlet total P 10.2 mg P/L), columns filled with small-size slag (6-12 mm BOF-slag; 5-16 mm EAFslag)removed >98% of inlet total P, whereas columnsfilled with big-size slag (20-50 mm BOF-slag and 20-40mm EAF-slag) removed 56 and 86% of inlet total P,respectively. Most probably, the smaller was the size ofslag, the greater was the specific surface for CaO-slagdissolution and adsorption of Ca phosphate precipitates.Field experiments confirmed that EAF-slag and BOF-slagare efficient substrate for P removal from the effluent of aconstructed wetland (mean inlet total P 8.3 mg P/L). Overa period of 85 weeks of operation, EAF-slag removed 36%of inlet total P, whereas BOF-slag removed 59% of inlettotal P. P removal efficiencies increased with increasing temperature and void hydraulic retention time (HRTv),most probably because the increase in temperature and HRTv affected the rate of CaO dissolution and Caphosphate precipitation. However, it was found that longHRTv (>3 days) may produce high pH of the effluents(>9), as the result of excessive CaO-slag dissolution. However, the results of field experiments demonstrated that at shorter HRTv (1-2 days), slag filters produced pH that were elevated only during the first 5 weeks of operation, and then stabilized below a pH of 9. Finally, a dimensioning equation based on the experimental results was proposed.
