Air-Cooled Blast Furnace Slag. II: Phosphate Removal from Simulated Rainfall Events

This paper is the second in a series on the potential use of air-cooled blast furnace (ACBF) slag for phosphate (${\mathrm{PO}}_4$) removal from passive stormwater runoff systems—that is, the ACBF slag aggregates function as reactive base/subbase layers beneath porous pavement systems. Batch tests containing simulated stormwater stock solutions of 1, 5, and $10\mathrm{mg}/\mathrm{L}$ ${\mathrm{PO}}_4$ were equilibrated with AASHTO Nos. 10, 67, and 3 ACBF slags for various periods up to 72 h and showed rapid uptake of ${\mathrm{PO}}_4$. More than 90% ${\mathrm{PO}}_4$ removal was achieved by the AASHTO No. 10 ACBF slag for all dosing levels and time intervals, and the AASHTO No. 67 ACBF slag attained similar performance after an hour of contact time. The AASHTO No. 3 ACBF slag media attained $\ge 50\%$ ${\mathrm{PO}}_4$ removal at the $1\mathrm{mg}/\mathrm{L}$ ${\mathrm{PO}}_4$ dosing level for all time intervals. Batch testing at the $1\mathrm{mg}/\mathrm{L}$ ${\mathrm{PO}}_4$ dosing level typically showed $\mathrm{pH}>9$ for all ACBF slags, and often $\mathrm{pH}\gg 9$ due to the strongly alkaline nature of ACBF slag. Conversely, a flow-through test mold containing a 15.2-cm (6-in.) thick compacted AASHTO No. 67 ACBF slag media subjected to 24 h wet/dry cycles of inundation by a $1\mathrm{mg}/\mathrm{L}$ ${\mathrm{PO}}_4$ solution maintained $\mathrm{pH}<9$ while removing $>50\%$ ${\mathrm{PO}}_4$ for 33 cycles. The measured and estimated total ${\mathrm{PO}}_4$ uptake for the AASHTO Nos. 67 and 3 ACBF slags, respectively, were used as inputs to a stormwater management model to estimate the service life of the ACBF slags for ${\mathrm{PO}}_4$ removal in three land use applications. An AASHTO Nos. 67 and 3 ACBF slag layered system with respective thicknesses of 15.2 cm (6 in.) and 76.2 cm (30 in.) was estimated to yield an average service life of 10 years for five selected cities on the Great Lakes evaluated in the model, and then also end with as little as 20% site coverage or equivalent gallery volume.