Phosphorus Removal From Stormwater Using Electric Arc Furnace Steel Slag

Date
2013-07
Authors
Okochi, Nnaemeka Chinonyerem
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Publisher
Faculty of Graduate Studies and Research, University of Regina
Abstract

The use of electric arc furnace steel slag (EAF) as a viable add-on technology to existing stormwater systems for the removal of dissolved phosphorus was investigated. Existing popular stormwater treatment measures such as grassed swales, detention ponds and constructed wetlands, while effective in removing certain components such as suspended solids, have not fared very well in dealing with dissolved nutrients and heavy metals. This can create a host of health and economic challenges for the communities that rely on these water supplies for their upkeep and well-being. Synthetic stormwater solutions of different metal to phosphorus concentration ratios observed in actual stormwater were used in the experiments. Parameters such as stormwater composition, phosphorus concentration, metal concentration, initial pH, temperature, slag mass and slag particle size were varied in batch and column experiments to determine the effects of different environmental and treatment system conditions on the phosphorus removal efficiency of the slag. EAF used in the experiments was also examined to uncover its mineral composition and propose a theory on the pathway for phosphorus removal. Observations from the sorption experiments and EAF characterization studies were subsequently considered to propose a theory for phosphorus removal. Results demonstrated that physisorption contributed to phosphorus uptake. However, it was not the key phosphorus removal mechanism. Instead, surface complexation and precipitation occurring on Fe sorption sites via ligand exchange were found to significantly influence the removal of phosphorus from solution.The presence of cadmium, lead and zinc had a minimal effect on the phosphorus removal process, while copper was a significant inhibitor of phosphorus uptake by the EAF medium. Reduced removal of phosphorus by slag was evident in the copperdominant stormwater solution. This was attributed to the formation of a stable complex between the copper and the slag at Fe sorption sites via the ion-exchange surface model. The selective occupation of these active Fe sorption sites, as well as the stability of the complex formed, inhibited further phosphorus uptake by EAF. Stormwater composition, phosphorus concentration, metal concentration, initial pH, temperature, slag mass and slag particle size were also found to significantly impact the effectiveness of EAF in removing phosphorus from a given stormwater system. Interactions between these factors were also significant in producing varying effects on the phosphorus removal response. An empirical model with an 87% correlation was proposed to predict the removal of phosphorus via EAF from stormwater with parameters in the following range: phosphorus concentration of 2 to 5 mg/L, copper concentration of 0 to 2.2mg/L, lead concentration of 0 to 8.3 mg/L, zinc concentration of 0 to 11.9 mg/L and a temperature of 7 to 22 °C. This study concludes that EAF is an adsorbent with the potential to effectively sequester dissolved phosphorus from stormwater. This means that EAF can act as a viable end-of-pipe add-on technology to existing stormwater treatment systems for improved effluent quality.

Description
A Thesis Submitted to the Faculty of Graduate Studies and Research In Partial Fulfillment of the Requirements for the Degree of Doctor of Philosophy in Environmental Systems Engineering, University of Regina. xxiii, 289 l.
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