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Preliminary evaluation of a constructed wetland treating extremely alkaline (pH 12) steel slag drainage

Lookup NU author(s): Dr Will Mayes, Jon Aumonier, Professor Adam Jarvis



High pH (>12) leachates are an environmental problem associated with drainage from lime (CaO)-rich industrial residues such as steel slags, lime spoil and coal combustion residues. Recent research has highlighted the potential for natural (‘volunteer’) wetlands to buffer extremely alkaline influent waters. This is ascribable to high CO2 partial pressures in the wetland waters from microbial respiration, which accelerates precipitation of calcium carbonate (CaCO3), and the high specific surface area for mineral precipitation offered by macrophytes. The research presented here builds on this and provides preliminary evaluation of a constructed wetland built in March 2008 to buffer drainage from steel slag heaps in north-east England. The slag drainage waters into the wetland are characterised by a mean pH of 11.9, high concentrations of Ca (up to 700mg/L), total alkalinity (up to 800mg/L as CaCO3) and are slightly brackish (Na = 300 mg/L; Cl = 400mg/L) reflecting native groundwaters at this coastal setting. Documented calcite precipitation rates (mean of 5g CaCO3/day/m2) from nearby volunteer sites receiving steel slag drainage were used to scale the constructed wetland planted with Phragmites australis; a species found to spontaneously grow in the vicinity of the discharge. The importance of biological activity in improving rates of calcite precipitation and thus lowering of pH is stressed. Secondary Ca-rich precipitates also serve as a sink for some trace elements present at low concentrations in the slag leachate such as Ni and V. The implications for scaling and applying constructed wetlands for highly alkaline drainage are discussed.

Publication metadata

Author(s): Mayes WM, Aumonier J, Jarvis AP

Editor(s): Billore, S

Publication type: Conference Proceedings (inc. Abstract)

Publication status: Published

Conference Name: International Conference on Wetland Systems Technology in Water Pollution Control

Year of Conference: 2008

Number of Volumes: 2

Pages: 307-315

Publisher: Vikram University, Ujjain