Constructed Wetlands for Treating Processed Mine Water - an Irish Case Study
Mine wastewater is characteristically elevated in metals and sulfate and conventionally treated with costly chemical applications. The development of passive treatment systems, employing both biotic and abiotic processes, has been recognized as an economically feasible, ecologically acceptable technology in the last decade. However, to-date most of these passive systems have been applied to abandoned mine waters for the primary purpose of increasing pH and removing metals. Two experimental-scale treatment wetlands were constructed and monitored at an active lead/zinc mine (Tara Mines) in Ireland, to treat alkaline mine seepage with elevated sulfate (and also metal) levels. Each system comprised three 12 m² (2 m depth) in-series surface-flow cells viz., inflow, vegetated and outflow and contained spent mushroom substrate (SMS). Typical aqueous concentrations of 830 mg L⁻¹ sulfate, 0.15 mg L⁻¹ lead and 2.0 mg L⁻¹ zinc entered the treatment wetlands at a flow rate of c. 650 mL min⁻¹. Anaerobic substrates, in which sulfate-reducing bacteria were indigenous, were conducive to biological reduction of sulfate to sulfide. Sulfide subsequently precipitated with metal cations. Monitoring of these wetlands over a 2-year period showed successful (maximum) removal of sulfate (29 g m⁻² day ⁻¹ (69%)), lead (6.6 mg m⁻² day ⁻¹ (64%)), and zinc (70 mg m⁻² day ⁻¹ (98%)). These contaminants were somewhat associated with the vegetation roots but more significantly so with the substrates. The interacting processes within the wetland ecosystems responsible for decontamination of the wastewater are currently being elucidated and quantified using a systems dynamic model. The communities of colonizing macroinvertebrates, macrophytes, algae and microorganisms also contributed to development of diverse ecosystems and a successful alternative treatment process.