Metal removal mechanisms in a short hydraulic residence time subsurface flow compost wetland for mine drainage treatment

The performance of an innovative pilot-scale subsurface flow compost wetland for the attenuation of zinc in mine drainage was investigated. The particular novelty of the system, which operated under ambient environmental conditions, was the short hydraulic residence time (HRT) of 7.5-14.5 h. Short HRT is crucial because it reduces absolute wetland size and construction cost, which can be major impediments to use of wetlands. Over a 2 year period mean treatment efficiency was 67.5% (total Zn) and 84.4% (0.45 μm filtered Zn). Mean volume-adjusted removal rates for total and filtered Zn were 0.92 g/m3/day and 1.05 g/m3/day respectively. Both water and compost analyses showed bacterial sulfate reduction to be the most important Zn removal process within the subsurface flow wetland: water analyses showed consistent decreases in sulfate concentration through the system, whilst Acid Volatile Sulfide − Simultaneously Extracted Metals (AVS-SEM) analysis of the compost revealed concentrations of Zn up to 12,227 mg/kg and AVS up to 9066 mg/kg. Geochemical modelling using PHREEQC also showed that biogeochemical conditions within the wetland favored Zn attenuation as its sulfide: ZnS was the only solid Zn phase that was super-saturated in the wetland effluent water (mean Saturation Index of +9.52). Preliminary investigation of the use of liquid waste carbon sources to further enhance performance were ambiguous, but the results of the research nevertheless show the potential of short hydraulic residence time subsurface flow compost wetlands as a low cost treatment option for metal-polluted abandoned mine drainage.