International Trials of Vertical Flow Reactors for Coal Mine Water Treatment

Vertical flow reactors (VFRs) were tested at coal mine sites in New Zealand, South Korea, and the USA. The objective was to evaluate the iron removal efficiency and iron removal mechanisms during field trials at low pH and circumneutral pH, and to evaluate the potential use of VFRs as stand-alone systems or in combination with other passive treatment technologies. Total iron and manganese removal efficiencies at circumneutral pH (6–8) often exceeded 90%, with effluent concentrations less than 1 mg/L. This is attributed to both homogeneous and heterogenous Fe(II) oxidation and filtration of the precipitated ferrihydrite. Iron removal efficiencies at moderately acidic conditions (pH 3–4.5) averaged close to 40%, with an average 71.0% removal in one of the trials after iron removal capacity was stabilized. Microbial Fe(II) oxidation and precipitation as schwertmannite together with aggregation of colloidal and nano-particulate Fe(III) are suspected to be the main removal mechanisms. Iron solubility limited removal under very acidic conditions (pH?<?3). The reproducibility of the results with respect to previous research confirmed that VFRs can be used as stand-alone passive treatment systems for iron removal from mine waters with a footprint less than half of the area required by a conventional aerobic wetland. A VFR can also provide useful iron pretreatment for other passive treatment systems under circumneutral conditions, but would have to be combined with alkaline generating systems to achieve full iron removal from acidic mine waters.