Development of Passive Treatment Systems for Treating Acid Mine Drainage at Stockton Mine
Acid mine drainage (AMD) at Stockton Coal Mine is generated from the oxidation of pyrite in carbonaceous mudstones exposed during surface mining. Acidity production causes metals such as Fe and Al to leach from overburden materials including feldspars. Water chemistry and flow were monitored at numerous AMD seeps at Stockton. Manchester Seep, which daylights at the toe of an overburden embankment, was identified as a suitable research site for trialling passive-treatment systems designed to neutralize acidity and sequester metals in AMD. Median dissolved metal concentrations from the Manchester seep were; 62.9 mg/L Fe, 32.5 mg/L Al, 0.0514 mg/L Cu, 0.175 mg/L Ni, 0.993 mg/L Zn and 0.00109 mg/L Cd. Treatment of this water is achieved downstream by the Mangatini fine limestone dosing plant, however in the interest of assessing other technologies this work investigated the use of bioreactors to assess the potential of passive treatment technologies to treat the Manchester Seep AMD. Geotechnical parameters, including hydraulic conductivity, were measured for various mixtures of organic and alkaline waste products suitable for use as bioreactor media. Seven mesocosm-scale bioreactors were fed aerated Manchester Seep AMD in a laboratory set-up for nearly four months. Bioreactors incorporating mussel shells performed better than limestone and were capable of sequestering >0.80 mol metals/m3 substrate/day (or neutralising acidity at rates >66 g CaCO3/m2/day) while removing >98.2% of all metals. Tracer studies were conducted on two bioreactor systems containing the same substrate composition but different reactor shapes. Results will be applied to reactor models to better ascertain the relationship between reactor hydraulics and treatment performance. Pilot-scale passive systems incorporating three treatment stages were designed and are currently being installed to treat a portion of Manchester Seep AMD on site. The first stage consists of a sedimentation basin to remove sediment. The second stage includes three bioreactors in parallel to test treatment effectiveness of different substrate mixtures, depths and hydraulic configurations. Data derived from the mesocosm lab study were used to optimise these designs. The final treatment stage consists of three different aerobic wetland configurations, also operated in parallel, to compare their effectiveness at providing oxygenation and tertiary treatment of metals (primarily Fe) from bioreactor effluent.