Passive treatment of acid mine drainage using waste mussel shell, Stockton Coal Mine, New Zealand

Acid Mine Drainage (AMD), a result of historical and current coal mining, and the associated oxidation of pyrite within the coal measures, is a significant environmental liability for Stockton Opencast Mine, located on the West Coast of the South Island, New Zealand. This liability is likely to persist for at least 100 years. With up to 6,000 mm of rainfall per year, often in intensive events (some up to 100 mm/hr), water management is critical for the control of adverse effects that include low pH, dissolved metals (including aluminium), and high suspended solids in drainage waterways. Traditionally, neutralisation of AMD at Stockton coal mine has been carried out by direct dosing of ultra-fine limestone (UFL) to affected waterways.

An alternative passive treatment system utilising mussel shell, generally considered a waste stream and dumped at landfill, offers a cheaper, less labour-intensive method of reducing the acid load reporting to the primary waterways on site.

An operational field trial was conducted on the Manchester Street Seep and indicated a payback period of 1,027 days (up to March 2012) when compared to the cost of UFL treatment. The bioreactor has been treating water with an initial pH of <3 to a pH >7; and achieving a metal removal efficiency of 96-99% for Al, Fe, Ni, Tl, and Zn. Low DO and initial high concentrations of CBOD5 and ammonia nitrogen in the discharge due to remnant mussel flesh had negligible effect on the receiving waterway. The removal of sediment and sludge build-up on the surface of the system is required approximately every two years to increase the system’s porosity and treatment capacity.