AMD Treatment in New Zealand – Use of Small-scale Passive Systems

The general goal of passive AMD treatment is to reduce levels of acidity and metals to acceptable levels. Most treatment systems use either an oxidizing or a reducing strategy. In oxidizing systems, alkalinity is added along with dissolved oxygen; in reducing systems, dissolved oxygen is removed (if present) and then alkalinity is added. To test the applicability of oxidizing and reducing strategies to treat AMD, small-scale passive treatment systems have been constructed and tested successfully at two sites on the west coast of the South Island, New Zealand. At the Sullivan Mine, a reducing system consisting of a vertical-flow wetland reduced the levels of acidity by 100%, iron by 97%, aluminum by 100%, and nickel by 66%. The Pike River Coal Company has shown foresight by using the same system to treat AMD at a small adit within the Pike River Coal Field. The system reduced acidity by 100%, iron by 99%, aluminum by 96%, nickel by 95%, manganese by 51%, and zinc by 99%. At a third AMD site, the Blackball Mine, a laboratory-based experiment was used to test the applicability of an oxidizing system for AMD treatment. In the experiment, AMD was passed through a limestone leaching column over a nineday period. Acidity was reduced by 100%, iron by 83%, aluminum by 82%, manganese by 8%, and zinc by 64%. To determine if this treatment method reduces the toxicity of the AMD to aquatic invertebrates, a 96-hour ecotoxicity experiment was conducted. The results show a significant decrease in mortality in the treated AMD compared to untreated. These small-scale systems and laboratory-based experiments suggest that a full-scale treatment system using a reducing strategy may be successful at the Sullivan and Pike River Mines and that a system using an oxidizing strategy may be appropriate for the Blackball Mine. Additional Key Words: AMD, acid rock drainage (ARD), Pike River, Sullivan Mine, vertical flow wetlands, successive alkalinity producing systems (SAPS), water treatment