Classification of Acid Mine Drainage Treatment Technologies
The AMD technologies can be classified according to three levels of development, i.e., laboratory scale, pilot scale, and proven technologies (DWA, 2013). Laboratory scale technologies include all technologies that have only been tested at a theoretical laboratory scale (DWA, 2013). There is still, however, insufficient data available to attempt to test or implement the technology at full scale. Quite a significant number of recently developed technologies fall under this category as the transition to full-scale development is associated with very high risk and the current state of the mining industry has made most companies very risk averse. Pilot scale technologies are technologies that have been simulated in pilot plants to prove the chemical, physical or biological principles on a larger scale. The risks associated with technology that has been implemented on a pilot plant scale may be less than the risks associated with laboratory scale technology (DWA, 2013). A proven technology is one that has been in operation at a scale comparable with the scale required for the application under consideration (DWA, 2013). The implementation of any novel technologies in a financial tight market requires that any new technology be completely proven in order to achieve its full market deployment. Whilst massive research in the AMD treatment options has resulted in the generation of a significant number of laboratory scale technologies, there are relatively few that have been proven and implemented on a commercial scale.
AMD treatment technologies are commonly categorised as either 'passive' or 'active', both potentially combining physical, biological and chemical approaches. The main purpose of both types is to lower acidity and toxic metal concentrations, raise pH and often lower sulphate concentrations and salinity (Taylor et al., 2005). The active treatment methods involve regular reagent inputs for continued operation, thus are labour intensive. The AMD treatment systems can be used for both operational mine sites and occasionally post-closure scenarios (Taylor et al., 2005). Within the two categories of AMD treatment technologies, the methods can also be classified as (1) traditional (or conventional) treatment methods (i.e., methods that follow the pattern of an ordinary wastewater treatment plant) and (2) methods taking a new and original approach (innovative treatment methods, e.g., anoxic limestone drains [ALDs], constructed wetlands, etc.). However, it is noted that most of the traditional methods tend to follow the active treatment route, whereas the other category tends to involve a more passive treatment approach.