Passive Biological Treatment of Acid Mine Drainage: Challenges of the 21 Century

Acid mine drainage (AMD), characterized by a low pH and high concentrations of sulphates and heavy metals, is a disquieting problem for the Canadian mineral industry and other industries elsewhere in the world. Traditional active systems, including lime neutralization, become costly in time or inapplicable in remote regions. Research has recently focussed on passive biological systems that have certain advantages such as low installation, operation and maintenance costs. The three groups of promising passive biotechnologies are wetlands, bioreactors, and permeable reactive walls. Their efficiency is sometimes limited as it depends on the activity of the sulphatereducing bacteria (SRB), which is in turn mainly controlled by the composition of the reactive mixture. The essential component of the reactive mixture is organic matter, which must be inexpensive, relatively biodegradable and available in the long term. The components of the reactive mixture must also allow for adequate flow within the system. Performance of the passive biological reactors is also related to the initial AMD load and the toxicity of the metals present. Several reactive mixtures were tested to find sources of organic matter that are both reactive and available in the long term. However, speciation of metals in effluents and in the reactive mixture and the toxicity of the treated effluents still need to be studied. Many challenges thus remain for a better prediction of the passive biological system efficiency.