Jarno Mäkinen, VTT Technical Research Centre of Finland Ltd, P.O. Box 1000, FI-02044 VTT, Finland, e-mail: jarno.makinen(at)vtt.fi
The other in-situ treatment technologies consist of chemical, physical and/or biologic mechanisms. The main technologies are permeable reactive barriers (PRB) and in-situ treatment of mining lakes with sulphate reducing bacteria (SRB). The PRB is constructed by excavating a trench to the soil and filling it with permeable and reactive material. Then, polluted water is flowing through the reactive material, which adsorbs or transforms pollutants to harmless form. Plenty of active materials have been introduced for PRBs, with metallic iron, organic matter and lime being used currently at pilot/industrial size in the field of mine water treatment (U.S. EPA. 1998). The in-situ treatment of mining lakes relies on sulphate reducing bacteria, which utilize organic substrate and sulphate in anoxic conditions and produce hydrogen sulphide. In addition to reduction in sulphate levels, produced hydrogen sulphide precipitates variety of dissolved heavy metals (e.g. Cu, Zn, Ni…) and degradation of substrate also increases pH and alkalinity (Vestola and Mroueh, 2008).
Other in-situ treatment technologies require initial work to obtain water purification effects. The strengths of these technologies are that after initial work (e.g. construction of PRB reactive zone or pumping the organic substrate to the bottom of the mining lake) very little work is needed to treat the water; technologies are closing the passive methods. The drawbacks related to these technologies are that they cannot be utilized in all cases. Moreover, they are not yet the major treatment method, and therefore there is lack of evidences in long term treatment efficiency. Also, if preliminary work is not done properly, they have to be modified, or even closed, increasing the costs.
In the following articles permeable reactive barriers and in-situ treatment of mining lakes with SRB have been evaluated:
Figure 1. Open pit of the Hammaslahti Cu mine, in which sulphate reducing bacteria have been used to treat the mine water after mine closure. Photo © P. Kauppila, GTK.
U.S. EPA. 1998. Permeable Reactive Barrier Technologies for Contaminant Remediation. United States Environmental Protection Agency.
Vestola, E. & Mroueh, U-M. 2008. In Situ Treatment of Acid Mine Drainage by Sulphate Reducing Bacteria – Guide to the pit lake treatment. VTT Research Notes 2422 (in Finnish).