Development of biological treatment for sulfate- and metals-containing cold mining-impacted waters

When discharged into the environment, acidic mining effluents containing sulfate and metals cause the acidification of natural waters and harm to aquatic organisms. Environmental permit conditions are being tightened to prevent pollution, reduce emissions and waste, protect biodiversity, and to promote sustainable use of natural resources. Thus, more sustainable and effective water treatment solutions are required. Biological sulfate reduction can be exploited for the removal of sulfate and the precipitation of many metals as sulfides.

In this thesis, a biological treatment process was developed for cold mining-impacted waters (MIWs) using sulfate-reducing bacteria (SRB). The SRB consortium was enriched from a sediment sample collected from a boreal area and acclimatized to cold conditions. Succinate was used as the carbon source for the bacteria. The ability of the SRB consortium to utilize different low-cost carbon sources – conditioned sewage sludge, peat, and whey – was also studied. The consortium was used for the removal of sulfate and metals from synthetic and actual acidic MIWs in a continuous up-flow biofilm reactor. In addition, the SRB consortium and a pure SRB culture isolated from it were both tested for the bioregeneration of sulfate-laden anion exchange resin. The resin was separately loaded with sulfate and incubated with the SRB cultures, when the resin-attached sulfate ions were replaced with the ions present in the SRB solutions.

The cold-tolerant SRB consortium was found to grow at temperatures as low as 6 °C. The bacteria were able to utilize low-cost organic carbon sources, and promising results were achieved in the removal of sulfate from synthetic MIW, especially with conditioned sewage sludge as the carbon source. When the sulfidogenic bioreactor was operated at a temperature of 11.7 °C for the treatment of actual MIW, the sulfate reduction rate reached 4500 mg/(L×d) and 87% of the initial sulfate content was reduced. Most of the metals present in the actual MIWs precipitated either off-line with hydrogen sulfide gas formed in the reactor, or inside the reactor vessel. Bioregeneration of the sulfate-laden anion exchange resin proved successful with both pure and mixed SRB cultures. In the column experiments, the capacity of the resin was almost completely restored in bioregeneration using the pure culture. The results of this study provide valuable information about the possibilities of using biological sulfate reduction in the treatment of MIWs at low temperatures and of combining the advantages of sulfate reduction and ion exchange.