Performance of a sulfidogenic bioreactor inoculated with indigenous acidic communities for treating an extremely acidic mine water

Mining contributes to water pollution through the generation and uncontrolled release of sulfuric acid and metalliferous drainage (AMD). While sulfate reducing bacteria have been used for the bioremediation of AMD, one of the main limitations to using sulfate-reducing bioreactors is that most species are highly sensitive to acidity; therefore it is necessary to avoid direct contact between acidic mine waters and the bacteria, which is normally achieved by using off-line systems. There have been few successful applications using acidophilic, sulfate-reducing bacterial populations, which tolerate low pH and high metal concentrations and allow direct metal sulfide precipitation to occur within a single bioreactor unit. Here, we describe the performance of a low pH sulfidogenic bioreactor inoculated with an indigenous microbial community to treat mine-impacted water. The inoculum was obtained from anaerobic sediments collected from an acidic river located in northern Chile. The sulfidogenic bioreactor system (2.3 L) was operated as a continuous flow mode unit for 99 days at 30 degrees C and fed with synthetic water based on the chemical composition of the acidic river, characterized by extremely low pH (2.1) and, zinc and iron as main transition metals (0.5 and 2 mM, respectively). The bioreactor pH was set to 4.5 initially and was increased in stages to pH 6.0 during the experiment. The results show that zinc concentrations in liquors draining the bioreactor were below the detection level in most of the samples analyzed. In addition, by progressively increasing the glycerol concentration, it was possible to increase the removal of iron (70% of the total present), though more acetic acid (from 1 to 5 mM) was generated. Analysis of the microbial populations showed that they changed with varying operation parameters, and a known acetogenic sulfidogen (Desulfoporosinus acididurans) became more dominant over time. The modular unit used for treating mine-impacted water demonstrated robust activity with low complexity engineering.