Passive water treatment technologies are increasingly being considered for mine site closure in the Yukon. Efforts are currently underway in Yukon to test, compare and contrast passive treatment technologies with conventional technologies. This study aimed to provide additional information about the effectiveness of passive treatment technologies for mine water treatment in cold climates. To test the hypothesis that bioreactors can effectively treat mine-impacted water at low temperatures, four bench-scale, continuous flow bioreactors were assessed for their potential to remove As, Se and Sb from mine effluent over a one year period. More specifically, the objectives of this study were to: 1) assess the efficiency of removal of As, Sb and Se from a synthetic drainage with relatively high initial concentrations of the three metals in cold conditions, as well as from actual leachate collected at the Eagle Gold site, a proposed gold mine in central Yukon, 2) evaluate the effect of using wood chips as part of the composition of the bioreactor, and 3) assess the effect of the freeze/thaw transition on the bioreactors’ performance.
Protocols were: Phase 1) the bioreactors were operated with the addition of liquid methanol as a carbon source, and in an environment with uncontrolled temperature during the fall until the bioreactors froze solid; Phase 2) the bioreactors were thawed in a fridge at a stable temperature of 6°C; Phase 3) the bioreactors were operated and monitored at 6°C; Phase 4) the bioreactors were operated at 6°C in the same conditions but without the carbon addition.
Results show that all bioreactors significantly decreased As, Sb and Se concentrations when carbon was added independent of influent concentration. Using the leachate produced on site with a 1% methanol addition, 38%, 90% and 95% of As, Se and Sb was removed over phases 1 to 3. Using the highly concentrated metals drainage with an average of 5 mg/L As, 0.5 mg/L Se and 0.03 mg/L Sb, the removal efficiencies were >85% , >87%, and >99% for Sb, As and Se, respectively.
In addition to metal removal performance, the effect of freeze and thaw was assessed. The results indicate that the use spruce chips in the bioreactor substrate improved As removal during Phase 1 and helped improve removal and mitigate the effect of freeze/thaw on As, Sb and Se removal. It is suggested that the wood chips provide an adequate support to either protect and/or favor biofilm growth, which may promote sulfate reduction and metal sulfides precipitation. Finally, the efficacy for adding carbon was assessed by comparing the results for a 153-day period (with 1% methanol added) to a 237-day period without methanol added. The results suggest that the effect of carbon addition on Se and Se removal performances was negligible, while there was measureable effect on As removal performance. We surmise that adding methanol provided an easily biodegradable carbon source, which is much needed by the microorganisms responsible for As removal at cold temperatures.
This study is one of very few studies reported in the literature that demonstrates Sb removal from water by an anaerobic bioreactor. Overall, it demonstrates the potential application of passive anaerobic bioreactors as a technique to remove As, Sb and Se from mine water effluent in a cold climate. It also suggests that the addition of easily biodegradable carbon to the bioreactor may be required for bioreactors built in Yukon for As removal where cold temperatures and freeze/thaw conditions occur.