Design and evaluation of an electrochemical bioreactor for water treatment

Water treatment issues are a problem worldwide. More and more gold mines are being opened in Africa and other parts of the world; these mines employ the carbon-inleach technique to recover gold from complex cyanide solution. The resulting process waste streams are high in cyanide, nitrate-N/ nitrite-N and in some cases arsenic. This thesis research looks at the design and evaluation of an electrochemical bioreactor for nitrogen and arsenic removal from contaminated water. Initial test results obtained by passing a mine's contaminated water through a conventional up flow bioreactor showed 100% nitrate-N and 85% arsenic removed for an initial arsenic and nitrate-N of 2,000 ppb and 50,000 ppb, respectively. The flow rate was 0.74 ml/min with a residence time of 2 days for the circuit of 3 reactors. Using the gathered information and improved reactor configurations, the design of the electrochemical bioreactor was tested. The performance of the electrochemical bioreactor was compared with a conventional up flow bioreactor with the same features as the electrochemical bioreactor but without applied potential. Results show that for a feed solution of 2,000 ppb of arsenic and 50,000 ppb nitrate-N; arsenic, nitrate-N and nitrite-N removal rates achieved was 100 % in both reactors at a nutrient addition of 4.0 g/ liter of contaminant solution at a contaminant feed rate to reactor of 5.042 liters per day. In a bid to optimize the reactors further with less nutrient addition, the arsenic concentration of solution coming out of the tail end of reactor without applied potential (Rl) stabilized at 75 ppb with nutrient addition at 2.0 g TSB per liter of contaminant solution. With the same concentration of 2000 ppb arsenic in the feed, the tail solution coming out of reactor with applied potential (R2) stabilized at 35 ppb with a batch nutrient addition rate of 2.0 g TSB per liter of contaminant solution and an applied potential of IV. Nitrate-N and nitrite-N removal rate remained at 100% in both reactors at 2.0 g TSB per liter of contaminant solution. The electrochemical bioreactor reactor with applied potential (R2) thus has an improved combined removal rate of arsenic and nitrate-N as compared to the conventional bioreactor in the two week period of testing. Additionally, it was also observed that arsenic remaining in solution after the passage of 2000 ppb arsenic contaminant solution through the reactors averaged 20 ppb, both at the bottom and the top of the conventional up-flow bioreactor. However, with the increasing reduction gradient from the bottom to the top due to the applied potential, a further reduction in arsenic at the top of the electrochemical bioreactor was the case with the bottom and top arsenic levels averaging 20 ppb and 0 ppb, respectively, for the nutrient addition rate of 4.0 g TSB per liter of contaminant solution. v