Factors affecting the efficiency of passive mine drainage treatment systems

Anaerobic bio-reactors were used to test the effect of influent pH on the removal efficiency of metals from acid-rock drainage. Two studies utilized a near-neutral pH, metal-laden influent to examine the metal removal efficiency and residence time requirements of the reactors. Another study used the more typical low pH simulated influent. The results suggest that bacterial sulfate reduction may be a zero order kinetic reaction relative to sulfate concentrations used in the experiments, and may be the factor that controls the metal removal efficiency in these treatment systems. The sorptive capacities of the substrate used had not been exhausted during the experiments as indicated by the loading rates of removal of metals exceeding the mass production rates of sulfide. Microbial sulfate reduction was less in the reactors receiving low pH influent during short residence time experiments. Sulfate reducing bacteria may have been inhibited by high flows of low pH water.