Nitrification-Denitrification of Thiocyanate, Ammonia, and Nitrates in Highly Contaminated Gold Mine Effluents Using Methanol as Energy Source

Chemical technologies efficiently remove cyanide from wastewater, but only marginally degrade thiocyanate (${\mathrm{SCN}}^{-}$); they also produce ammonia nitrogen (${\mathrm{NH}}_3\text{-}\mathrm{N}$) and nitrates (${\mathrm{NO}}_3^{-}$), which require complementary treatment. Biological nitrification-denitrification is environmentally friendly and of low cost, but little is known about the treatment efficiency for highly contaminated gold mine effluents. This study assesses the performance of a nitrification-denitrification pilot-scale system using methanol as an energy source. A gold mine effluent is sampled at the inlet of an operating treatment plant. The effluent is alkaline (pH 7.5–8.4) and contains high concentrations of ${\mathrm{SCN}}^{-}$ ($435\pm 53\mathrm{mg}/\mathrm{L}$) and ${\mathrm{NH}}_3\text{-}\mathrm{N}$ ($41\pm 8\mathrm{mg}/\mathrm{L}$). Physicochemical characteristics of the effluent, both before and after the treatment, are evaluated over a 140-day period while varying the hydraulic retention time (HRT) and dilution factor. The results indicate the presence of biological activity. The low HRTs and dilution factors adversely influence the ${\mathrm{NH}}_3\text{-}\mathrm{N}$ removal. ${\mathrm{SCN}}^{-}$ and ${\mathrm{NO}}_3^{-}$ are efficiently removed ($>97$ and $>80\%$, respectively), demonstrating the satisfactory performance of the nitrification-denitrification approach and the effectiveness of methanol as an energy source.