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

Abstract

Chemical technologies efficiently remove cyanide from wastewater, but only marginally degrade thiocyanate (SCN−); they also produce ammonia nitrogen (NH3-N) and nitrates (NO3−), 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 SCN− (435±53 mg/L) and NH3-N (41±8 mg/L). Physicochemical characteristics of the effluent, both before and after the treatment, are evaluated over a 14-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 NH3-N removal. SCN− and NO3− are efficiently removed (>97 and >8%, respectively), demonstrating the satisfactory performance of the nitrification-denitrification approach and the effectiveness of methanol as an energy source.