Undetonated ammonium nitrate - based explosives are readily soluble in water and quickly enter into the mine water and process water at a mine site. Process water is eventually discharged to a surface water body and, without treatment, can lead to eutrophication in the recipient. In order to investigate the application of nitrate removal by denitrification in the cold climate of northern Sweden, laboratory and field experiments have been conducted. Laboratory column experiments were designed to determine denitrification rates at relevant temperatures for northern Sweden. Based on the column experiments at low temperature, a hydraulic residence time of ca. 24 hours was one of the design criteria used for dimensioning a barrier system in the field. In autumn 2009, a pilot-scale barrier system was constructed of sheet metal at the Malmberget iron ore mine. The barrier (9m x 2m x 1.5m) appears as an open basin with three inner dividing walls, and is filled with a reactive mixture consisting of crushed rock, sawdust, and sewage sludge. A small fraction of the water discharging from the Malmberget clarification pond was fed by pump to the barrier at a rate of ca. 5 m3 day-1.
In the pilot-scale barrier system, analytical results from 2010 indicated that nitrate (NO3-) removal generally lay in the range between 11 and 77% of influent nitrate concentrations. In order to achieve a higher removal rate, the barrier system needed to be supplemented with an additional carbon and energy source for denitrifying bacteria. In 2011, a sodium acetate solution as an external electron donor was continuously added to the barrier system along with the influent water. Analytical results from 2011 indicate that the barrier system in Malmberget removes >95% of the incoming nitrate during summer months (average daily temperature 14oC for 2011) with the addition of acetate. The barrier did exhibit decreases in performance toward the end of the study period in 2011, but this is likely the result of a lower hydraulic residence time in the barrier at this time.
Nitrite (NO2-) concentrations in effluent waters were initially in excess of influent values, indicating the production of nitrite in the barrier. However, both nitrite and ammonium (NH4+) concentrations decreased to below detection limits after the addition of acetate (CH3COO-), suggestion the occurrence of anaerobic ammonium oxidation (anommox) in the barrier substrate. During this period, alkalinity values peaked at 500 mg HCO3- L-1 in effluent waters, indicative of organic carbon (acetate) oxidation through denitrification:
5CH3COO- + 8NO3- + 3H+ → 4N2(g) + 10HCO3- + 4H2O
Stable isotope analyses of δ15N and δ18O in nitrate demonstrate an enrichment in 15N and 18O along the flowpath, supporting the conclusion that denitrification is responsible for nitrate removal.
22nd V.M. Goldschmidt Conference