Nitrogen removal from waste rock drainage in northern Sweden with denitrifying woodchip bioreactor

Although nitrogen is not a traditional contaminant when considering the detrimental impacts of mine waste leachate on aquatic ecosystems, it is a common pollutant of concern in underground iron ore mining where waste rock leachate has a neutral pH and a low metal content. This is the case in northern Sweden, where environmental authorities, supported by the EU Water Framework Directive, have imposed strict regulations on nitrogen discharges to oligotrophic surface water systems. Requirements for lower nitrogen releases has driven the development and application of a bioreactor technology for nitrate removal at LKAB’s Kiruna iron ore mine.

A full-scale woodchip denitrifying bioreactor has been in operation since September 2018 in Kiruna for the removal of nitrate (NO₃^-) from waste rock leachate. Drainage from the waste rock pile is intercepted in a subsurface groundwater collection reservoir at the toe of the waste rock pile and pumped at an average rate of 22 m³/d into the bioreactor. Leachate from the low-sulfur waste rock is characterized by neutral pH (average pH 7), moderate alkalinity (108 mg/L HCO₃^-), and elevated concentrations of sulfate, NO₃^- and chloride (average concentrations 670, 61 and 102 mg L^-1 respectively).

During 2019, and average nitrogen removal efficiency was 77%: during the 165 day sampling period, 189 kg NO₃-N were removed in the bioreactor, which is primarily attributed to denitrification. A net production of 26 kg of NO₂-N was measured. Nitrous oxide (N₂O) emissions in gas and aqueous phase are low from the bioreactor and the primary product of denitrification is assumed to molecular nitrogen (N₂). Dissolved N₂O concentrations were on average greater at the bioreactor inlet (277 µg L^-1) than at the outlet (179 µg L^-1), although variations were substantial during the summer months with a net dissolved N₂O export from the bioreactor on one occasion in late summer. The flux as N₂O from the bioreactor surface varied from 1 – 28 mg N₂O m^-2 d^-1. In addition to nitrate removal, zinc concentrations were reduced by, on average, 88%.