A European EIT Raw Materials project, ‘NITREM‘, has achieved positive results from a pilot scale implementation at LKAB’s Kiruna mine and is planned to be scaled up to full-scale. Nitrate concentrations of around 70 mg/litre in the leachate from waste rock piles was reduced to less than 10 mg/litre (an efficiency of 77%) after passing through a bioreactor. Nitrate is formed from the residual of ammonium nitrate-based explosives used in mining, and causes unwanted growth of algae leading to autrophication of waterways.
The bioreactor is a passive design, requiring little intervention. An oblong pit is constructed, the walls sealed with a geomembrane, and filled with wood chips and a dose of activated sewage sludge containing nitrogen-reducing bacteria. The bioreactor is covered an operates under anaerobic (oxygen-free) conditions. The bacteria utilise carbon from the wood chips to transform the nitrate to nitrogen gas, while the leachate flows through the reactor.A trial during 2019 recorded an average nitrate removal efficency of 77% with input nitrate contents of 61 – 87 mg/litre. During the operating period the average temperature was 3 °C, confirming that the microbial process is suited for use in cold climate conditions.
The process is proposed to be scaled up and implemented at further mining sites to reduce the impact on the environment. Further details are available at the NITREM project webpage. The project consortium consists of Uppsala University, WSP Sverige AB, Cedervall Arkitekter, Agencia Estatal Consejo Superior de Investigaciones Centificas, Boliden Mineral, LKAB, LTU Business and the Swedish University of Agricultural Sciences.
Passive wastewater treatment systems have been widely reported for the reduction of heavy metals, a review is given here. These results that the technology is also suitable for reduction of anions such as nitrate in the wastewater.