The bioreactor technology developed within the NITREM project was previously evaluated in a pilot-scale system at LKAB’s Kiruna mine, and was tested in a full-scale operation at the same mine during the project time (2018-2021). The image below depicts the water treatment setting, where an underground reservoir collected groundwater runoff from a waste rock deposit. Waste rock drainage was then pumped to three separate bioreactors. With NITREM’s bioreactor technology, nitrogen (in the form of nitrate) is removed from the waste rock leachate through the denitrification process. Denitrifying bacteria reduce dissolved nitrate in the treated water to harmless nitrogen gas. The treated water is then discharged to a local recipient.
The denitrifying bioreactors built within the project each consisted of a large oblong excavated pit (ca 40 m long, 7 m wide, 2 m deep). The pit was lined with an impermeable geomembrane and was then filled with woodchips and a small amount of activated sewage sludge. The large amount of organic material in the bioreactor provided a carbon and energy source for the denitrifying bacteria. These bacteria required oxygen-free conditions in order to conduct denitrification; these conditions were promoted by directing water flow to the deeper sections of the bioreactor with the help of inner walls and a soil cover.
Water was led into the bioreactor for treatment along one of the shorter sides. Once the water entered the bioreactor, it flowed through the porous material and was released from the other side.
The three bioreactors were constructed and brought into operation on 19 September 2018.