The technology development has been driven by EU’s Water Framework Directive and will enable the industry to meet current and future discharge requirements. The outcome is a low cost and low maintenance technology ready for market introduction and customer testing. The project consortium consists of Uppsala University, WSP Sverige AB, Cedervall Arkitekter, Agencia Estatal Consejo Superior de Investigaciones Cientificas, Boliden Mineral, LKAB, LTU Business and the Swedish University of Agricultural Sciences.
The bioreactor technology was previously evaluated in a pilot-scale system at LKAB’s Kiruna mine, and is currently being tested in a full scale operation at the same mine. In the ”triangle area” at the Kiruna mine, a water collection system has been constructed, leading leachate from a waste rock pile to three bioreactors. NITREM’s bioreactor technology removes nitrogen (in the form of nitrate) from the waste rock leachate. A microbial process occurs in the bioreactor, where denitrifying bacteria reduce dissolved nitrate in the treated water to harmless nitrogen gas.
In brief, the bioreactor consists of a large oblong excavated pit, about two meters deep. The pit is lined with an impermeable geomembrane and is then filled with woodchips and a small amount of activated sewage sludge. The large amount of organic material in the bioreactor provides a carbon and energy source for the denitrifying bacteria. These bacteria require oxygen-free conditions in order to conduct denitrification; these conditions are promoted by directing water flow to the deeper sections of the bioreactor with the help of inner walls and a soil cover. Water is led into the bioreactor for treatment along one of the shorter sides. Once the water enters the bioreactor, it flows through the porous material and is released from the other side.
In order to optimize bioreactor efficiency, the water quality at the inlet and outlet are continuously monitored with on-line sensors or by manual sampling. In order to predict the performance of the bioreactor under different conditions (e.g. different flows and temperatures), computer simulations are used.
Operations at the current bioreactor system in Kiruna started on 19 September 2019 with a flow of 0.5 l/s through the bioreactors. The leachate from the waste rock pile is collected in a pumping chamber and is then pumped to the bioreactor. Monitoring data from the fall of 2018 indicate a nitrate removal of > 95 % with initial concentrations of 70-110 mg/l nitrate-nitrogen. Nitrate removal has continued at this level during 2019. The average temperature in the bioreactors is c. 3oC, and such a high level of nitrate removal is very good for a biological process that is temperature-dependent. The discharge water from the bioreactor is also analysed for nitrogen bi-products such as nitrous oxide, ammonium and nitrite. Concentrations of these compounds are generally low but the operation of the bioreactor system is being optimized in order to minimize the release of bi-products.
In addition to the bioreactor technology, NITREM includes a ”sustainable landscape design”. NITREM’s landscape design focuses on the construction of waste rock piles from mining operations. The waste piles are constructed so that they blend into the landscape in a natural way, while surface water and groundwater are collected and led into bioreactors for nitrate removal. This type of landscape design is not only conducive to the removal of nitrate, but also in promoting biodiversity and increasing the potential for land reclamation after ending mining operations.