NITREM is a three-year innovation project, funded by EIT RawMaterials, with the aim of developing a service that combines geomorphic landscape design for waste rock deposits with a nitrate-removing bioreactor technology that reduces nitrogen levels in waste rock leachate.
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.
A geomorphic approach to waste rock dump design creates landforms that mimic the function and aesthetics of more natural environments. The basis of a geomorphic design is to create a landform that does not require ongoing treatment to prevent soil erosion, is hydrologically stable, is visually appealing and promotes a self-sustaining ecosystem. The goal is to design landforms that adhere to geomorphic principles and behaviours of water flow on natural landscapes. By mimicking the function of nature, such landforms are designed to efficiently convey water without excessive erosion or sediment loading on the surrounding environment. The hydrological control also helps to augment bioreactor performance.
Such landforms offer more topographic variation than conventional designs. The valleys that result from establishing a functional drainage system provide a diversity of microclimates including slopes and slope aspects that result in varied water harvesting, soil conditions, sunlight exposure and ecological niches, which ultimately promote the development of a more resilient and diverse ecological composition. Geomorphic landform design has been identified as Best Available Technique (BAT) in the European Union.
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.
Within the framework of the NITREM project, three bioreactors were constructed and brought into operation on 19 September 2018. Each bioreactor has treated flows up to 0.5 l / s. Leachate from the waste rock deposit is collected in a underground reservoir before being pumped to the bioreactors. Monitoring data from 2019 has shown a average nitrate removal of 77% at an average temperature of 3 ºC in the bioreactors with input nitrate contents of 61-87 mg / l nitrogen. Such a high nitrate removal at such low temperature is very good for a biological process that is temperature dependent. The discharge from the bioreactor is also analyzed for nitrogen by-products such as nitrite, ammonium and nitrous oxide. The levels of these substances are generally low and the operation of the bioreactor system is being optimized to minimize emissions of by-products.