Ammonium-nitrate based explosives (NH4NO3) used within the operations of Kiruna iron ore mine release nitrate (NO3-) into the environment, potentially having adverse effects on local river-systems. One way of reducing NO3- impacts to the environment is through a woodchip denitrifying bioreactor (DBR). Waste rock leachate is collected and passed through the bioreactor, where denitrifying microbial communities reduce NO3- to nitrogen gas (N2) using a carbon energy source. However, the efficiency of the DBR present in Kiruna iron ore mine has declined since the start of its operation leading to lower values of NO3-removal throughout the years.  Denitrification being a temperature dependent process, a heating device was installed to warm up the water prior to the DBR treatment to counterbalance this decrease. The effect of which has been assessed within this thesis. Chemical analyses encompassing NO3-, nitrite (NO2-), ammonium (NH4+), total organic carbon (TOC), phosphorus compounds (tot-P, PO4-P), and bacterial abundance were then investigated along a flowpath in the DBR. Overall, the results have shown that with an increase in temperature prior to the treatment, TOC, tot-P, PO4-P release was improved. Moreover, NO3- removal doubled compared to the previous year. TOC, tot-P and PO4-P are the result of the hydrolysis process, transforming the woodchips in available carbon source and providing nutrients for the bacteria to perform denitrification. Similarly, the bacterial abundance presented a significant increase with temperature. This suggest that both hydrolysis and bacteria growth enhancement with temperature ultimately participated in the improvement of the denitrification reaction. Moreover, a long-lasting effect of temperature on NO3- removal was observed during a following cold period as NO3- removal stayed above 45% after two months without heating. It is suggested that the cost of heating can be reduced by inducing “heat pulse” instead of continuous heating. Adding a heating system prior to treatment represents a promising solution for the future of sustainable mining, particularly for mines located in extreme climates such as Kiruna. / NITREM