Solutions for control of nitrogen discharges at mines and quarries

The growth of Finnish extractive industry increases the need to study and monitor different environmental impacts and to integrate the environmental issues more tightly into the overall framework of the activities. Nitrogen released from explosives or from mining processes and ending up in the water system can have negative environmental effects. The MINIMAN project, financed by the Tekes Green Mining Programme and the industry aimed at comprehensive understanding on the nitrogen issue in the extractive industry. The project collected essential data on nitrogen compounds present in the environments of mines and quarries, and developed technologies for the treatment of nitrogen containing mine water. Based on a technology review, selective sorption, electrochemically assisted membrane separation and biological treatment were selected to be studied and developed in the project. Denitrification and nitrification studies on synthetic mine wastewater in laboratory scale revealed that denitrification was possible at low pH (2.5) and temperatures as low as 7°C. Nitrification was more pH sensitive. Both processes tolerated metals (Fe, Ni, Co and As). In the adsorption tests with zeolite, complete ammonium removal from real mine wastewater was achieved with a hydraulic retention time of 2 minutes, with complete regeneration. In addition, a novel electropervaporative nitrogen capture technology was developed. Further larger scale testing and development of all technologies is required to ensure their feasibility in a real mining site. A follow-up period in lysimeters showed that the explosives originated nitrogen content of left over stones from natural stone quarrying is relatively low and ca. half of the nitrogen is leached within the first weeks after detonation. The main sources of nitrogen are process and dewatering waters, irrespective of the scale of extractive activity. The total potential nitrogen load to the environment depends on the scale and type of the activity as well as the type of explosives used. In addition to factors related to the activity itself, the overall nitrogen management should take into account the background concentrations and sensitivity of the local ecosystem.