Passive water treatment in the mining sector

Popular technologies

Over the years, several mining clients have turned to passive technologies to treat water released into the environment, especially for closed mines, and more recently for active mines.

In this article, passive technologies or treatments are defined as those that do not use chemicals or mechanical equipment (or very few) to clean mine impacted water. These technologies rely more on natural processes and mechanisms for mine impacted water treatment.

While mining companies choose these technologies for obvious economical reasons, they are also seeking to reduce their environmental footprint. Passive water treatment technologies can come in many forms, including:

• Technologies for neutralizing water acidity, used for example in anoxic limestone drains system both to neutralize water acidity and to increase water alkalinity and pH.
• Aerobic and anaerobic constructed wetlands (vertical and horizontal flow).
• Phytoremediation systems with carefully selected plants that accumulate contaminants, such as Alyssum bertolonii (hyperaccumulator of nickel) and Nicotiana tabacum.
• Bioreactors that use the actions of sulfate-reducing bacteria to reduce sulfates in water to hydrogen sulfide and increase pH levels. The hydrogen sulfides then combine with metals to produce metal sulfides that are precipitated and retained in the reactor.

Passive technologies have been explored in Alberta and British Columbia’s coal mines over the past few years, with technology such as Saturated Backfill Zone technology, or SBZ. In this process, part of the waste rocks (backfilled in the pit) saturated with water could generate anoxic conditions by using dissolved carbon available in the effluent or by adding an external carbon source such as methanol. These conditions along with the actions of microorganisms allow selenium attenuation by reducing selenate to selenite and to selenium element which will be retained in the pit. In the same way, because reactions take place under anoxic conditions, denitrification can be carried out by the microorganisms responsible for reducing nitrates to nitrogen gas.

Other passive technologies or semi-passive technologies (especially when chemicals are used), evaluate the sequestration and settling of certain metals in waste rock piles.

In short, these technologies, imitating natural phenomena for impacted mine water treatment, seem to produce in general, interesting results at relatively low costs compared to active water treatment technologies.

Risk management

However, there are risks associated with these technologies that must be assessed based on, for example, the project type and scope, as well as the mine life. An analysis of all operational, environmental and technological risks must be carried out on a case-by-case basis to ensure the technology can be applied.

Few factors to consider are :

• The need to treat water on a year-round basis;
• The availability of space to store water to remedy the effect of rainfall peaks and to allow controlled flow distribution;
• The maximum contaminant absorption capacity of the plants used;
• The rate at which the waste rock pile’s pores clog or foul;
• The robustness of the water treatment technology under bad weather or extreme conditions;
• The short-, medium- and long-term chemical stability of the extracted contaminants and the possibility of releasing them;
• The need to use a geomembrane for certain technologies or in certain cases;
• The coating of alkaline materials used in drains by iron precipitates.

BBA takes a holistic approach that considers all these factors, drawing on the know-how of its biology, water treatment, geotechnical, automation, geochemistry and hydrotechnical experts.

Conclusion

Passive water treatment is a socially and environmentally responsible approach to treat impacted mine water. It replicates natural processes and mechanisms that are advantageous and beneficial to reproduce. However, this approach is not suitable for every project, so it is important to conduct a holistic risk analysis and consider all factors at play. In many instances, chemicals must be used to optimize the action of microorganisms, as is the case with removal of nitrate and attenuation of selenium in anoxic environments. This is one of the reason these technologies are referred to as semi-passive treatment in some cases.

BBA supports a systemic approach that brings together all necessary expertise and offers a sustainable, adapted solution based on the knowledge and experience of biologists, geochemists and water treatment specialists. Mechanical, civil and electrical engineering capabilities also allow BBA to properly review and document limitations, estimate capital expenditures (CAPEX) and operating expenditures (OPEX), and then select the optimal technology.

This allows BBA to choose the right technology based on the project constraints for the short, medium and long term conditions. Finally, regardless of the passive or semi-passive treatment chosen, regular monitoring is recommended due to the sensitivity of these technologies to external climate factors, particularly for long-term conditions, after mine closure.

[1] SOR/2002-222, under the Fisheries Act.