Passive treatment of acid mine drainage water: a sustainable solution

Acid mine drainage occurs when rocks containing sulfide minerals, such as pyrite (FeS₂), interact with water and oxygen. This reaction produces sulfuric acid and dissolves metals such as iron, aluminum, manganese, etc. The acidic water can further dissolve nearby minerals, releasing toxic elements such as arsenic, cadmium and zinc. AMD lowers water pH and raises metal concentrations. It disrupts aquatic ecosystems and also generates solid waste, destabilizing fish habitats and polluting waterways.

Passive treatment systems reproduce natural processes to address AMD without relying on constant chemical inputs or energy. These systems use oxidation and reduction processes and rely on microbial activity to neutralize acidity and remove contaminants. They perform best under conditions with moderate acidity, low flow rates and fewer contaminants. While not universally applicable, passive treatment systems are cost-effective and environmentally sustainable, particularly when managing post-closure mine sites.

1. Aerobic wetlands

These shallow, vegetation-rich wetlands are designed to oxidize and remove metals, mainly iron and manganese, from slightly acidic to alkaline mine drainage water. Ferrous iron (Fe²⁺) oxidizes to ferric iron (Fe³⁺) in the presence of atmospheric oxygen, forming ferric hydroxide (Fe(OH)₃) through hydrolysis. Manganese requires a higher pH, typically above 8, to precipitate. However, aerobic wetlands are most effective when paired with pretreatment systems that ensure water alkalinity. These wetlands are also suitable for refining and improving water quality.

2. Anaerobic wetlands

These wetlands combine organic matter with limestone to create conditions that favour sulfate-reducing bacteria, which increase alkalinity. These sulfate-reducing bacteria convert sulfates into hydrogen sulfide, which reacts with dissolved metals to form stable metal sulfides. Limestone raises pH levels and provides bicarbonate alkalinity. However, the efficiency of anaerobic wetlands depends on the quality of their organic matter and limestone. Over time, precipitate buildup can clog the system, reducing performance.

3. Anoxic limestone drains

Anoxic limestone drains are buried limestone channels that prevent oxygen from entering, reducing the risk of clogging from iron hydroxides. Acidic water flows through the limestone, dissolving calcium carbonate, which neutralizes acidity and increases alkalinity. These drains are less effective for AMD with high aluminum concentrations, as aluminum can cause clogging. They are best suited for AMD from coal mines, which generally exhibit lower acidity and iron levels.

4. Vertical-flow wetlands

This system mixes organic materials and limestone in a layered, vertical flow design. The water flows through the organic layer, where oxygen is removed, and then flows through limestone, which raises the alkalinity. These systems are effective for treating highly acidic mine drainage but require regular maintenance to clear metal precipitates.

5. Open limestone channels and limestone leaching beds

These systems route AMD across or through limestone to neutralize acidity by raising pH levels, allowing metals to deposit as hydroxides. However, these open limestone channels are prone to clogging, as precipitates coat the limestone, diminishing long-term effectiveness.

6. Permeable reactive barriers

Permeable reactive barriers are underground barriers filled with reactive materials such as limestone or organic matter. These barriers intercept AMD, raising alkalinity and causing metals to deposit as sulfides or hydroxides. They work well for onsite treatment, particularly in remote or cold areas. However, their effectiveness can decline over time as precipitates fill up the reactive spaces.

Passive treatment systems offer the following benefits:

• Cost-effective : These systems typically involve lower installation and operating costs than active treatment systems.
• Eco-mindful : They blend naturally into their surroundings, improving biodiversity and complementing natural landscapes.
• Low-maintenance : While most passive systems require minimal upkeep, periodic inspections are necessary to maintain longevity.

Passive treatment systems are not suitable for AMD with high flow rates, extreme acidity or excessive contaminants.

Over time, precipitate buildup can clog these systems, requiring regular cleaning or material replacement.

Passive treatment systems offer an eco-mindful and cost-effective approach to managing AMD, particularly in post-mining restoration projects. Although not suitable for every scenario, these systems serve as a viable alternative to energy-intensive active treatments, promoting environmental recovery and supporting a more long-term mining legacy.

The BBA team includes engineers, chemists, geochemists and biologists who are capable of carrying out comprehensive AMD studies and projects for AMD challenges and passive treatment options.

J. Taylor, S. Pape, N. Murphy. A Summary of Passive and Active Treatment Technologies for Acid and Metalliferous Drainage (AMD).

C. Zipper, J. Skousen, C. Jage. Passive Treatment of Acid-Mine Drainage.

K. L. Ford. Passive Treatment Systems for Acid Mine Drainage.

J. Skousen, C. Zipper, A. Rose, P. F. Ziemkiewicz, R.t Nairn, L. M. McDonald, R. L. Kleinmann. Review of Passive Systems for Acid Mine Drainage Treatment.