One of the major issues facing the Canadian mining industry is the treatment of effluent during and after closure of a mining property. Effluent treatment may be complicated by the presence of acid mine drainage (AMD) which even under the best reclamation scenario may require long term collection and treatment. While chemical treatment or some other form of active effluent treatment has traditionally been conducted in Canada, greater consideration has been given recently to forms of passive treatment.
Passive treatment systems utilize the chemical, biological and physical removal processes that often occur naturally in the environment to modify the influent characteristics. Passive treatment systems were initially considered attractive to treat acid mine drainage due to their lower costs of construction, operation and maintenance, and their ability to operate at remote locations with limited operational requirements. The objective of this project was to review passive treatment systems, and make recommendations on their applicability to treat acid mine drainage in Canada.
Although passive systems have been proven at many locations around the world, the Canadian climate and aquatic environments present considerable challenges to large-scale use in Canada. Biologically driven systems have low activity in cold temperatures and drought, while storm and spring “freshet” events demand flexible, strong systems.
In this review, four major types of passive technologies for the treatment of acid mine drainage have been examined:
• anoxic limestone drains;
• constructed wetlands;
• microbial reactor systems; and,
• biosorption systems.
In accordance with the scope of work, this document provides:
• summary of known passive treatment technologies;
• maintenance and monitoring requirements;
• life expectancy and long term implications (>100 years);
• implications of treatment product disposal;
• estimate of costs for the technologies described based on generic cases;
• ability to meet Canadian Metal Mining Liquid Effluent Regulations, and to control toxicity;
• descriptions of case studies including: range of flow, temperature, water chemistry, and identification of limiting conditions where available in the literature (Appendix A); and,
• general assessment of the applicability of current passive treatment systems to Canadian mine sites.
ANOXIC LIMESTONE DRAINS (ALD’S)
The basic design of an ALD is a trench filled with high quality crushed limestone, sealed under plastic and geotechnical fabric, covered by soil, through which an unaerated, contaminated effluent stream flows by gravity. As it flows through the system, the acid mine drainage gradually dissolves the limestone, releasing calcium as bicarbonate, thus raising the pH.
Based primarily on studies conducted at coal mines in the United States, ALD’s have been shown to be most effective for influent with dissolved oxygen, ferric iron (Fe3+) and aluminum concentrations of less than 1 mg/L, and sulphate concentrations below 2,000 mg/L. At higher concentrations the limestone may become armoured with oxides or gypsum, reducing the rate of limestone dissolution or plugging the system. In either instance, the ability of the ALD to generate alkalinity may be significantly reduced, and failure of the system may occur.
As a result of these strict influent requirements, ALD’s are expected to have only limited application to treatment of acid mine drainage at Canadian metal mines.
CONSTRUCTED WETLANDS
Constructed wetlands are ecological systems designed to optimize a variety of natural physical, chemical, microbial and plant-mediated processes. In a constructed wetland, influent AMD drains by gravity through the wetland, progressively undergoing metal removal and neutralization. Metals are removed by precipitation, chelation and exchange reactions, while neutralization is primarily achieved by the activity of sulphate reducing bacteria (SRB), or the increase in alkalinity from the chemical and microbial reactions including limestone dissolution.
Passive systems for AMD treatment have commonly used combinations of natural or constructed wetlands, Sphagnum peat and open ponds, supplemented by chemical amendments (mostly limestone) and organic substrate to increase alkalinity and reduce acidity. Sequential treatment of AMD to remove iron by oxidation, hydrolysis and settling in the aerobic stage, followed by SRB activity in an anaerobic stage to raise pH, is an effective combination.
For either aerobic or anaerobic cells, the design must maximize contact with the matrix, which can either be aerated water, or anaerobic substrate. It is essential that constructed wetlands are managed in terms of their individual components and their mutual interactions to gain a desired overall efficiency. Many of the metal removing mechanisms in a wetland are temporary and reversible, and can reach saturation; thereby reducing the wetland’s efficiency and decreasing their cost effectiveness. In addition, para-reversibility represents a challenge for treatment product monitoring or disposal.
Constructed wetlands have the potential to address AMD treatment at some Canadian sites where sufficient surface area is available, and can form the preferred alternative in terms of costs, efficiency and environmental safety. The ‘black box’ design approach that has been used in the past and is still being suggested, is not recommended. The design should be based on an understanding of the interactions between the chemical, microbial and plant-mediated components of the system and the engineering, climate and hydrogeological realities of the treatment site.
A well designed, constructed wetland is an efficient accumulator of metals and reaction products. Key to its efficiency, is the continuous physical, chemical and biotic matrix in the wetland. This capacity will be limited during freezing or high flow conditions. As a result, constructed wetlands may be most applicable to Canadian mines having shorter and milder winters, and at sites where a constant rate of flow can be maintained. An alternative treatment method may be required during the winter and during spring runoff conditions, or else large retention ponds are required.
MICROBIAL REACTOR SYSTEMS
Microbial reactor systems or bioreactors may be in an open or closed configuration, referring to whether they are exposed to the atmosphere. In either instance, the microbial reactor shell contains a biodegradable substrate (usually agricultural products such as mushroom compost or straw) which supports the growth of micro-organisms, which in turn treat acid mine drainage.
The cellulose of the agricultural products is degraded by cellulolytic bacteria to generate free sugars and other metabolites, which are further metabolized to provide substrate for the fermentative anaerobes. Under anaerobic conditions these free sugars are fermented to short chain organic acids or short chain fatty acids, which are suitable substrates to support the growth of sulphate reducing bacteria (SRB). SRB reduce sulphate to hydrogen sulphide which precipitates metal ions as low solubility metal sulphides. Concurrently, the sulphate reducing bacteria consume hydrogen ions and produce carbon dioxide during their metabolism, causing an increase in the pH of the solution due to reduced concentration of free hydrogen ions and the buffering effect of the CO2/bicarbonate buffer system.
Pilot plant data available suggests that bioreactors are a feasible technology for treatment of small AMD streams. Open bioreactors are expected to only be applicable at Canadian mines with mild or moderate winters. Closed bioreactors can operate anywhere that a fairly constant temperature can be maintained.
BIOSORPTION SYSTEMS
Micro-organisms, including bacteria, algae, fungi and yeasts, can efficiently accumulate heavy metals and radionuclides from their external environment. Biosorption systems in a wide variety of configurations rely on this ability to treat acidic drainage. Living cells can be used to treat effluent where metal concentrations are below toxic levels. The use of dead biomass in the form of commercial biosorbents eliminates the problems of metal toxicity, adverse climatic conditions, and the costs associated with nutrient supply and culture maintenance.
Only a limited number of studies have been conducted to date in regards to treating AMD with biosorption systems. While it does not appear that biosorption systems are an effective stand-alone treatment system for AMD, with further study they may become an alternative form of treatment of parts of an effluent stream, or as a final polishing step. The success of biosorption systems using living biomass during the winter is expected to be limited. Treatment efficiency will be lowered under poor growth conditions. Systems employing dead biomass are expected to have greater applicability, and may not be compromised by winter conditions as long as flow is maintained.
OVERVIEW
Passive treatment of acid mine drainage has a future in Canada, but is limited to applications where:
• flows are of relatively constant volume
• water temperature is greater than 7C (eg. mine water or embankment seepage)
• water chemistry of low to medium strength acidity and metal concentration
• low concentrations of aluminum and iron
• low sensitivity of the receiving environment to upsets in the passive treatment system.
Further research and field experience is needed to more precisely specify passive treatments for AMD in Canada to ensure that metal mining liquid effluent regulations are met all the time. No “passive treatment” system is truly passive. All systems require monitoring and replacement of consumed alkalinity or organic-based nutrients for bacteria. Also, metal precipitates need to be removed and in some jurisdictions sludge falls under “hazardous waste” regulations and disposal may be a significant challenge.
