James J. Gusek, P.E.
Knight PiŽsold and Co.
1050 17th Street, Suite 500
Denver, CO 80265-0500
Phone: 303-629-8788
Email: jgusek@knightpiesold.com

ABSTRACT

There are hundreds of passive treatment systems accepting mining influenced water (MIW) throughout the world. Some systems do not perform to design expectations while others, including volunteer systems, have successfully operated relatively unattended for decades. The primary reasons for this situation include the common misconceptions that 1) a "cookbook" approach to design is valid for a wide array of MIW chemistries and site conditions and 2) low-maintenance means "no maintenance." Passive treatment systems for MIW are typically man-made ecosystems that are designed to handle a specific range of metal loading conditions and MIW geochemistry. Thus, when design conditions are exceeded, the suite of microbial to macroscopic ecosystems may be slow to recover or mature. This should be no surprise to designers. But when a particular system fails, it may be inappropriately attributed to the technology, not the design. This paper presents a standard "phased" design protocol that appears to work and provides examples of sub-par performance of selected passive treatment systems.

INTRODUCTION AND BACKGROUND

Natural systems have been removing metals from water for eons; examples include pyrite fixed into coal beds and bog iron ore deposits. For the past dozen years, wetlands and bogs have been the natural method of choice of engineers at many abandoned mine land sites for improving water quality. Contaminant reductions are being seen through the precipitation of hydroxides, sul-fides, and carbonates, and pH adjustments. Local conditions, oxidation state, and water and soil chemistries dictate whether such natural reac-tions occur under oxidiz-ing (aerobic) or reducing (anaero-bic) conditions. Man-made or constructed wetlands/bioreactors employ the same principles as natural wetlands but are designed to optimize processes occurring in natural wetland ecosystems. The key goal of bioreactors/wet-lands is the long-term immobilization of metals in the substrate materials. Metals are precipitated as carbonates or sulfides in the "bioreactor" substrate (anaerobic cells) and as oxides and hydroxides in aerobic (rock filter) cells.

Anaerobic bioreactors have been successful at substantially reducing metal concentrations and favorably adjust-ing pH on metal mine drainages. It is generally recognized that the bacteria commonly found in cattle and other domestic animal intestinal tracts include sulfate reducers and a consortium of other bacteria. Hence, manure from cows or other animals has been frequently used as bacterial inoculum for anaerobic biotreatment cells. These same bacteria are found in many natural wetlands and bogs and in lakes and ocean water. Aerobic biotreatment systems are similar to "natural" wetlands in that they typically have shallow depths and support vegetation in the form of algae and emergent plant species.

Since 1988, there have been rapid advancements in understanding the functioning of wetland/ bioreactor systems. The first large-scale aerobic system (7.6 m3 per min. or 2,000 gpm capacity) was built in 1992 by TVA in Alabama (Brodie, et al., 1993). The West Fork Unit system (4.5 m3 per min. or 1,200 gpm capacity) was constructed in Missouri in 1996 and is the first large-scale anaerobic biotreatment system (Gusek, et al., 1998 and Gusek, 2000). At West Fork, an aerobic "rock filter" cell provides polishing treatment for manganese and other parameters.

The passive treatment technique holds promise over typical chemical treatment methods because large volumes of treatment residuals are not generated; in fact, residual disposal may be delayed for decades or longer. One volunteer passive treatment system outside an abandoned metal mine has been identified in Ireland that has operated unattended since about 1889, over a century (Beining and Otte, 1997). This volunteer passive treatment system reportedly has 70% of its total metal removal capacity remaining.

METALS REMOVAL MECHANISMS IN PASSIVE TREATMENT SYSTEMS

Many physical, chemical, and biological mechanisms are known to occur within passive treatment systems to reduce the metal concentra-tions and neutraliz-e the acidity of the incoming flow streams. Notable mechanisms include the following:

• Sulfide and carbonate precipitation catalyzed by sulfate reducing bacteria (SRB) in anaerobic zones.
• Hydroxide and oxide precipi-tation catalyzed by bacteria in aerobic zones.
• Filtering of suspended material.
• Metal uptake into live roots and leaves.
• Adsorp-tion and exchange with plant, soil and other biologi-cal materials.

Remarkably, some studies have shown that plant uptake does not contribute signifi-cantly to water quality improve-ments in passive treatment systems (Wildeman, et al., 1993). However, plants can replenish systems with organic material and add aesthetic appeal. In aerobic systems, plant-assisted reactions appear to aid overall metal-removal performance, perhaps by increasing oxygen and hydroxide concentrations in the surrounding water through photosynthesis-related reactions and respiration in the plant root zone. Plants also appear to provide attachment sites for oxidizing bacteria/algae. Research has shown that microbial processes are a domi-nant removal mechanism in passive treatment systems (Wildeman, et al., 1993). One anonymous re-searcher considered a passive treatment system as a "bioreactor with a green toupee," referring to the substrate where most of the bioreactions occur and the collection of plants that grow on top of the treatment cells.

THE DESIGN PROCESS

The design of passive treatment systems is a somewhat inexact science due to the variety of water chemistries requiring treatment and the variety of materials that can be used in construction. For chemically simple coal drainage (relatively mild pH water containing iron and manganese and little or no aluminum), engineers and scientists at the former US Bureau of Mines developed "cookbook" design criteria (Hedin, et al., 1994) for aerobic systems that are still being followed (sometimes inappropriately) today. Wildeman, et al., (1993) developed a phased design protocol that is appropriate for more complex acidic as well as neutral to net alkaline drainage chemistries.

These two approaches represent end points in a design philosophy continuum. The inherent danger in any "cookbook" design approach is a typical inability to properly address situations lying outside the range of conditions that were originally used to develop the standardized design criteria. The treatment of low pH water containing dissolved aluminum is especially problematic and outside the original US Bureau of Mines design criteria which addressed the issue by suggesting restrictions in the application of anoxic limestone drains (ALDs). A precise and reliable aluminum design guideline has yet to be developed for ALDs and probably should not be even considered. That is because of the complexity of aluminum chemistry. While iron can be more or less be precipitated aerobically as ferric hydroxide or anaerobically as a sulfide or carbonate, the list of aluminum mineral species found in nature (and thereby possible in a passive treatment system) is extensive.

The "cookbook" design challenge represented by the individual case of aluminum is multiplied many fold when additional heavy metal contributions are considered, as may be the case for some MIW sources at metal mines. Adding the effects of varying anionic concentrations and water temperature further reinforces the futility of considering cookbook approaches to passive treatment design. Still, the design engineer must start somewhere.

The situation is not as bleak as it may sound. The mining, chemistry and other industries have used a phased design process, probably since the dawn of engineering. The concept is simple: start small, learn from failures, and build on successes until the data required to properly design a full-scale treatment system is obtained. With that data, the risks of the full-scale system failure or less than optimum performance are significantly reduced. Wildeman, et al. (1993) proposed a design protocol that included laboratory-, bench- and pilot-scale phases. The approach has been used at over three dozen mine drainage sites.

A phased-approach design project typically begins in the laboratory with static tests, graduating to final testing phases (bench and pilot) performed at the site on the actual MIW. Bench scale testing will determine if the treatment technology is a viable solution for the MIW and will narrow initial design variables for the field pilot. A proper bench scale test will certainly reduce the duration of the more costly field pilot test. Field pilot test duration can range from days, to months, to years, depending on the nature of the technology. Depending on the nature of the equipment and personnel needed, significant costs may be incurred during the field pilot tests: about $500 to $1,000 per week, mostly for sampling and analysis. Compare this to $5,000-$10,000 per week for active treatment pilot tests. More detailed descriptions of testing phase activities follow.

TESTING PHASES

• Lab Scale Testing - This phase of testing is usually conducted in the laboratory. It might include:
  • Paste pH and redox testing of passive treatment material substrates,
  • Static bottle tests to isolate and identify beneficial bacteria for a given cell type (aerobic or anaerobic), and
  • Static limestone "cubitainer" tests for limestone consumption/alkalinity determination.
    
    
• Bench Scale Testing - This phase of testing is typically performed in the controlled environment of a laboratory but can be conducted in the field. It is most appropriate for evaluating the dynamic response of different mixtures of organic substrates, system configurations or metal loading rates. This level of testing should be relatively inexpensive to set up; most of the cost should be allocated to sampling and analysis. To keep costs down, bench-scale test units can be constructed with off-the-shelf items like trash cans and kiddie wading pools, items typically found at do-it-yourself/home improvement stores and gardening centers. Once the range of dynamic variables has been narrowed, one should proceed to onsite pilot testing.
  
  
• Field Pilot Scale Testing - This phase of testing is performed at the site, on the actual MIW. Information gathered during these tests should provide an accurate operating cost estimate as well as final capital cost data. If the field pilot study does not meet the necessary discharge standards, another treatment technology should be considered or added on. It is also important to determine the sludge characteristics during this phase, will the sludge be a hazardous or non-hazardous? Can the treatment sludge be disposed of on the mine site? Sludge management and organic substrate replacement may comprise the principle "operating" costs of a passive treatment system.

Upon completion of the field pilot test, full-scale design should take into consideration seasonal fluctuations in flow rate and seasonal fluctuations in chemical composition that may not have occurred during a shorter pilot test. Equalization ponds or tanks should be included in the design to handle these fluctuations.

It is important to note that there are two equally important aspects of full-scale passive treatment system design: bio-geochemistry and filtration. The bench and pilot test results should have yielded the conditions necessary to establish the proper bio-geochemistry or dominant geo-ecosystem in a given treatment cell to develop stable chemical precipitates. However, constructing an ideal bio-geochemical environment is a wasted effort if the metal precipitates formed are flushed out of the system because of inefficient filtration. Among other factors, this aspect of a proper system design is influenced by the grain-size distribution and compacted density of organic substrates, the settling and flocculating characteristics of the precipitates, and the retention times of the settling cells.

WHY SOME SYSTEMS FAIL

There are four major reasons why some passive treatment systems do not function as intended:

• No Design - e.g., "Just build a swamp here, fill that pond over there with manure and call it good."
• Inadequate Design - undersized for load, applying the wrong geochemical approach, phased design lacking, complex geochemistry, improper startup and operational procedures.
• Inadequate Maintenance - (low maintenance does not mean "NO" maintenance).
• Last Minute Design Changes - departure from well-conceived construction specifications to respond to a field fit conditions can affect system performance - experience helps.

Brief discussions of these reasons follow.

Inadequate Designs: Given the wealth of technical information available in the scientific literature, it is rare to find a passive treatment system based on a "seat of the pants" design. However, without much design background, any person with a strong recollection of his or her high school chemistry can construct a system that will function successfully at some level and thus provide some proof that yes, the concept can work in principle. This level of effort is insufficient, however, for designing a system that will work continuously for many years. Professional assistance should be sought from experienced engineers and academia to avoid frustrating failures.

Although they may be slow to admit it, professionals are not immune to failure. This is why it is prudent to:

• Experience failures and eliminate design uncertainties during lab, bench and pilot testing (phased design).
• Clearly determine the range of expected metal loading (the product of flow times concentration) for the treatment situation to avoid under-sizing.
• Evaluate startup procedures (being ecologically based, passive treatment systems typically should not be "turned-on" at full flow; bacteria may need time to incubate or acclimate).
• Develop clear operational plans and designs that allow future maintenance without total system shutdown.

A success story worthy of note, the 1,200-gpm capacity West Fork system in Missouri (see Gusek, et al. 1998 and Gusek, 2000), has met stringent NPDES permit requirements for the last five years without a single violation despite experiencing minor problems. In this case, the heart of the system was two anaerobic sulfate reducing bacteria (SRB) bioreactors, plumbed in parallel. Each cell was sized (based on the results of pilot testing) to accept the full flow from the mine for up to several months in case maintenance was required.

When suspended sediment from mine hoisting operations inad-vertently choked the surface of the anaerobic cells (despite an intermediate settling pond), the mine elected to replace the organic substrate with fresh materials (Murphy, 2001). This was undertaken in the summer, when bacterial activity was high, by diverting all the mine flow through one of the SRB bioreactors while the other cell was being retrofitted. The mine personnel were supported in this endeavor by an "operator's manual" that accompanied the original plans and specifications; the original design consultant was not even contacted.

Inadequate Maintenance: With minor exceptions, passive treatment systems consist of biological populations that include many suites of living things ranging from bacteria to plants. While somewhat resilient to minor, short lived changes, the biological populations in passive treatment systems cannot sustain overloading without suffering sometimes permanent damage. Overloading may not be apparent at startup. In concert with the definition of "loading" previously provided, the term "overloading" extends beyond the concept of excess flow rates (perhaps in response to storm events). It also applies to increases in metal concentration while flow remains fairly constant. Addressing this is a water management issue, solved by including surge/ equalization capacity and flow controls in the system design.

Short term changes in mineral acidity can be dealt with using limestone amendments that are periodically replenished as needed. This was a lesson that was learned at the Wheal Jane pilot system in Cornwall, England (unpublished data). An anaerobic SRB bioreactor was sized to receive flow from a series of aerobic cells that were designed based on USBM criteria to remove iron at low pH. These cells were also expected to and did remove arsenic. At the time, all flow from the aerobic cells was routed to the anaerobic cells, including direct precipitation. The prevailing thought (in 1993) was that rainfall would dilute the metals remaining and even at increased flow rates, the loading would stay constant. However, a number of conditions combined to overwhelm the SRB cell receiving the effluent from the aerobic cells. First, the aerobic cells were not as efficient as expected in neutralizing mineral acidity and rainfall dilution did not significantly affect the mineral acidity of the water, a critical design parameter for SRB cells. Second, overloading occurred during the winter, when SRB bacteria activity was stressed already due to the low water and air temperatures. Third, and lastly, the organic substrate did not contain any inherent buffering capacity (bench scale tests had not been performed due to schedule restrictions). In summary, the stressed SRB were hit with an acidity overload and there was no-self buffering component in the substrate to counter it. Consequently, the metal removal performance of the cell suffered. Fortunately, this was a pilot test and the situation was corrected by excavating the anaerobic substrate, amending it with limestone, re-inoculating with manure and installing a flow restriction device (orifice) on the aerobic cell outfall that helped to manage the flow peaks. The cell responded favorably and was subsequently more successful at zinc (and iron) removal.

Another similar situation occurred at the Burleigh Tunnel in Colorado (EPA, 1999), but the outcome was different. This drainage typically has neutral pH and about 50 to 60 mg/L of dissolved zinc. Two pilot-scale cells, each capable of handling about seven gpm, were constructed in 1994. Like the Wheal Jane SRB cells, the Burleigh Tunnel SRB cells were exposed to a high flow/high concentration event (pH 4.1 [estimate], Zn @109 mg/L, flow @20 gpm - loading was estimated to be three times the design rate) in 1995 in response to the spring snowmelt. The acidity loading also increased and despite some self-buffering capacity of the substrate, the cells' performance suffered. Unlike the test protocol at Wheal Jane, there was no intervention response to the overloading event such as reducing the flow to allow the SRB to recover or re-inoculation with fresh SRB. Consequently, the cells limped along for another year before the test was terminated. In the view of this author, the results of this test could have been markedly different (and more positive) had some effort at system maintenance been made.

Last Minute Design Changes: As stated earlier, a properly designed passive treatment system should be based on a phased testing program of laboratory-, bench- and pilot-scale experiments. These experiments and the subsequent design must take into account the physical availability of some construction materials. Bench testing may have identified a superior type of organic component that the SRB favored, but it may not be available in sufficient quantities to warrant including it in the final design. Local farmers in particular are notorious for offering to give away animal manure during the testing phase of the project only to boost the price to capitalize on a captive market when large quantities need to be procured. Contractors and project owners seek relief from these situations by substituting "similar" but less expensive sources which are virtually the same. Again, the West Fork Project in Missouri provides a couple of instances where minor digression from the pilot design caused subsequent problems in the full-scale system.

As reported in more detail by Gusek (2000), the first problem related to the use of geotextile in the organic substrate column which was six feet (2.9 meters) thick in both the pilot and final design. To allow better flow control/system throttling in the full scale SRB cells during the summer, intermediate layers of perforated pipes were installed in the substrate at the two-foot and four-foot depths. To facilitate water collection/ dispersion, the pipes were sandwiched between a layer of geonet and two layers of geotextile. Due to project scheduling, there was not time to test this concept on a pilot scale; the design change appeared to be minor. Another minor design change occurred during construction of the full-scale system. Alfalfa hay that was used in the construction of the pilot was in short supply; a source of spoiled alfalfa pellets was offered as a substitute and approved by the field engineer.

The two combined changes above had significant impacts on the ultimate hydraulic performance of the SRB cells. While the geochemical characteristics of the substrate mix met the design specifications, the physical situation caused by the changes was a significant departure from the pilot design. First, the geotextile trapped some of the gases evolved from the biological activity and created a "gas-lock" condition that restricted fluid flow through the cell. Second, the substitution of the alfalfa product in place of the baled source yielded a substrate with a slightly lower saturated permeability than that measured in the pilot. The net result was a system that was geochemically sized to temporarily treat elevated flows but the flow restrictions prevented this design feature of the system from being used. The condition was ultimately fixed, but a valuable lesson was learned. Even minor deviations from bench or pilot scale configurations or design can result in major changes in system performance and should be avoided as much as possible.

SUMMARY

Passive treatment technology has been proven to be effective in a variety of geochemical, flow and climatic situations (Gusek, 2000). However, "cookbook" design approaches should be implemented on a full-scale basis with caution; it would be more prudent to use cookbook designs as a starting point for bench or pilot scaled passive treatment systems. Conclusively, many system failures can be avoided by using phased testing of system designs and attention to detail during construction, operation and maintenance.

REFERENCES

Beining, B.A., and M.L. Otte, 1997. "Retention of Metals and Longevity of a Wetland Receiving Mine Leachate," presented at the 1997 National Meeting of the American Society for Surface Mining and Reclamation, Austin, Texas, May 10-16.

Brodie, G.A., 1993. "Staged, aerobic constructed wetlands to treat acid drainage: Case history of Fabius Impoundment 1 and overview of the Tennessee Valley Authority's Program", In: G.A. Moshiri (ed.), Constructed Wetlands for Water Quality Improvement, pp. 157-166, Lewis Publishers, Boca Raton, Florida, 632 pp.

Gusek, J.J., T.R. Wildeman, A. Miller, and J. Fricke,A. Miller, and J. Fricke, 1998. "The Challenges of Designing, Permitting and Building a 1,200 GPM Passive Bioreactor for Metal Mine Drainage, West Fork Mine, MO", presented at the 15th Annual Meeting, American Society of Surface Mining, Reclamation (ASSMR), St. Louis, MO, May 17-21.

Gusek, J., C. Mann, T. Wildeman, and D. Murphy, 2000. "Operational Results of a 1,200 GPM Passive Bioreactor for Metal Mine Drainage, West Fork, Missouri", presented at the Fifth International Conference on Acid Rock Drainage (ICARD 2000), Denver, Colorado, May, Proceedings published by SME, Littleton, Colorado, ISBN 087335-182-7.

Gusek, J.J., 2000. "Reality Check: Passive Treatment of Mine Drainage An Emerging Technology or Proven Methodology", Presented at the Society for Mining, Metallurgy and Exploration Annual Meeting, Salt Lake City, Utah, February 28-March 1, Preprint No. 00-43.

Hedin, R.S., R.W. Nairn, and R.L.P. Kleinmann, 1994. Passive Treatment of Coal Mine Drainage, USDI, Bureau of Mines Information Circular IC 9389, Pittsburgh, Pennsylvania.

Murphy, Denis, 2001. Personal communication.

U.S. Environmental Protection Agency, 1999. "Draft Anaerobic Compost Constructed Wetlands System Technology: Innovative Technology Evaluation Report". Site Program Contract No. 68-C5-0037.

Wildeman, T. R., G. A. Brodie, and J. J. Gusek, 1993. Wetland Design for Mining Operations. BiTech Publishing Co., Vancouver, B.C. Canada.

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