Sponsored by: Canadian Centre for Mineral and Energy Technology, Centre de recherches minérales, Denison Mines Limited, Environment Canada and INCO Limited

July 1993

EXECUTIVE SUMMARY

The MEND Project 3.11.1: "Treatment of A cidic Seepages Employing Wetland Ecology and Microbiology has reached the completion of its fourth year. The project has been financially supported by Inco, Denison, Environment Canada, CANMET and by the Centre de Recherche Minérales (CRM). The objectives of the project are to determine the conditions which will lead to the treatment and amelioration of acid mine drainage (AMD) through the use of ecological microbial processes. Those occur naturally in wetlands, lake and ocean sediments. The Makela Test Cell System was intended to provide flow control for a typical seepage from a base metal tailings dam. Under flow control, natural Fe3+ hydroxide precipitation and acidification rates were determined. Conditions which are required to promote microbial sulphate reduction and alkalinity-generation were to be identified. The microbially-mediated treatment of acid mine drainage is referred to as ARUM (Acid Reduction Usina Microbioloav}.

The construction of retention cells at the perimeter of the phreatic line of the tailings dam was complex. This terrain is hydrologically unstable and dikes are prone to slumping. Permeable dikes were used to provide sheet flow, and impermeable dikes, providing flow control, were required to separate the retention cells. Frequent repairs on the cell system were needed from the beginning of construction in 1989 until summer 1991. By the end of the summer of 1991 flow control was achieved, and a prototype of a floating cattail cover, which allows the ARUM system to develop, was finally installed. Project activities are given below for each year.

Year 1 (1989/1990): In the first year, the Test Cell System was constructed and hydraulic adjustments were made to control flow. Test work in 200 L drums (ARUMators), containing organic amendment and equipped with sampling ports, showed that microbial alkalinity-generation in tailings seepages is possible. ARUMator 3, a 12 m3 fibreglass tank with an 800 L inner sleeve containing the organic amendment, was installed at the end of the Test Cell System. This would facilitate testing of the ARUM processes under completely controlled conditions.

Increases in pH in the ARUMators were reported from 2.5 to 5.7. Decreases in nickel concentrations from 91 mg/L in a sample from a surface port to 1.7 mg/L in a bottom sample of the same ARUMator were noted. These observations lead to the recognition that the process is sediment-bound and that a floating cattail cover was needed. Cattails rooted in the organic amendment would rapidly deplete the nutrients required for the ARUM ecosystem. A floating vegetation mat, however, would not only provide organic matter to the sediment below, but also enhance reducing conditions in the water column between the sediment and the floating cattail mat.

The research of the first year was reported in June 1990 in a report entitled: MEND Project 3.1 1.1 "Treatment of Acidic Seepages Employing Wetland Ecology and Microbiology, Final Report", by M. Kalin, June 1990. DSS Contract Number 23440-8-9264. A peer review was carried out on the report, and the project was found technically sound .

Year 2 (1990/1991): After the first winter, the Test Cell System required readjustment of the hydraulic conditions. It was established that the lowest controllable flows were 3 - 5 L/min. The maximum flow, which the system could sustain without structural failure, was determined to be 300 L/min. Baseline chemistry of the system was defined in the second year.

Ground water contributions, amounting to less than 1 L/min, were found to have no detectable effects on the water chemistry. The conditions under which precipitation of ferrous (Fe2+) and ferric (Fe3+) hydroxide takes place in the precipitation cell (Cell 1 ) were defined. A baffle system was installed in Cell 1 which facilitated settling of the hydroxides. This cell discharged a clear, acidic solution with low iron concentrations to Cell 2.

Organic amendment was placed in Cells 3 and 4, between snowfencing curtains. Flax bales mixed with hay bales were used to provide the substrate on which the microbial ecosystem would grow and where alkalinity would be generated. Through the activity of the sulphate reducers, hydrogen sulphide is generated, which results in the precipitation of metal sulphides. An extensive microbiological investigation was carried out in the laboratory to define the growth requirements of the alkalinity generating microbes.

A report on the work completed in the second year was submitted in March 1991. MEND Project 3.1 1.1 "Treatment of Acidic Seepage Employing Wetland Ecology and Microbiology, Final Report", by M.Kalin, March 1991. DSS Contract Number 23440-0-9065.

Year 3 (1991/1992): In the first two years of the project, the ecological conditions required for microbial alkalinity-generation were defined. Floating cattail mats were installed on Cells 3 and 4 in 1991. The third year was, therefore, the first opportunity to demonstrate the ARUM process under defined flow conditions. The optimum configuration required for the establishment of the ARUM process had only been achieved by late July, due to problems encountered with bank stability in late May 1991. Slumping of the tailings dam blocked the bypass ditch, preventing regulation of the flow to the Test Cell System.

The ARUM process works from the sediment upwards, and thus, its effects would first be seen in the lower part of the water column in Cells 3 and 4. The flow was adjusted to 1 L/min by mid July. By mid September 1991, differences in metal concentrations of water on the surface and in the lower parts of the water column were large. In Cell 4, the nickel concentrations at the surface ranged from 43 mg/L to 74 mg/L. The range in the lower part of the water column (50 - 60 cm) was 12 mg/L to 33 mg/L. In Cell 3, the first ARUM cell receiving the low pH AMD, the nickel concentrations ranged between 23 and 51 mg/L at the surface, while the lower part of the water column had concentrations between 15 and 24 mg/L. This represents approximately a 50 % reduction of the nickel concentrations. Copper was present in both cells at the surface in concentrations ranging from < 1 to 4 mg/L and reduced by the ARUM process in the lower part of the cells to 1 or < 1 model.

At a flow rate of 1 L/min, the water in Cells 3 and 4 has a retention time of just over 4 months. The surface water, however, short-circuits and therefore, the pH of the discharged water had only slightly increased from 2.5 to 3.2. However, 27 kg of alkalinity has been generated in water leaving Test Cell 4 after passing over the actively ARUMating lower water column, where the pH is as high as 6.0. If water were to be discharged from the bottom of Cell 4, reduced metal concentrations with a high pH water would leave the system by the end of the third year.

The results of the third year indicated that, in the Test Cell System, alkalinity generation had taken place. The work of the third year was reported in March 1992. MEND Project 3.11.1 "Treatment of Acidic Seepages Employing Wetland Ecology and Microbiology, Final Report", by M. Kalin, March 1992. DSS Contract Number 23440-0-9065.

Year 4 (1992/1993): Due to dam stability problems during spring thaw and freezing of the dikes along with the control valve, the system was closed during the winter of 1992 to 1993 As the cattail rafts were planted late in 1991 growing season, growth was restricted to a few plants. By the beginning of the 1992 growing season, adjustments were made in the root zone. The floating cover was functional by July 1992 and the system was ready to be monitored.

In 1989 and 1990, there was no flow control and flows were very variable. With short retention times (4.2 days in Cells 1 and 2 and 3.26 days in Cells 3 and 4 at 40 L/min) Fe3+ hydroxide precipitation occurred throughout the system. When flow control was established at 1 L/min, retention time could be increased to an estimated 168 days in Cells 1 and 2. In 1992, Fe3+ hydroxide precipitation facilitated the removal of at least 94 % of the iron load in Cell 1 and produced an acidity loading of 100 to 600 g/day in the water entering the ARUM cells (Cells 3 and 4).

The final configuration, established by the end of 1991, allowed for the establishment of ARUM in Cells 3 and 4. In 1992, with a retention time of 131 days, the ARUM system (Cells 3 and 4) removed 80 - 87 % of the nickel loading, 77 - 98 % of the copper loading, 10 - 20 % of the sulphur loading, and 47 - 73 % of the acidity loading from the seepage water.

This report presents the summary of those components of the microbial ecosystem which play major roles in the ARUM process. The relationships between wetland ecosystems and ARUM processes are given in Section 2. In Section 3, the Test Cell System is described, outlining the events which finally lead to flow control and floating cattail rafts in 1992. The water chemistry, the hydrology with and without microbial activity, as well as the iron hydroxide precipitation, are described in Section 4. In Section 5, the data obtained in the research program are used to define the operating parameters, such as nutrient supply and chemical conditions. The expected performance and the applications of the process are discussed in Sections 6 and 7. In Section 8, the limitations of the microbial approach are outlined. Some economic considerations are presented in Section 9. It is concluded in Section 10 that the project has provided the technical basis to define the conditions required to utilize microbial amelioration of AMD in decommissioning seepage collection ponds, open pits and polishing ponds.

Last Modified: 2003-11-26		Important Notices

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