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Bioremediation - Case History: AMD Treatment at Wheal Jane, Cornwall, UK

 

Case History: AMD Treatment at Wheal Jane, Cornwall, UK

 

Wheal Jane is an abandoned tin mine located in Cornwall, UK. As shown in Figure 15 it is located next to the Carnon River. In the winter of 1991/1992, just one year after closure, AMD from Wheal Jane contaminated the Carnon River (Whitehead & Neal, 2005a).

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Figure 15 – Wheal Jane location (Whitehead & Prior, 2005b)

 

AMD is a big problem in the UK with South Wales, Yorkshire and Durham suffering the most. It is also an issue in popular mining areas of the US including Pennsylvania and eastern Appalachia (Whitehead & Prior, 2005b). A pilot study of bioremediation methods using passive wetlands was implemented and studied water pre-treated in three ways: Lime Dosing (LD), Anoxic Limestone Dosing (ALD) and Lime Free (LF). After treatment AMD was allowed to pass through aerobic cells and then into anaerobic cells. Figure 16 shows a schematic of the system (Whitehead & Prior, 2005b). This case study will look into the anaerobic cells only since it is the key technology that can be considered a bioremediation technique. The system at Wheal Jane is the largest of its kind in Europe and cost around £1 million ($1.6 million) to set up (Whitehead & Prior, 2005b).

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Figure 16 – Schematic of treatment system at Wheal Jane (Whitehead & Prior, 2005b)

 

The anaerobic cells were 87.5x8.75x1 meters (length x width x depth) in dimension (Johnson & Hallberg, 2005b). The compost consisted of 95% sawdust, 5% hay and a small amount of manure to introduce bacteria to the cell. The compost was contained in polyethylene membranes. The main function of the cells was to generate alkalinity (i.e. to raise the pH) and to remove toxic heavy metals that included zinc, copper and cadmium by making use of SRB. Water that entered the anaerobic cell had already been pretreated in aerobic cells to remove iron and arsenic (Johnson & Hallberg, 2005b). Hydrolysis of the iron in the aerobic cells meant that influent into the anaerobic cells had a low pH approximately around pH 3. The cascading of water through the 5 aerobic cells also caused the water to have a high oxygen content. Both low pH and high oxygen content are known to be bad for SRB. Because of the large surface area lots of rainwater also entered the anaerobic cells.

Water was sampled in the rock filters and results of the study showed that (Johnson & Hallberg, 2005b):

  • pH

Water flowing out of the LD and ALD cells was actually lower than the water that entered with a mean of pH 5.32 and pH 5.40, respectively. The pH of the water flowing from the LF cells continued to increase with a mean value of pH 6.05 (Johnson & Hallberg, 2005b).

  • Sulfate reduction

Sulfate redcution for the LD, ALD and LF cells had mean reduction values of 27%, 23% and 62%, respectively (Johnson & Hallberg, 2005b).

  • Iron removal

After flowing through the aerobic cells 95% of the ferrous iron had been oxidised to ferric iron. However, for the LD and ALD cells concentrations of soluble iron flowing out of the anaerobic cells was found to be higher than that flowing in. The reason for this is discussed later (Johnson & Hallberg, 2005b).

  • Heavy metal removal

Zinc was removed from the LD, ALD and LF cells at percentages of 55%, 67% and >99%, respectively. Copper and cadmium were removed at similar levels (Johnson & Hallberg, 2005b).

 

Lessons

Placing the anaerobic cells after the aerobic cells was found to be counterproductive, the water entering the anaerobic cells was found to have lower pH and higher oxygen concentrations that it would if it drained from the mine directly (Johnson & Hallberg, 2005b). Large surface area for the anaerobic cells was also found to be an issue since it allowed large volumes of rain water to enter the system. Around 0.2 l/s of water flowed through the cell of which AMD made up 56% and rainwater 44% (Johnson & Hallberg, 2005b). This severely limited the amount of water that could be effectively processed. Studying the soluble iron content of the LD and ALD cells showed that for the soluble iron coming from the AMD was 29% and 54% respectively. The remaining iron was found to be flushed from the sumps that connected cells. Removal of iron before it entered the anaerobic cell was found to be essential, since if it was allowed to enter the cell it would flow through, enter the rock filters and undergo hyrdrolysis and produce significant acidity (Johnson & Hallberg, 2005b).

The key factor that determined why the LF system worked but the LD and ALD did not had nothing to do with the influent water since for all three systems it was more or less identical. The main contributing factor was the fact that the LF system had to be shutdown for 10 months due to technical isssues. This downtime allowed the anaerobic cells in the LF system to “mature” (Johnson & Hallberg, 2005b). The harsh environment produced by the AMD likely killed any native bacteria in the manure but the shutdown time allowed resistant SRB to grow into sizable populations in the anaerobic cells (Johnson & Hallberg, 2005b).

 

Recommendations for future anerobic wetlands

Anaerobic cells should always be placed ahead of aerobic cells to raise the pH and lower the oxygen content of influent into the cell (Johnson & Hallberg, 2005b). This also limits the rain water entering the cells. The cells should be allowed to “mature” for a period of time before AMD is treated to allow robust SRB populations to develop in significant numbers (Johnson & Hallberg, 2005b). The optimum maturation period is still a topic of ongoing research. It is clear that bacteria from an innoculating material such as manure will not survive, it may be better to introduce a slurry from another “mature” anaerobic cell to boost the SRB population (Johnson & Hallberg, 2005b).

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