TREATING ACIDITY IN COAL PIT LAKES USING SEWAGE AND ...

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Treating acidity in pit lakes using sewage and greenwaste A review by Gibert et al. (2002) found that organic matter type was the prime determinate of the efficacy of SRB treatment systems, providing both a suitable redox environment and carbon source (Harris & Ragusa, 2000, 2001; Gibert et al., 2002; McCullough et al., 2006). The availability of carbon from plant matter is dependent upon the rate of decomposition, which can be extremely limited in acidic and anoxic conditions (Harris & Ragusa, 2001). Castro and Moore (1997) and Hard et al. (2003) observed that in many sites, the viability of this approach required an organic matter source that was effective, economical and locally available. This issue is particularly acute in remote mine sites, which prompted Lund et al. (2006) to trial Local Council mulch for remediation of a regional Australian pit lake. In many remote mining locations, the only bulk carbon sources likely to be available in large quantities are sewage from the site or support town, and green waste (a broad range of plant material collected through clearing of areas on the mine or in the towns from domestic and municipal lawns and gardens and also plant material from clearing of native vegetation as part of open cut mining process) (McCullough et al., 2006). Testing similar bulk materials in permeable reactive barriers, Waybrant et al. (1998) found they successfully reduced sulfate, with sewage sludge the fastest to achieve high levels of sulfate reduction. Testing in bioreactors, Harris and Ragusa (2000) also found a mixture of sewage sludge and plant material (fresh rye grass) was effective in laboratory experiments at increasing pH (2.3 to >3), reducing acidity and divalent metal concentrations of AMD waters through sulfate reduction within 30 days. The combined mixture proved more effective in ameliorating pH and metal concentrations than either sewage sludge (little response) or fresh plant material (nil response) on their own. Potential problems with in situ treatment of AMD affected pit lakes include Postgate’s (1984) assertion that SRB activities only occur at a pH>5. However, Herhily and Mills (1985) found SRB activity in a lake at pH 2.5, and Gyure et al. (1987) in a lake at pH 2.7. McCullough et al. (2008) recorded remediation of pH 2.4 pit lake waters by SRB activity in a sewage evaporation pond. It appears that less acidic microenvironments in the system can allow SRB activity to occur and this activity sustains and increases the size of the microenvironment (Küsel & Dorsch, 2000; Küsel et al., 2001). Peine et al. (2000) found that in German pit lakes that acidification of the lakes was maintained by the establishment of an acidity driven iron cycle. This cycle is dependant on the formation of the mineral Schwertmannite, constant 38
39 McCullough, Lund and May (2008) input of Fe 2+ and no SRB activity below pH of 5.5. In this model, the benefits of addition of organic matter to the sediment would eventually be overcome as the cycle became established. Other issues that might reduce the effectiveness of this approach to treatment are large inputs of acidity from the catchment. Very little bioremediation work has been carried out on mining pit lakes in Australia (Harries, 1997). As an arid continent, the effects of drought and climate change pose particular challenges for mining companies as heavy water users such that in some areas water is becoming as valuable as the ore (P. Lilly, AUSIMM, pers. comm.). Therefore there is a growing demand for new sources of water to meet a variety of end uses (Doupé & Lymbery, 2005; McCullough & Lund, 2006). At CCP, water availability for the operation is a priority concern due to competing demands for the main water source (Bowen River). Treatment of pit lake water even to only marginal quality would reduce pressure on other sources of water (including the Bowen River) for uses such as dust suppression (McCullough and Lund, 2006). In Chapter 6, we demonstrated in a pilot study in Collinsville that sewage and greenwaste in combination had the potential to successfully treat GAE mine lake water. The aim of Experiment 2 in Chapter 7, which was conducted in Perth (under more controlled conditions) was to demonstrate that the positive effects were reproducible, determine whether reduced quantities of organic material would be equally effective and investigate the mechanisms responsible for the improvements more closely. Furthermore in Experiment 1 the sewage sludge used was from Collinsville Waste Water Treatment Plant. Due to the low production from the Collinsville Plant, the sludge had been baked in the sun for several months. In Experiment 2 we wanted to test a combination of sewage from Bowen and Collinsville, as Bowen sewage is wetter and larger quantities are available.
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Page 67 and 68: 8.2.1 Risk Assessment 65 McCullough
Page 71 and 72: 8.2.5 Pit lakes 69 McCullough, Lund
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89 McCullough, Lund and May (2008)
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Concentration (mg L -1) 93 McCullou
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8.4 Discussion 95 McCullough, Lund
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9 Conclusions 99 McCullough, Lund a
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11 References 103 McCullough, Lund
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