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Treating Acidity in Coal Pit Lakes Using Sewage and Green Waste: Microcosm and Field Scale Trials at Collinsville Coal Project (Queensland)

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Published: March 08Project Number: C14052

Get ReportAuthor: Clint McCullough, Mark Lund, Joel May | Edith Cowan University, Xstrata Coal Qld

This research project involved, collaboration and technology transfer from research in Collie (Western Australia) to Collinsville (Queensland).   Coal mine lakes represent a potentially valuable resource to both the environment and the community in inland Australia, if the water can be remediated to an appropriate standard for its proposed end use. Beneficial end uses include: aquaculture, water for irrigation, recreation, and for nature conservation. Research in Collie coal mine lakes and internationally has found there is considerable potential for utilising biological processes to reverse acidity generating processes. This approach focused primarily on additions of organic material to support sulfate reducing bacteria (SRB) which convert sulfate back to sulfides, removing acidity and metals in the process. The approach also fosters a range of other biological processes which can increase alkalinity and pH.   

Previous ACARP funded research in Collie had tested the use of SRBs for treating acidic mine waters with limited success. Limiting the effectiveness of this approach in Collie are low sulfate concentrations that occur in these pit lakes. Nevertheless, there are a number of highly acidic mine lakes in the Collinsville Coal Project (CCP) with high sulfate levels.   

CCP discharged 4 ML of highly acidic mine water into a 80 ML sewage evaporation pond. CCP and later the authors monitored the effects of this discharge on water quality in the ponds. In approximately 18 months water quality in the evaporation pond had returned to pre-addition conditions. It appeared that a combination of processes, including SRB activity were responsible for the remediation of the mine water. The results of this study suggested that SRB activity might be useful in the treatment of pit lakes on the CCP site.   

This project aimed to establish a full scale demonstration of the application of passive biological remediation in a pit lake on the CCP lease. This pit lake is located near the Bowen Shire's Collinsville Water Treatment Plant (CWTP) and green waste transfer station. Organic substrate in the form of primary treated sludge from the plant were added to the lake. This wastewater provided a ready source of, BOD (Biochemical Oxygen Demand), available carbon, and nutrients. All of these wastewater components have been identified as key factors promoting SRB activity. Monitoring of this experimental lake and control lakes both before and after the addition enabled an assessment to be made of the success of this innovative remediation method.   

The first two experiments using acrylic tubes (microcosms) containing sediment and pit lake water were established firstly in Collinsville and then in Perth. These experiments were used to gain a better understanding of the processes responsible for mine water remediation and to provide estimates of the quantities of organic matter that were likely to be required in a full-scale treatment of a pit lake.   

The microcosm experiment involved testing of greenwaste only, sewage only, and greenwaste and sewage for their effectiveness in remediation of pit lake water, in typical Collinsville conditions.   

This first experiment clearly showed remediation of pit lake water pH from 2.2 to 5.5 in 145 days, with commensurate declines in iron, aluminium and toxic metal concentrations. The presence of greenwaste appeared to be important for the effectiveness of the treatment. This was mainly believed to be due to the bulk of the greenwaste which extended the area of SRB activity up through the water column.   

The second experiment repeated the first experiment in design but added different quantities of organic material and a second sewage type. Another important aim of this experiment was to demonstrate the repeatability of the initial results given the heterogeneous nature of the organic materials being used.   

This second experiment demonstrated that results were generally reproducible from the first experiment. However the performance of greenwaste only was poorer with a pH of >4 reached after 210 days, while the other treatments produced circum-neutral pH (~7) after 120 days with the exception being low levels of sewage only which produced a pH of ~6 after 210 days. Associated with the pH improvements were reductions in electrical conductivity, sulfate and metal concentrations. Key drivers for rapid remediation appeared too be the high tropical temperatures, the combination of greenwaste and sewage, and a 'fresher' type of sewage from Bowen rather than Collinsville. This experiment enabled the team to determine the quantities of organic materials required for a field trial.   

Comprehensive risk assessments and stakeholder consultation identified significant occupational health and safety concern of using sewage and greenwaste at field scale was of high levels of faecal coliforms in the water creating a risk for operators. The second experiment revealed that after 180 days there were no faecal coliforms in the water, although total coliforms were high.   

These latter coliforms were believed to be associated with decomposition process and were not believed to pose a contact risk to operators. Nevertheless, precautions were developed for operators involved in the project.   

In August 2006 to January 2007, the smaller lake (GAEW) was filled with dried sewage sludge (60 t), liquid sewage sludge (3,190 t) and municipal green waste (980 t). Monitoring of this new treatment lake and the remaining control lake and other control lakes then continued for another 6 months at monthly intervals.   

Due to groundwater influx and heavy cyclonic rainfall events, it was often unclear what contribution sulfate reduction process have made to changes seen in water quality. Nevertheless, physico-chemical changes to control lakes during monitoring could generally be explained as a result of these two external influences. However, after four months of filling ceasing, GAEW ORP began to decline from around 600 mV to 200 mV starting from the benthos. Also beginning at the benthos, pH increased soon afterwards reaching a pH of 3.7 across the lowest 3 m of water column by July 2007. The lower 3 m mean pH of the GAEE lake was 2.2 at this time. Similarly, at the end of the experiment electrical conductivity was reduced to 9.0 mS cm-1 compared to 9.4 mS cm-1 in the GAEE lake.   

These field chemistry observations suggest that addition of low-grade organic materials for remediation of acid mine waters at field scale shows promise. However, remediation has only just begun and further monitoring is required to access the degree of treatment that can be achieved and how long this treatment will continue.

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