4.4.3 AMD from historical mining activities – additional considerations for treatment
The same flow charts and selection criteria presented earlier in this chapter can be used to select treatment options for pre-existing AMD. However, data can be collected from the AMD over a period of time to add confidence to the selection of appropriate treatment techniques.
AMD water chemistry and flow rate should be measured monthly for at least 12 months preferably via data logger. Water chemistry parameters should include pH, total acidity, dissolved Fe, dissolved Al, dissolved Mn, Fe2+:Fe3+ ratio, DO, and TSS. It is strongly recommended that the sequential titration acidity procedure described by Hilton (2004) be used on multiple occasions (see Appendix F.5). For flow rate, the range of flow rates and response of flow to precipitation events should be determined either by spot sampling or preferable via data logger. Any correlation of flow with water chemistry should also be determined.
Active treatment
If active treatment is considered, laboratory bench-scale tests should be conducted to help in system selection and design. For the dosing-with-alkali step (DA), bench-scale tests should be conducted using various chemicals to determine dosing rates and effectiveness (Younger et al. 2002) and a sequential titration acidity analysis should be conducted as described in Appendix F.5.1 (Hilton 2005). For the oxidation step (O), the various oxidation techniques can be tested in the laboratory and it can be determined if chemical oxidants are required and, if so, at what dosing rates. For the sedimentation step (S), coagulants and flocculants can be tested in the laboratory on treated AMD to determine if they are necessary to aid in settling precipitates formed during neutralisation.
Once a neutralisation chemical and coagulants/flocculants have been selected, AMD can be treated in the laboratory to generate sludge for leach testing.
Passive treatment
AMD at closed and abandoned mines typically has a more stable chemistry and flow rate than AMD at active mine sites and land is usually more readily available for treatment systems – factors that fit well with passive treatment.
If passive treatment is considered, once potential treatment solutions have been identified, small-scale field trials should be constructed on site to test the effectiveness of the various options before investing in full-scale system construction. See Appendix F.6 and Trumm et al. (2006, 2009) for examples of small-scale trials. Even if only one option is indicated through the use of Figures 19 and 20, field trials should still be conducted because unknown factors can influence the effectiveness of treatment systems. The choice of the full-scale system should be based on the results of the field trials and a review of the cost, effectiveness, limitations and risk of failure for each option (see Appendix F.3). A computer program such as AMDTreat (Means et al. 2003; Appendix F.4) can be used to design specific components of various treatment options to determine potential costs.
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