English

Abstract

This paper provides an approach for roughly estimating the capital and annual operating costs, and the longevities (time to substantial replacement or upgrade), of alkali-based water-treatment plants for ARD. Under most combinations of flow and acidities, these plants will be the only cost-effective option for long-term consistent adherence to water-quality restrictions. Median and mean capital costs were roughly US$3.8 and $4.5 million, and there was a correlation with average annual flow rate. There were no strong correlations among total annual operating costs, flows, and acidity concentrations due to highly variable individual contributions from factors like power and reagents. Nevertheless, an average unit-volume treatment cost was US$0.27/m 3 for acidities less than 800 mg/L, and was US$2.24/m 3 for acidities above 4000 mg/L. The reported longevity of these water-treatment plants is around twenty years, attributable to factors such as increasing chemical loadings, increasing flows, and improved technology. A replacement period of 20 years can have a significant, but not dominant, effect on net present value over 100 years. Passive treatment systems and soil covers do not usually attenuate concentrations consistently to non-toxic discharge levels, so additional treatment can be required. If this involves a water-treatment plant, an interplay of combined costs, longevities, durations, and risks leads to a myriad of waste-management scenarios. For example, a soil cover that lessens annual acidity loadings can extend the number of years a treatment plant operates; thereby increasing (1) the length of time the mining company must maintain a site presence and thus (2) the risk posed by a greater probability of intense storm events or other problems leading to an accidental release of contaminated water. Long-term costs for passive treatment systems, like wetlands, and soil covers are not as well defined. This can give the false impression that water-treatment plants are more expensive in the long term. ______________________ 1 Paper presented at the 7 th International Conference on Acid Rock Drainage (ICARD), March 26-30, 2006, St. Louis MO. R.I. Barnhisel (ed.) Published by the American Society of Mining and Reclamation (ASMR), 3134 Montavesta Road, Lexington, KY 40502 2 Minesite Drainage Assessment Group (www.mdag.com), 8035 Redtail Court, Surrey, British Columbia, Canada V3W 0N4 Proceedings America Society of Mining and Reclamation, 2004 pp 1333-1344 DOI: 10.21000/JASMR06021333