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Lookup NU author(s): Dr Tobias Roetting
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Acid mine drainage (AMD) and other metal-polluted anthropogenic discharges are a major cause of water contamination world-wide. Conventional treatment plants are expensive to operate, and application at remote sites may be impractical. Passive treatment systems, which only require naturally available energy sources and infrequent maintenance, may be an economical option to decontaminate these waters. Nevertheless, they are prone to clogging and passivation (loss of permeability or reactivity, respectively) when used to treat water with high metal concentrations or high acidity loads. Many passive treatment systems are based on calcite dissolution that removes trivalent metals such as Fe(III) and Al, but not divalent metals (e.g. Zn, Mn, Cu, Pb, Ni, Cd). To overcome these problems, a Dispersed Alkaline Substrate (DAS) was developed and tested in this dissertation. DAS consists of a fine-grained alkaline reagent (e. g. calcite or MgO [caustic magnesia] sand) mixed with a coarse inert matrix (e. g. wood chips). Calcite-DAS was capable of treating Acid Mine Drainage with an inflow net acidity of 1350-2300 mg/L as CaCO3, removing an average 1200 mg/L as CaCO3 in laboratory columns. The substrate functioned without clogging during over one year at an acidity load of 150 g acidity/m2·day, five times the loading rate recommended for conventional passive treatment systems. Al, Fe(III), Cu and Pb were virtually eliminated and some Zn, Ni and Cd were removed at low flow rates. The system was also tested at a field pilot plant constructed at Monte Romero mine (Huelva Province, SW Spain), where a 3 m3 calcite-DAS tank eliminated a median net acidity of 900 mg/L as CaCO3 at an acidity load of 600 g acidity/m2·day and an Al-load of 30 g/m2·day. However, the tank eventually clogged due to formation of an Al- and gypsum-rich hardpan within the substrate. Clogging by Al-precipitates probably could be retarded by lower Al-loads. Comparison with the laboratory columns and other publications show that acceptable Al-loads may be in the order of 5-10 g Al/m2·day. MgO-DAS laboratory columns depleted 310 mg/L Zn and 30 mg/L Mn below detection limit during over one year without clogging at a median Darcy flow rate of 0.1 m/day. In DAS containing only 12.5 % (v/v) of MgO with median particle size of 0.15 mm, 95% of the applied MgO dissolved in the zone where Zn and Mn accumulated. DAS performs better than other passive treatment systems based on gravel-sized alkaline reagents. DAS reactivity is greater than that of gravel-based treatment systems, because mixing with a coarse inert matrix allows the use of fine-grained alkaline reagents which provide a higher reactive surface. Due to their small size, the grains are dissolved before passivation. This ensures that, contrary to most gravel-based systems, most of the reactive is consumed, minimizing economic costs of passive treatment with DAS. The high acidity removal is possible because metals accumulate intentionally in DAS. Hydrolysis and precipitation release protons which promote further dissolution of reactive. The large pores of the inert DAS matrix and the dispersion (separation) of the alkaline grains minimize clogging problems. Therefore, DAS combines high reactivity, excellent metal removal, good hydraulic performance and efficient use of the alkaline reagent.
Author(s): Rötting T
Publication type: Report
Publication status: Published
Series Title: Department of Geotechnical Engineering and Geo-Sciences (ETCG-ETSECCPB)
Type: PhD Thesis
Year: 2007
Pages: 140
Institution: Technical University of Catalonia (UPC)
Place Published: Barcelona, Spain
