Field and pilot-scale geochemical investigations into using sewage sludge for sulphidic mine waste remediation

Abstract: Successful mine waste reclamation is a fundamental aspect in mine plan development and is legislated through the European Parliament’s ‘Management of Waste from Extractive Industries Directive (2006/21/EC)’. Field and pilot-scale trials were utilised to evaluate different applications of an alternative cover material, sewage sludge, in its ability to remediate and prevent acid rock drainage formation from sulphidic mine tailings derived from the Kristineberg Zn-Cu mine, northern Sweden. Sewage sludge is an organically-rich waste material generated from the treatment of domestic waste water. It may function suitably as a low permeable barrier against oxygen ingress when compacted or as a surface vegetation substrate. The first study focused on evaluating the effectiveness of a sewage sludge sealing layer, part of a composite dry cover design system. Data on tailings, leachate water and pore gas geochemistry during eight years from two experimental pilot-scale test cells revealed that the sludge was an effective barrier to oxygen influx, which prevented sulphide oxidation and acid rock drainage formation. Sludge-borne metals (Cd, Co, Cu, Fe, Pb, Zn) precipitated and were retained in the underlying tailings due to the reduced conditions produced, resulting in low concentrations of dissolved metals (<10μg/L Cd, Co, Cu, Ni, Pb, Zn) in the drainage, several orders of magnitude lower than that from an uncovered tailings reference cell. However, a 19.6% mass reduction of the sludge due to organic matter degradation may compromise the effectiveness of the cover in the long-term. The second study evaluated the geochemical impact of a field-scale surface application of sewage sludge on the groundwater quality of a formally-remediated sulphidic-tailings impoundment. Thirteen years after initial remediation, 12 000 tonnes of sludge were applied to a depth of 0.3m to re-vegetate and stabilise the pre-existing water-saturated and composite dry cover design systems. After two years, a 17% reduction of the sludge volume occurred due largely to aerobic degradation of the organic matter fraction, leaching of the constituents Ca and S, and partially due to the leaching of Cu, Ni, Pb and Zn. Groundwater data indicated that sludge-borne constituents derived from the dry covered areas were prevented from infiltrating through the low permeable sealing layer. Instead they drained laterally through the protective layer to the impoundment toe and did not reach nor have a negative impact on the tailings groundwater. Oppositely, a major plume of sludge-borne constituents derived from the water-saturated area entered the tailings groundwater vertically unhindered, then dispersed laterally through the tailings due to the dominating hydrogeological regime. The plume migrated underneath the dry composite covered area after 2.2 years, where concentrations of Cu (188µg/L), Fe (254mg/L), Ni (263µg/L), Pb (95µg/L) and Zn (1.55mg/L) peaked. The plume included elevated nitrate concentrations (167mg/L) released from nitrification of ammonium in the sludge,which oxidised pyrite in the tailings. However, the effects were temporary due to declining sludge-borne nitrate and metal release from the sludge due to vegetation establishment after 2 years. The expected duration of the plume to disperse and exit the tailings impoundment is estimated to take another 4 years. In conclusion, these studies have found that sewage sludge can be effectively utilised as a suitable alternative cover material within the study periods observed as a sealing layer and as a final vegetation substrate on composite dry covers, but not water-saturated covered systems for the remediation of sulphidic mine tailings. Further investigations will have to verify the effectiveness of these applications in the long-term.