Compacted sewage sludge as a barrier for tailing impoundment

The feasibility of compacted sewage sludge serving as a barrier for tailing impoundment was evaluated by the batch test and hydraulic conductivity test with respect to heavy metal retardation and impermeability. The batch test results showed that the effective removal of heavy metals approached 97.8 and 93.4% for Zn and Cd, respectively. Formation of precipitation of oxy(hydroxide) and carbonate minerals was mainly responsible for the attenuation of heavy metals in the early period of the test. Nevertheless, the further removal of heavy metals can be attributed to the sulfate reduction. The hydraulic conductivity test indicated that almost all of the heavy metals contained in simulated acid pore water were retarded by compacted sewage sludge. The hydraulic conductivity of the compacted sewage sludge ranged from 3.0 × 10⁻⁸ to 8.0 × 10⁻⁸ cm s⁻¹, lower than 1.0 × 10⁻⁷ cm s⁻¹, which is required by regulations for the hydraulic barrier in landfill sites. Thus, this study suggested that compacted sewage sludge could be used as a bottom barrier for tailing impoundment.