Characterization of Passive Treatment Sy

Treatment of mine drainage, during operation and following abandonment, can represent a substantial cost to mine operators and governing authorities. Recovery of valuable metals from mine drainage as part of the treatment process may offset costs, yet this is rarely seen in practice. Industrial-scale metal recovery from mine waters is the preserve of active treatment systems, where reactor conditions are carefully controlled to ensure consistent quality. In many circumstances, passive treatment is preferred, but close process control is not feasible. This study analyzes substrate from a pilot-scale compost-based passive treatment system, operational for two years, using bacterial sulfate reduction to remove metals as their sulfides. The results indicate that the used substrate is categorized as 'hazardous' under UK regulations due to accumulated zinc and requires pre-treatment. Notably, zinc concentrations were highest in the upper substrate layer, suggesting selective harvesting may reduce waste volumes and facilitate recovery. Batch-scale leaching tests showed recovery rates exceeding 83-96% of zinc from the upper-layer substrate, depending on acid strength, within a 100-hour leach test. These findings highlight the potential economic benefits of metal recovery in larger-scale passive treatment systems.