The effectiveness of liquid carbon additions to enhance zinc removal in laboratory-scale short hydraulic residence time (19 hours) compost bioreactors receiving synthetic mine water with a high influent zinc concentration (45 mg/L) was investigated. The unique combination of short hydraulic residence time, high strength wastewater and carbon additions was designed to investigate the potential for application of this approach to mine water treatment. Effective removal of zinc could not be sustained by sulfate reduction and / or other attenuation processes, in the presence of such elevated zinc concentrations, without carbon supplementation. Propionic acid addition resulted in improved and sustained performance (mean zinc removal 99%). Comparison of bioreactors receiving continuous propionic acid with those in which carbon addition ceased after a period of time demonstrated distinct differences in the microbial communities. The addition of propionic acid promoted the activities of sulfate reducing bacteria with the compost substrate becoming a net sink for sulfate, which led to efficient zinc removal via bacterial sulfate reduction. Upon cessation of propionic acid addition, carbon limitation resulted in oxidising conditions and the growth of sulfur oxidising bacteria with the compost substrate becoming a net source of sulfate, compromising zinc removal by bacterial sulfate reduction. These research findings show the potential for modest liquid carbon additions to compost-based passive treatment systems to enhance rates of metal attenuation in a short hydraulic residence time, enabling remediation of highly polluting mine drainage at sites with limited land availability.
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