The Influence of Engineering Scale and Environmental Conditions on the Performance of Compost Bioreactors for the Remediation of Zinc in Mine Water Discharges

Abstract

The effects of engineering scale on the performance of a compost-based system for the remediation of a discharge from an abandoned metal mine was investigated by simultaneous operation, under field conditions, of a laboratory-scale column and a pilot-scale system. The two systems contained identical reactive substrate, comprising limestone gravel, compost, wood chips and activated sludge from a municipal waste water treatment plant, and had an initial hydraulic residence time of approximately 19 h. The influent mine water contained around 2–2.5 mg/L zinc and had a circumneutral pH. Clear differences in the performance of the systems were seen, demonstrating the importance of engineering scale in the remediation of zinc. The laboratory-scale column was most effective at removing zinc, with approximately 96 % of the influent zinc attenuated within the system, while the pilot scale system removed, on average, 84 % of the influent zinc. The poorer performance of the pilot-scale reactor may, in part, be due to preferential flow, as indicated by a greater reduction in hydraulic residence time than in the laboratory-scale system. Early indications are that temperature also plays an important role in the attenuation of zinc within such systems, possibly linked to reduced microbial activity during periods of low temperature. Despite an apparent decrease in sulphate concentration within both systems, it is unclear whether bacterial sulphate reduction is the dominant mechanism for metal removal or whether sorption processes prevail. Implications for full-scale design of these treatment systems are discussed.