Removal of iron and manganese ions from abandoned neutral or alkaline mine drainage via ozone oxidation and micro-sand filtration: a pilot-scale operation

Abstract: The objective of this study was to evaluate the ozone oxidation of dissolved Fe and Mn ions in abandoned neutral or alkaline mine drainage (NAMD) and the subsequent micro-sand filtration by conducting a pilot-scale operation with several ozone doses in an ozone reactor. The effects of the hydraulic retention time on the ozone oxidation were also investigated. The pH values were either neutral or slightly alkaline and the average Fe and Mn concentrations in the abandoned NAMD were 25.5 and 2.06 mg/L, respectively. The experiment results indicated that six-hour ozone oxidation with an ozone dose of 24.0 g O₃/h (i.e. initial ozone concentration: 3.53 mg O₃/L) and subsequent micro-sand filtration could meet the drinking water quality standard (less than 0.3 mg/L) for both Fe and Mn in the micro-sand filter effluent. The hydraulic retention time of the ozone reactor at this ozone dose was 106 s, and the media of sand grains were between 0.4 and 0.7 mm in diameter. The total dissolved solids, alkalinity, and (Formula presented.) concentrations of the influents were virtually unchanged in this pilot-scale operation. However, the oxidation-reduction potential of about 18 mV in influents increased rather gradually in effluents, and was recorded in the range of 185–210 mV during ozone oxidation. The ozone oxidation and subsequent micro-sand filtration under the operating conditions, in this study, appear to be among the desirable alternatives for the effective removal of dissolved Fe and Mn in the form of metal precipitates during ozone oxidation. This alternative could prevent metal precipitates from settling at the bottom of local water channels by removing them before they are discharged into streams.