Effects of batch and continuous-flow operation on biotreatment of Mn(II)-containing mine drainage

The biotreatment of mine drainage containing dissolved manganese (Mn) using Mn(II)-oxidizing bacteria is challenging. Sequencing-batch (SBRs) and continuous-flow reactors (CFRs) packed with limestones and inoculated with the mine-drainage microbial community were compared to determine the removal efficiency of Mn(II) from mine drainage. Mn(II) removal in CFRs was 11.4%±0.0% (mean ± standard deviation) in the first two weeks and; it slightly increased to 13.6%±0.0% after four weeks, and more than 94% of Mn(II) was removed under the steady-state treatment phase. The performance of SBRs was more effective, wherein 24.4%±0.1% of Mn was removed in the first two weeks, and in four weeks, surpassed 66.6%±0.2%. Rapid Mn(II) removal observed in the start-up of SBR resulted from higher microbial metabolic activities. The adenosine triphosphate (ATP) content of the microbial community was four-fold more than in CFR, but comparable during the steady-state phase. The Mn-oxide deposits occurring in the SBR and CFR at steady-state were mixed phases of birnessite and woodruffite, and the average Mn oxidation valence in the SBR (+3.73) was slightly higher than that in the CFR (+3.54). During the start-up treatment, the closest relatives of Methyloversatilis, Methylibium, and Curvibacter dominated the SBR, whereas putative Mn oxidizers were associated with Hyphomicrobium, Pedobacter, Pedomicrobium, Terricaulis sp., Sulfuritalea, and Terrimonas organisms. The growth of potential Mn-oxidizing genera, including Mesorhizobium, Rhodococcus, Hydrogenophaga, Terricaulis sp., and ‘Candidatus Manganitrophus-noduliformans’ was observed under the steady state. The SBR operation was effective as a prior start-up treatment for mine drainage-containing Mn(II), through which the CFR performed well as posterior bio-treatment.