Biofilters are effectively used for drinking water treatment. However, the long ripening time of virgin media for manganese (Mn) removal is a major concern. In this study, the influence of the flow regime on the ripening time of virgin pumice medium was investigated. For this purpose, pilot-scale experiments were performed to compare the start-up of flow-through and recirculating filter columns using inherent inoculation with the same groundwater source. The systems were operated at 2 m·h¹ with gradual flow increments up to 5 m·h¹ and avoiding Fe-loading. Effective Mn removal (.90%) in flow-through and recirculating columns was achieved after 8 and 23 days, respectively. Flow-through columns reached compliance with a local drinking water criterion (Mn, 0.1 mg·L¹) at 15 cm filter depth in 11 days. Recirculating filter columns required 32 days to reach compliance at 30 cm depth. The start-up in recirculation regime resulted in a water consumption reduction of about 50% compared with flow-through regime. The intermittent provision of the Mn-loading in recirculating regime impacted the Mn-oxidizing bacteria (MOB) concentration in the pumice stone medium. Both flow regimes required a similar total Mn-loading (0.16 and 0.11 kg·Mn·m², respectively), suggesting that Mn-loading was the limiting factor for the ripening of pumice.

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Biofiltration for Mn removal has not been proven in the tropics (18-29 °C). Also, the use of pumice as an alternative filter medium for Mn removal is still poorly known. In this study, a bench-scale biofiltration experiment, using virgin pumice, silica sand, and anthracite, was conducted for 107 days using tropical groundwater at 22 °C. Characterization of manganese oxide (MnOx) on filter media was carried out by Raman spectroscopy, X-ray diffraction (XRD), and scanning electron microscopy (SEM). The ability of culturable bacteria to oxidize Mn was verified by the leucoberbelin blue dye assay (LBB). The microbial activity on filter media was studied by ATP analyses. Identification of the MOB was performed by 16S rRNA sequencing. Results showed that the ripening time in each column was similar (∼80 days); therefore, the filter media and water temperature do not seem to accelerate the start-up period. The MnOx present on all ripened media was of the birnessite type. The MnOx morphology, influent water parameters, and microbiological activity suggest a start-up period likely assisted by biological oxidation. Therefore, biofiltration for manganese removal is feasible under tropical conditions. The similarity in the performance of pumice with the other media confirmed its suitability for biotic Mn removal. All materials presented similar known MOB at the genus level; however, different closest related species colonized selectively the filter media. Two strains of Pseudoxanthomonas sp., a not recognized genus on matured biofilters, look promising as inoculums in pumice and sand. Proper application of biofiltration in the tropics needs operational and bioaugmentation strategies.

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The presence of manganese (Mn) in drinking water causes aesthetic, operational, and health problems. Removal of Mn(ii) by the use of Mn oxidizing microorganisms in biofiltration processes has been studied leading to a more economical and environmentally friendly process than conventional treatments with chemical products. There are multiple reports in temperate regions, but little information under tropical conditions, concerning Mn(ii) removal by microorganisms. In this study, 13 autochthonous culturable manganese oxidizing bacteria isolated from water of two wells with ∼0.6 mg L-1 and ∼0.2 mg L-1 Mn(ii), respectively, were evaluated in vitro to determine their potential as inoculums for application in biofiltration processes. The oxidation capacity of Mn(ii) was evaluated by a qualitative test with Leucoberbelin blue I and the Mn(ii) removal was quantified by measurements of the residual Mn(ii) in a culture broth with an initial concentration of 5.9 mg L-1. The biofilm formation capacity of the strains was also determined by crystal violet staining and by quantification of polysaccharides of extracellular polymeric substances. Three strains showed the highest percentage of Mn(ii) removal in the culture in broth, two belonging to the genus Aeromonas (HAC-12 and RUE-1), with removal values of 42.5% and 40.1%, respectively, and one belonging to the genus Stenotrophomonas (S-3), with 40.3% removal. Besides, HAC-3B and RUE-6A showed the highest production of exopolysaccharides. These results allowed the selection of an Aeromonas strain (HAC-12) and one Stenotrophomonas strain (S-3) as possible candidates to be used as inoculums in biofilters for the removal of Mn(ii) from groundwater. @en