TheMicrobiological Oxidation and Removal of Manganese from Drinking Water

Bya Continuous Recycle Fluidized Bioreactor

ReportNo WSAA 124

July 1997

SUMMARY

Thiswork extends the results of previous research in which a technique wasdeveloped for the immobilization of manganese-oxidizing cells of Pedomicrobium sp. ACM 3067 on magnetiteparticles and for their use in a continuous recycle fluidized bioreactor (CRFB)for the oxidation and removal of manganese from water. In this previousresearch a laboratory model CRFB was operated for 22 weeks with removal ratesof greater than 74% and up to 98% for Mn²⁺ concentrations in therange 0.25 to 8.5 mg/l when operated at a residence time of 21 hours. Themajority of the manganese in the effluent was residual Mn²⁺ withonly low levels of oxidized and adsorbed manganese present. The bulk of theoxidized manganese remained attached to the immobilized cells in the fluidizedcolumn. Under controlled conditions the optimal pH was pH 7.8-8.0 but activitysharply declined to very low levels at pH 7. As low activity at pH7 indicated apotential problem with the process for treatment of neutral surface waters, thecurrent research investigated the effect of influents in the range of pH 6-8without pH control. Under uncontrolled pH conditions manganese oxidation andremoval was found to be unaffected as the cells maintained an optimal pH of7.9-8.0 by metabolic activity. Manganese removal was also found to be enhancedat lower dissolved oxygen conditions with the optimal level being 2-4 ppm.Pilot plant trials operated over a period of four months confirmed theeffective treatment of raw drinking water at residence times as low as 15 min.The Pedomicrobium cells appeared toremain dominant and manganese oxidation was unaffected by the presence ofcompeting natural microorganisms in the influent water. At a residence time of15 min soluble manganese in treated water met the 0.1 mg/l guideline level forinfluent soluble manganese concentrations up to 0.6 mg/l, the 0.05 mg/l guidelinefor influent concentrations up to 0.3 mg/l, and the 0.02 mg/l guideline forinfluent concentrations up to 0.1 mg/l. The ability to remove insolublemanganese from influent water varied and apparently depended on the nature ofthe particulate matter. It is recommended that the microbiological process formanganese oxidation and removal be placed at the beginning of the treatmentplant and be followed by a filtration step to remove particulate material, anda chlorination step for disinfection. Further research is required toinvestigate manganese oxidation in waters containing high levels of ammonium,nitrite, and nitrate which may affect manganese removal rates. Further work isalso required to develop a continuous process to recover and dispose of the manganeseconcentrate and biomass from the fluidized bed.

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