MIKROBIOLOGISK OCH KEMISK OXIDATION AV MANGAN I RÅVATTEN / Microbial and chemical oxidation of manganese in raw water

The oxidation kinetics of manganese in a sandfilled column has been studied. The oxidation pathways can be both chemical and microbiological. The rate limiting step during chemical oxidation of manganese is the autocatalysis of manganese (II). The time to half the manganese(II) concentration from 1,1 to 0,55 mg/l at pH 8,5 was calculated to 30 days at 25°C with a proper kinetic model. In a sandfilter, manganese is removed
totally within 20 minutes.
Microbial catalysis of manganses oxidation may follow according to different mechanisms. Precipitation of manganese can take place at cell walls of dead microorganisms. Both living and dead microorgansims may contribute
to the oxidation in sandfilters. Typically, manganese oxidation in sand filters is microbiologically mediated, since the reaction rate is much faster than the corresponding chemical oxidation rate. The oxidation process may be described with the Michaelis-Menten kinetics.
This theory can also be used for interpreting the reaction mechanisms in a sand filter. If manganese oxidation is assumed to take place homogeneously in the sand volume, different detention times of water will give rise to the same operational constants KM and vmax for the Michaelis-Menten kinetics under the condition that the oxygene supply not is limited. If the biofilm in the reactor changes with the depth, both KM and vmax will
vary. One reason why the biofilm may vary with depth can be different manganese concentrations. The variations in measured KM and vmax in this report are minor and of no significant importance for the reaction rate. According to the results, vmax ~ 8 mg/(l•min) and KM ~ 1 mg/l could be calculated. This conforms that microorganisms
have great concequence for the oxidation of manganese in sand filters. The Michaelis-Menten constants can be used for calculating sand thickness and reactor volume in a filter.