Kinetics of microbial landfill methane oxidation in biofilters

A methane oxidizing biofilter system fitted to the passive venting system of a harbor sludge landfill in Germany was characterized with respect to the the methanotrophic population, methane oxidizing capacity, and reaction kinetics. Methanotrophic cell counts stabilized on a high level with 1.3×108 to 7.1×109 cells g dw−1 about one year after first biofilter operation, and a maximum of 1.2×1011 cells g dw−1. Potential methane oxidizing activity varied between 5.3 and 10.7 μg h−1 g dw−1. Cell numbers correlated well with methane oxidation activities. Extrapolation of potential activities gave methane removal rates between 35 and 109 g CH4 h−1 m−3, calculated for 30 °C. Optimum temperature was 38 °C for freshly sampled biofilter material and 22 °C for a methanotrophic enrichment culture grown at 10 °C incubation temperature. Substrate kinetics revealed the presence of a low-affinity methane oxidizing community with a high Vmax of 1.78 μmol CH4 h−1 g ww−1 and a high KM of 15.1 μM. KMO2 for methane oxidation was 58 μM. No substantial methane oxidizing activity was detected below 1.7–2.6 vol.-% O2 in the gaseous phase. Methane deprivation led to a decrease in methane oxidation activity within 5–9 weeks but could still be detected after 25 weeks of substrate deprivation and was fully restored within 3 weeks of continuous methane supply. Very high salt loads are leached from the novel biofilter material, expanded clay, yielding electric conductivity values of up to 15 mS cm−1 in the leachate. Values>6 mS cm−1 were shown to depress methane consumption. Water retention characteristics of the material proved to be favourable for methane oxidizing systems with a gas permeable volume of 78% of bulk volume at field capacity water content. Correspondingly, no influence of water content on methane oxidation activity could be detected at water contents between 2.5 and 20 vol.-%.