Investigation of aerobic and anaerobic oxidation of methane in hotspots of microbial methane oxidation systems

Landfill gas, in which methane is included, is produced by the biological degradation of untreated organic waste, and if released in the atmosphere can contribute to global climate change. As a complementary or replacement measure to gas extraction systems, microbial methane oxidation systems (MMOS) can control methane emissions from landfills. This thesis work focuses on the research to improve MMOS performance; both aerobic and anaerobic methane oxidation in landfill covers are investigated. Particularly, anaerobic methane oxidation in landfill covers is presented, as it is a novel research field. The laboratory experiments performed are mainly two, in order to determine aerobic and anaerobic methane oxidation potential of a compost material, which was sampled in a biowindow eight years after the installation. The material was sampled at two different depths: the shallow part – which represents the main methane oxidation horizon – and the deeper part – which might have been exposed to anaerobic methane oxidation – at a hotspot area, where the MMOS was temporarily overloaded with methane. The first experiment was performed under aerobic conditions in four soil columns. Two of them were filled with soil samples from the shallow part of a biowindow hotspot, while the other two with the deeper part of the same hotspot. Pure methane was supplied from the bottom of the columns and air from the top; three different gas phases were investigated, in which both air and methane flow rates were changed. The second experiment was performed under anaerobic conditions, using two of the columns of the aerobic test, filled with the different samples. To simulate anaerobic conditions, pure nitrogen was supplied at the top and methane was provided from the bottom. The parameters monitored during the tests were the composition of the exhaust gas and of the column interior gas at various depth levels, together with chemical, physical and maturity parameters, checked at the beginning and end of both experiments. Concurrently with the main experiments, Fourier Transform Infrared Spectroscopy (FTIR) analysis was performed in order to compare the IR spectrum of biowindow field samples with material from column experiments, under aerobic and anaerobic conditions. Results from the columns under aerobic conditions show the high oxidation rate of the material, even after eight years of temporal methane overload, under optimised laboratory conditions. A slight difference is found between the samples. In particular, only when the methane flow rate was increased, in the second phase of the experiment, the material of the shallow part of the hotspot shows a bigger decrease in the methane oxidation rate, with respect to the deeper part, indicating a higher sensitivity to methane load, perhaps due to the formation of extracellular polymeric substances (EPS) – execrated by the microorganisms under stress conditions – or to the higher adaptation to methane load of the microbial community present in the lower layer. Regarding the columns under anaerobic conditions, just three measurements are reliable, due to technical problems during the experiment. Nonetheless, these results show a decreasing profile of methane concentration from the inlet to the outlet gas; this could imply the presence of methane oxidation also under anaerobic conditions. This experiment should be performed for a longer period, with a higher number of samples, in order to have a statistically significant number of measurements to evaluate anaerobic methane oxidation process in MMOS. The results from the FTIR comparisons, show no differences in the composition of the samples. The only differences found were in the concentrations of ammonium ion, organic sulphates and EPS, which poorly varied. Also in this case the comparison should be performed with a substantially higher amount of samples, to obtain statistically significant results.