H2/CO2 (FED-BATCH) FERMENTATION FOR BIOLOGICAL PRODUCTION OF CH4 BY PURE CULTURE OF HYDROGENOTROPHIC ARCHAEA

With the recent approval of the Paris agreement, we achieved another step in favour of sustainable energy production, declaring once again that fossil fuels must be replaced by a new generation of fuels from renewable end recycled sources. However, new processes for producing green energy need to be competitive and appealing not only at an environmental level but also economic. The biological methane production (BMP) process owns a great potential due to its ability to convert directly gaseous substrates simply using the natural metabolism of methanogens which act as bio-catalysts. This study wants to present results related to CH4 production from H2 and CO2 in exponential fed-batch fermentations using pure culture of Methanothermobacter marburgensis in a 2L reactors system. By the support of design of experiment (DoE) methods, different experiments were performed varying the value of exponential factors of liquid dilution rates, gas feeding rates and H2/CO2 ratio. The quantitative analysis reveals that by increasing the gas feeding rate and the trace elements dilution rate the culture performances improved. The highest value of methane evolution rate (MER) achieved here is the highest ever published among the studies concerning fed-batch fermentation of H2 and CO2 by Methanothermobacter marburgensis. Furthermore, high trace element (TE) feeding was decisive for the biomass growth. A second hydrogenotrophic archaeal strain, Methanothermococcus okinawensis, was also applied for the BMP process and tested in the 2L bioreactors in fed-batch mode. The first preliminary results are presented here and they show that Methanothermococcus okinawensis was able to grow and to produce methane, but the conditions in the reactor were limiting the culture growth and the productivity. On the base of the considerations on the TE importance, Methanothermococcus okinawensis was also cultivated at increasing concentration of TE in closed batch mode in order to examine the effects on its performances. It was found that higher TE amount boosted both growth and CH4 productivity in closed batch cultures compared to standard concentration of TE. Therefore, varying the TE concentration within the tolerance thresholds could be a turning point for producing high biomass concentrations and high MER