Two methane utilizing bacteria strains, GB 21 and WSB 874, were cultivated with methane as sole carbon and energy source in a submerged continuous fermentation process. The aim of the investigations was to reach high biomass concentrations (> 25 g BDM . kg-' medium) and high productivities (> 3 g BDM . kg-' . h-I). A precondition for the high-performance fermentation process was to ensure a transfer rate of methane and oxygen in quantities required by the microorganisms. For this purposes a high performance stirred pressure fermenter was used. The fermentation process is characterized by the transfer of two gaseous substrates. In order to develop a technical process it was necessary to investigate both the correlations between the two gases and the productivity of the fermentation process and the influence of system pressure on the microorganisms.Within a pressure range of up to 0.7 MPa a biomass concentration of up to 50 g BDM . kg-' medium and a productivity of up to 9 g BDM . kg-' . h-' was reached in a continuously running fermentation process under nonstenle conditions. Under these fermentation conditions the microbial population consisted of a dominating main culture GB 21 or WSB 874, respectively, and an accompanying flora (type 11). The microbial population was very stable in its composition during the fermentation process running continuously for weeks and months.

Using a methanol utilizing acidophilic MB 58 bacteria strain (IMET 109453, a stable fermentation process was performed in a pilot plant pressure fermenter of 250 I total volume for more than four months. Under the fermentation conditions (system pressure and retention time) used, productivities ranged between 2.3 and 9.4 BDM kg-l. h-l and growth yields between 0.31 and 0.42. Models describe the correlations between yield and retention time. The mean value of growth yield amounts to 0.41 at a retention time of 6.5 h.