As already indicated in Figure . 2a, the CO2 consumption is highest at the beginning of the first subsequence and then decays, until reaching a quasi steady-state situation at the end of this subsequence, where we have only a very small, constant CO2 consumption due to CH4 formation (see above). This behavior repeats in the second and third subsequence, only with a lower CO2 consumption. Reasons for that will be discussed in section 3.4.