The effect of temperature on the production of methane from dry
CM was examined at 37, 55, and 65 °C for 9 repeated batches. Although
production of biogas was observed at all temperatures, methane was
not produced in any of the repeated batches at 55 and 65 °C (data not
shown). Therefore, only the results of experiments at 37 °C are
described, as follows. At 37 °C, the highest production of biogas was
observed in the last 2 batches, the 8th and 9th batches, which were
15 l kg− 1 and 19 l kg− 1 CM, respectively (Fig. 1). Production of biogas
decreased during the 5th and 6th batches, and this decrease probably
occurred because the used CM was stored in the refrigerator for a long
time. This could happen since the organic matter present in fresh CM
contains a significant concentration of organic nitrogen, a large
percentage (40–90%) of which is lost (20) depending on storage
conditions. Moreover, there was a significant decrease in the activity
of the anaerobic consortia, so when it was replaced by fresh CM
(which contains more organic matter and more active anaerobic
consortia), production of biogas was restored in consequent batches.
Gas content was also measured in all the batches, and CO2 was the
most dominant gas in all the batches, until the end of the 6th batch at
37 °C, when methane production started.