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.