An interesting correlation between methane production rates and archaea cell density during anaerobic digestion with increasing organic loading

Changes in the cell density of bacteria and archaea during anaerobic digestion producing methane gas with increasing the organic loading were determined using real-time polymerase chain reaction (PCR). It was confirmed that the methane production rate (MPR) was proportional to the cell density of archaea, whereas no clear relationship was observed between the cell density of bacteria and MPR. When the organic loading rate (OLR) increased, the bacteria activated quickly to produce volatile fatty acids (VFAs). However, the enhancement of archaeal activity to consume VFAs and produce methane gas was relatively small. This was because the high oxidation-reduction potential (ORP) and VFA concentration associated with the high OLR were harmful to the archaea. Consequently, when the OLR was very high, VFAs accumulated at high concentrations, the cell density of archaea was reduced, and finally the fermentative methane production deteriorated. (C) 2015 Elsevier Ltd. All rights reserved.