The changes in the microbial population of a two-stage anaerobic diges
tion system in terms of number and composition were studied during the
start-up phase. The numbers of methanogens and non-methanogens slight
ly decreased in the upflow filter, whereas in the pre-acidification st
age the number of acidogens was reasonably constant. Variations in the
morphology of the methanogens occurred along the filter, for example
the numbers of Methanosarcina and filamentous bacteria species decreas
ed considerably in the drain, possibly becoming attached to the biofil
m along the reactor, whereas in the pre-acidification stage only a few
Methanosarcina species were identified. Slight variations within the
bacterial populations occurred in terms of composition and numbers in
the filter. The number of viable methanogens varied in the filter effl
uent depending on the OLR changes but this did not adversely affect th
e COD removal efficiency of the upflow filter. Among the autofluoresce
nt methanogenic population, Methanococcus, Methanobacterium, Methanobr
evibacteria, and Methanosarcina species were tentatively identified in
the filter.