Determination of the microbial population in thermophilic anaerobic reactor: Comparative analysis by different counting methods

This paper describes the determination of the microbial population, in terms of the number, biomass and composition, of single and two-phase, laboratory-scale thermophilic (55 degreesC) anaerobic reactors, under steady-state conditions. Epifluorescence microscopy with DAPI (4 ' ,6-diamidine-2-phenylindole) as fluorochrome was used to determine the total number of micro-organisms in the reactors, and autofluorescence microscopy for the number of the autofluroescent methanogenic populations. The results obtained by the direct count methods were compared to the quantity of biomass contained in the system, determined by volatile suspended solids. The viable bacterial population was determined by plating techniques using an anaerobic chamber. The total bacterial and F420 autofluorescent populations of single-stage digesters increase when the hydraulic retention time decreases; nevertheless, the percentages of the autofluorescent methanogens remain constant at 13%. In the two-stage reactors, the percentages of this group are 99% and 26% of the total population in the acidogenic and methanogenic factors, respectively. In the single-stage reactors, biomass determinations can be used to estimate microbial concentrations, and vice versa, as there is a high positive correlation between microorganism concentration and biomass. It was obtained a high correlation between direct counts by epifluorescence microscopy and viable plate counts for the combined system studied. (C) 2001 Academic Press.