Feeding granular methanogenic sludge for prolonged periods with acetate as sole energy source resulted in weaker, more fluffy pellets.Incubation of granular methanogenic sludge in batch culture under growth- supporting conditions generally resulted in the overgrowth of the original seed granules by dispersed growing bacteria and development of fluffy pellets. Cultivation of granular methanogenic sludge in batch culture was only possible in experiments where propionate was stoichiometrically converted to methane and acetate (and probably carbon dioxide).By studying the potential methanogenic activities of granular methanogenic sludges (chapter 4) it was shown that the biomass consisted for a significant part of acetoclastic methanogens. These potential specific activities showed that obligately acetoclastic methanogens were quantitatively more important than hydrogenotrophic methanogens. These conclusions were drawn from a comparison between the potential methanogenic activities measured in the present experiments, and literature data on the potential activities of various groups of anaerobic bacteria. This approach provided a reliable tool for the estimation of the microbial composition of methanogenic consortia and was faster and more accurate than MPN counts. The resulting estimates of the acetoclastic biomass with respect to the occurrence of obligately and facultatively acetoclastic methanogens were corroborated by microscopic observations and indicated that Methanothrix soehngenii -like organisms are the most numerous acetoclastic methanogens in granular sludge.An indirect method for the determination of the potential methanogenic activity of anaerobic biomass as suggested by others, viz. the estimation of the amount of F 420 present in methanogenic consortia was tested with granular sludge in the present study. It was shown (chapter 5) that a good correlation existed between the F 420 content of different granular consortia and their potential methanogenic activity on formate. No correlation, however, was observed between the F 420 content and the potential acetoclastic methanogenic activity of such consortia. This indicates that F 420 content is not a reliable parameter for assessing the overall potential methanogenic activity of anaerobic biomass.In densely packed biolayers mass transfer limitation will influence the conversion capacity of the biomass at low substrate concentrations. The results reported in chapter 6 show that the effect of mass transfer limitation in granular methanogenic sludge is dependent on the composition of the biolayer, viz. its potential specific activity, the thickness, and on the type of substrate. To detect significant effects methanogenic granules had to be cultivated on specific substrates which resulted in the development of uniform biolayers. 
### Language ###
English

### Assessment ###
The text discusses microbial composition in methanogenic sludge, with a clear mention that Methanothrix soehngenii is the most common type of acetoclastic methanogens found in granular sludge.

### Query ###
What is the most common type of acetoclastic methanogens found in granular sludge?
What is the most common type of acetoclastic methanogens found in granular sludge?