The aggregation of bacteria into granular sludge would also be conducive to the creation of cell associations that are obligatory for the utilization of certain substrates. For example, the degradation of propionate and butyrate is thermodynamically unfavorable, unless the hydrogen produced during the oxidation of these two substrates is removed by an H2-consuming species. Close associations between organisms participating in interspecies hydrogen transfer were observed in the miniaggregates that formed in the liquid phase of a coculture of a butyrate oxidizer and an H2-consuming methanogen. Thus, it was not surprising when the close juxtaposition of a propionate degrader and an H2-consuming methanogen was shown to occur in granular material.