Five different sources of inocula were studied to determine its influence on biodegradability tests. Inocula were characterized determining granulometry, specific methanogenic activity, solids content, and volumetric sludge index. Also, the fermentative, aceticlastic, hydrogenophilic, OPHA, and sulfate-reducing groups were determined by the most probable number technique. Anaerobic biodegradability tests were conducted with two different substrates, one easy to degrade (glucose) and a toxic one (phenol). The best performance, in terms of percent of biodegradation and lag time, for both substrates, was obtained with the inoculum from a brewery industry UASB. The results can be explained in terms of the initial activity of the inoculum. The influence of the significant variations found in the specific methanogenic activity of the five inocula studied is discussed, in terms of the microbial composition of the samples. The results emphasized the importance of the selection of an appropriate source of inoculum in order to obtain reliable results.

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