In the present work the characterisation by image analysis of anaerobic biomass under organic and hydraulic shocks was performed. The digester was fed with a synthetic substrate, containing 50% oleic acid (as COD). Organic and hydraulic shocks were performed by stepwise increasing the substrate concentration or by reducing the hydraulic retention time. In both cases the organic loading rate changed from 6 to 30 kg COD/m3.d. Hydraulic shock induced a fast decrease in the number of free filaments and in total filament length, which attained a minimum value 51 hours after beginning the shock. The initial filament values were not recovered 340 hours after the hydraulic shock. In the organic shock, the minimum values of these parameters were detected 200 hours after beginning the shock and initial values were recovered 840 hours after. During the hydraulic shock the methanogenic acetoclastic activity was directly correlated to the number and length of free filaments. This result suggests that filaments are predominantly acetoclastic bacteria, probably Methanosaeta.

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