This study is focused on evaluating two modes of anaerobic digestion, the Chemostat mode and the SBR mode, on mesophilic conditions, and tests the performance of this discontinuous mode in comparison of the conventional CSTR (Completely Stirred Tank Reactor) digestion. Thus, for this study a list of tools and methodologies were used in order to characterize the model substrate, its stability and this reaction. The first step was to characterize the hydrolysis of proteins, lipids and sugars and their transformation in VFAs (volatile fatty acids). Then, to see these intermediate reactions producing methane (reaction of acetogenesis and methanogenesis).A BMP (Biochemical Methane Potential) assays were done to analyses the biodegradability of the sludge and then compare the efficiency of methane production for both digesters. For HRT of 20 days there was observed an efficiency of 39 and 38 %, for Chemostat and SBR mode, respectively, with methanisable COD biodegradability of 57 %. To analyze the SBR mode, a monitoring of the VFAs and particle size were done. Under the operative conditions of the digester, it is interesting to see the evolution of consumption of the organic acids and define the hydrolysis reaction. Hydrolysis constant was estimated with a first exponential order with a value of 0, 07 days-1 for slow hydrolysis and 3 days-1 for fast hydrolysis. Another interesting point was the particle size distribution of the flocs. For input and output sludge, the distribution was unchanged and the settling test failed, because the substrate not settles and creates dense aggregates. Also a condition of HRT of 10 days was tested. There was no COD fell seen for both digesters, so they were operating with almost the same methane production and digesters efficiency. Kinetics of SBR mode were unchanged, even doubling the input flow rate, there was not observed an accumulation of organic acids.

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