In the anaerobic digestion of solid wastes, hydrolysis is the rate limiting step and physico-chemical pretreatment is often required to promote solubilization of organic matter. As an illustration, anaerobic digestion was limited by the substrate solubilization step during continuous cultures performed with an industrial microbial biomass. In optimal operating conditions determined for the hydrolysis-acidogenesis step (T=35°C; pH=8.5; OLR=5.4 g COD/l.d) 50.6% COD solubilization was achieved. A thermochemical pretreatment based on sodium hydroxide addition, was used in order to enhance COD solubilization. Optimal conditions for COD solubilization were pH=12, T=140°C for 30 minutes. In these conditions, 70% COD solubilization was achieved. However, anaerobic biodegradability of the pretreated substrate was not improved and remained near 40%. The poor anaerobic biodegradability performances were attributed to the soluble molecules generated during the thermochemical pretreatment that were refractory and/or inhibitory to anaerobic microorganisms. Fractionation of the soluble pretreated microbial biomass by two methods (treatment with adsorbent resins and precipitation by pH adjustment) demonstrated that high molecular weight compounds (>100 kDa) are involved in the poor biodegradability and in the biotoxicity observed. Partial decolorization through resin use and acid precipitation remove these compounds. The consequence of their removal was an increase of the production of biogas.

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