CO-DIGESTION OF MUNICIPAL SEWAGE SLUDGES AND PRE-HYDROLYZED WOODY AGRICULTURAL WASTES

Material balance has been used to evaluate the COD behaviour and the t ime required for fed-batch digestion of mixtures of domestic sludges a nd pre-hydrolysed agricultural wastes. Pre-hydrolysis of the feed mate rials has been used to penetrate the strong lignocellulosic structure of these wastes as well as to increase the fraction of soluble organic substances in the mixture. The influence of the organic loading rate on the main process parameters (methane, carbon dioxide, total biogas productions and their respective conversion yields) has also been inve stigated. The organic load has been varied from 0.8 up to 6.1 g(COD) d m(-3) day(-1), corresponding to a range of volatile solids load of 0.6 -4.5 g(VS) dm(-3) day(-1) for the material under consideration. These values are slightly higher than those usually employed in conventional digester for domestic sewage sludges. However, methane production rea ched a maximum rate of only 5.6 mmol dm(-3) day(-1) at an organic load ing rate of 4.6 g(COD) dm(-3) day(-1), while both CH4 content and prod uction of biogas rapidly fell over 2.2 g(COD) dm(-3) day(-1). On the w hole, these results suggest that removal of lignin is necessary in ord er to carry out the continuous anaerobic digestion of pre-hydrolysed a gricultural wastes rich in woody materials.