Anaerobic digestion of the vegetable fraction of municipal refuses: mesophilic versus thermophilic conditions

The phenomena limiting the anaerobic digestion of vegetable refuses are stu died through batch tests carried out using anaerobic sludge previously sele cted under either mesophilic (37 degrees C) or thermophilic (55 degrees C) conditions. The compositions of the hydrolysed cellulosic and hemicellulosi c fractions of these materials are simulated by starch and hemicellulose hy drolysates, respectively. Non-hydrolysed mixtures of vegetable waste with s ewage sludge are used to ascertain whether the hydrolysis of these polymeri c materials is the limiting step of the digestion process or not. The exper imental data of methane production are then worked out by a first-order equ ation derived from the Monod's model to estimate the kinetic rate constant and methane production yield for each material. Comparison of these results shows that passing from mesophilic to thermophilic conditions is responsib le for a slight deceleration of methane production but remarkably enhances both methanation yield and methane content of biogas. The final part of the study deals with the fed-batch digestion of the same residues in static di gester. Working under thermophilic conditions at a loading rate threshold o f 6.0 g(COD)/l.d, the hemicellulose hydrolysate ensures the highest methane productivity (60 mmol(CH4)/l.d) and methane content of biogas (60%), while unbalance towards the acidogenic phase takes place under the same conditio ns for the starch hydrolysate. The intermediate behaviour of the non-hydrol ysed mixture of vegetable waste with sewage sludge demonstrates that hemice llulose hydrolysis is the limiting step of digestion and suggests the occur rence of ligninic by products inhibition on methane productivity.