Impacts of thermal pre-treatments on the semi-continuous anaerobic digestion of waste activated sludge

Thermal pre-treatments can be used in order to enhance the efficiency of anaerobic digestion of waste activated sludge. The objective of this work was to study the effects of thermal treatment on the semi-continuous anaerobic digestion of the main sludge compounds (proteins, carbohydrates and lipids). Thermal treatment at 190 degrees C was more efficient than treatment at 135 degrees C in terms of total COD, lipids, carbohydrates and protein removals and methane production. However, treatment at 190 degrees C produced refractory soluble COD. In all cases, with or without pre-treatments, lipids degradation yield (67% without pre-treatment and 84% with 190 degrees C treatment) was higher than carbohydrates (56% without pre-treatment and 82% with 190 degrees C treatment) and proteins (35% without pre-treatment and 46% with 190 degrees C treatment) degradation yields. Methane production increased by 25% after the 190 degrees C treatment. This moderate enhancement could be explained by a good initial biodegradability of tested sludge and by the sludge storage which was done for a long period. Enzymatic reactions may occur even at 4 degrees C. However, this methane production enhancement was enough to produce energy to pre-heat the sludge. (c) 2006 Elsevier B.V. All rights reserved.