Influence of the operating conditions of the intermediate thermal hydrolysis on the energetic efficiency of the sludge treatment process.

The application of steam explosion between two stages of anaerobic digestion may improve energy recovery from sludge while increasing organic matter removal. The influence of the operating conditions of the thermal process: temperature (130-210 °C), retention time (5-45 min) and TS concentration (5.4-10.8%), on the efficiency of VS removal, the biochemical methane potential of hydrolysed sludge and the kinetic constant of the degradation were evaluated using a Taguchi design. Increasing temperature and time increased the removal of VS and the potential of methane production but the kinetic constant was higher at lower temperatures. An optimal operating scheme was found at 170 °C (6 barg), 25 min at the greatest TS concentration in the feeding. Under such conditions, the thermal energy obtained from biogas combustion in a CHP covered the requirements for vapour generation and a profit of 3.54 € m of sludge was estimated.