PILOT SCALE CONTINUOUS THERMAL HYDROLYSIS OF ORGANIC WASTES FOR INCREASED BIOGAS PRODUCTION
Thermal hydrolysis is an established method of pre-treating organic wastes prior to anaerobic digestion or as an intermediate step between two anaerobic digesters. The process can be likened to pressure cooking, where material is subjected to high temperatures (usually 100-200°C) and respective pressures for a defined period of time (usually <1 hour). For many full-scale and, to the best of our knowledge, all pilot and laboratory-scale applications, the equipment used usually operates in batch mode due to technical difficulties of adding and removing material to a pressurised continuous process. The advantages of a continuous (or semi-continuous) process at full-scale over a batch process include a smaller footprint and lower operational costs. At pilot or laboratory-scale, batch thermal hydrolysis reactors suffer from problems of slow heating and cooling times which leads to poor definition of true treatment times. This work describes the development and operation of a pilot scale (treatment volume of 1.02 litres) thermal hydrolysis system that operates semi continuously. By using a high electrical heating power of 7.2 kW and a high heating surface to volume ratio, the system can treat materials for periods of just a few minutes. The system has been tested using the liquid fraction of cattle manure and biogas batch tests have shown that methane yields were increased by up to 40.5% at four days digestion, but the improvement was less pronounced at longer digestion times, with 12% increased methane yield after thirty two days.