Abstract: Provided is a method for performing thermal hydrolysis of sludge in a cyclic manner. A sludge stream is directed into one reactor during a first cycle and in a second cycle sludge is directed into another reactor. Each reactor has a sludge recirculation loop associated therewith. Flash steam from one reactor is transferred to the other reactor to pre-heat the sludge. The sludge in each reactor is recirculated through the circulation loop during each cycle of thermal hydrolysis. Live steam is directed into each reactor to raise the pressure and temperature within the reactor in order to enable the thermal hydrolysis process to take place.