How does thermal hydrolysis work?

Thermal hydrolysis is a process technology applied in wastewater treatment plants with anaerobic digestion. Thermal hydrolysis exposes sewage sludge or other types of wet organic waste to high temperature and pressure. The process is similar to preparing meals using steam.


From the wastewater treatment plant’s primary and secondary treatment units, raw sewage sludge is collected and dewatered to 16-18% dry solids.

This thickened sludge is continuously fed into the pulper. The pulper has the role to homogenise and pre-heat the sludge to a temperature close to 100°C, using steam recovered from the flash tank.


From the pulper, the warm sludge is fed continuously to the reactors, in a sequential process that ensures sealed batches of sludge in each reactor. Once a reactor fills up, sludge flows to the next available one. There are typically between 2 and 5 reactors in a Cambi thermal hydrolysis train, depending on sludge volume, reactor size and hydraulic retention time.

When the reactor is full and sealed, steam is pumped to raise the temperature to 160 to 180°C at a pressure of about 6 bars. The thermal hydrolysis process is typically set at 20 to 30 minutes for each batch, to ensure pathogen kill.


From the reactor, the now sterilised and hydrolysed sludge is passed to the flash tank, which operates at atmospheric pressure. The sudden pressure drop leads to substantial cell destruction for the organic matter in the sewage sludge. The steam generated by the pressure release is returned to the pulper to preheat the incoming sludge.

Leaving the flash tank, the sludge is cooled to the typical temperature for anaerobic digestion, in heat exchangers. Then it is fed to the anaerobic digesters.