THP between digesters

Thermal hydrolysis between digesters

  1. From the wastewater treatment plant's primary and secondary treatment units, thickened raw sludge is fed into the first stage digester(s)
  2. Sludge is pumped out of the first stage digester(s) and pre-dewatered at about 16.5% DS and fed into the pulper. In the pulper, sludge is pre-heated by steam recovered in the flash tank
  3. From the pulper, the pre-heated sludge is fed to the reactors in a batch mode. After filling the reactors, steam is added to bring the sludge to the required temperature and pressure 
  4. 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
  5. Leaving the flash tank, the sludge is cooled to the typical temperature for anaerobic digestion, partly by adding dilution water and partly in heat exchangers. Then it is fed to the second stage anaerobic digester(s)
  6. The end product after anaerobic digestion is pathogen free and meets the most stringent regulations for organic waste treatment. The result is excellent quality biosolids that can be land applied


When applied between two anaerobic digester sstages, thermal hydrolysis has the following benefits:

  • Significantly improves the biodegradability of activated sludge
  • Higher biogas production compared to that of THP before anaerobic digestion
  • Sterilises sludge providing pathogen-free biosolids with low odour
  • High dry solids content in final biosolids
  • Significantly reduces downstream requirements for drying and other thermal processes, when applicable

Thermal hydrolysis process

How does thermal hydrolysis work?

Short description of how the thermal hydrolysis process works
Where does thermal hydrolysis fit

Where does thermal hydrolysis fit?

Thermal hydrolysis is integrated in wastewater treatment plants
Sewage treatment plant with sludge

An introduction to sewage sludge and sludge treatment

Learn about the basics of sewage sludge and sludge treatment, its primary steps, actors, and significance for public health and the environment.