THP before anaerobic digestion

Thermal hydrolysis before anaerobic digestion

  1. 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. 
  2. 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.
  3. From the reactor, the now sterilised sludge is passed rapidly 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.
  4. 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 anaerobic digesters
  5. The end product after anaerobic digestion is sterilised and meets the most stringent regulations for organic waste treatment. The result is excellent quality biosolids that can be land applied.


When applied before anaerobic digestion, thermal hydrolysis has the following benefits:

  • Significantly improves the biodegradability of activated sludge
  • Allows significantly higher loading rates resulting in smaller digestion plants. This can save tremendous amounts in upfront capital investment in: brownfield expansions, digester renewal projects and greenfield projects
  • Increases rate of biogas production
  • Sterilises sludge providing pathogen-free biosolids with low odour
  • Significantly reduces downstream requirements for drying and other thermal processes