What is Thermal Hydrolysis?

Thermal hydrolysis is a pre-treatment to the conventional anaerobic digestion of organic waste. In general terms, it is a two-stage process:

  1. The sludge is heated up under high-pressure conditions
  2. The high-pressure material is flashed

This combined action fractures the cell structure and makes the waste more biodegradable, improving the anaerobic digestion performance and yielding more biogas. In addition, thermal hydrolysis sterilizes the sludge. The destruction of pathogens, or pasteurization, results in high-quality sludge that is suitable for land application as fertilizer or compost even under the most stringent regulations. Finally, this pre-treatment adjusts the rheology of the material to such an extent that loading rates to the anaerobic digester can be doubled. The dewaterability of the sludge is also significantly improved to up to 40% total dry solids, resulting in less biosolids. While research and development of this technology started back in the 90s, the technology has come long ways since evolving from the initial batch processes into the cutting-edge TH4+ process.

What is TH4+?

TH4+, or Thermal Hydrolysis for +, is teCH4+ innovative process that comprises the following steps:

1. Feed conditioning

The organic waste is fed into dosification vessels and heated up with vapors from the heat recovery section (Step 3) for optimum heat integration and energy efficiency. Two parallel dosification vessels sequentially feed the organic material to pressurization tanks, where the waste is pressurized using compressed air or steam. This allows the organic material to flow through the process without the need for pumps or any other mechanical means.

2. Hydrolysis

The pressurized material enters a mixer, where live steam is injected to achieve the temperature setpoint in an extremely rapid manner. In conventional processes, the material to be hydrolyzed is kept at high temperatures for approximately 30 minutes, long enough for the material to undergo secondary reactions that reduce its methanogenic potential. This limits standard hydrolysis temperatures to 180ºC. The TH4+ process overcomes this limitation by means of an exceptionally quick heating time of below 5 seconds. This greatly minimizes the impact of the secondary reactions even at temperatures as high as 220ºC. The hot material is flashed to a regulation tank that provides a stable pressure throughout the system.

3. Heat recovery

The content of the regulation tank is fed to the flash vessel through a series of flash valves. This second sudden decompression further fractures the cells structure, making the material more soluble

Why Thermal Hydrolysis?

  • Enhances biogas yields by 30% approx.
  • Improves biogas quality (more CH4, less H2S)
  • Decreases GHG emissions and carbon footprint, as the additional biogas is a renewable energy source
  • Reduces sludge volume, cutting drying and handling costs
  • Improves sludge quality producing pathogen-free, odor-less biosolids
  • Enables a foam-free, smoother anaerobic digestion downstream process
  • Debottlenecks existing digesters, doubling their loading capacity and avoiding significant capital expenditure

Why TH4+?

  • Best-in-class technology
  • Continuous hydrolysis process that overcomes the issues linked to batch processing
  • Super quick heating enables higher temperatures and pressures
  • No need to pump the viscous, highly abrasive organic material:
    • Higher reliability as eliminates recurrent mechanical problems
    • Lower maintenance cost
  • Regulation tank ensures stable pressure control throughout the system
  • Optimized heat integration for lower specific energy consumption
  • Compact design with reduced footprint

Technologies Comparison

CompanyTechnologyTime (min.)Temp. (ºC)FlashReactorHeat exchangersPumpsMaintenanceFootprint
teCH4+TH4+< 1220-160DoubleNew
H: High / L: Low / Y: Yes / N: No

Economic Comparison

Based on a 400,000 population equivalent (PE) thermal hydrolysis plant, the main economic benefits are:


Savings Increased biogas production k€/year


Savings Higher-quality biosolids k/year


Savings Lower CO2 emissions, k€/year


Energy cost, k€/year


Maintenance cost, k€/year

Total savings, k€/year

Total investment, k€


Return Payback, years


Return IRR, %


Return NPV, k€